Characteristic Features of Rotary Kilns

Characteristic Features of Rotary Kilns

A rotary kiln is a cylinder which rotates around its cylindrical axis and acts as a device to exchange heat. The kiln is for high temperature applications where it is necessary to change the ‘state’ of the material in a continuous process or in a batch type process. The construction, position, and alignment of kiln is an essential factor for the smooth operation. Slight inclination with the horizontal axis makes the movement of the solid bed in the kiln towards the discharge head. The operational efficiency of the kiln is based on various parameters like inclination angle, temperature, rotation speed, and material flow and discharge rate.

The rotary kiln is a horizontal circular cylinder lined with refractory material supported by support stations and driven through a girth gear and drive train. The drive train consists of DC (direct current) electrical motors and gear boxes with hydraulic packs. The kiln cylinder is located at an angle to the horizon and rotates at low revolutions around its longitudinal axis and operates essentially as a heat exchanger, dryer, calcinator, and incinerator. The inclination of cylinder makes an axial displacement of the solid bed, which moves towards the discharge end.

The rotating cylinder acts simultaneously as a conveying device and stirrer by the use of internal fins which helps to mix and rotate the material in radial direction. Inclination angle of the cylinder, operating temperature, rotating speed, material flow rate, and discharge rate are the important parameters for the performance of the kiln. Kiln control is one of the most vital parts and the kiln is very sensitive for operation. Control of the kiln during its operation, the assemblage of various components, and process parameters is essential one in the rapid fast developing environment.

Rotary kiln is an advanced thermal processing equipment used for processing solid materials at extremely high temperatures in order to cause a chemical reaction or physical change. It is normally used to carry out processes such as (i) calcination, (ii) thermal desorption, (iii) organic combustion, (iv) sintering / Induration, (v) reduction, (vi) heat setting, and (vii) several more processes. While rotary kilns have been originally developed for use in the cement industry, because of their flexibility, they can be now found throughout a variety of industries, aiding in both processing commodities, as well as in highly specialized applications. Rotary kilns have been synonymous with cement and lime kilns probably because of the history of their evolution and development.

Some of the most common kiln applications of rotary kilns in use today include (i) roasting a wide variety of ores, (ii) sintering of materials such as dolomite, and magnesite etc., (iii) reduction of ores such as in sponge iron production, (iv) calcination of limestone, gypsum, and bauxite etc., (v) incineration of waste materials, (vi) desorption of soil contaminants, (vii) upgrading of phosphate ores, (viii) waste lime recovery, (ix) catalyst activation, (x) activated carbon production and re-activation, (xi) plastics processing, (xii) ceramics processing, (xiii) activated coal regeneration, (xiv) lignite degasification, (xv) scrap tyre recycling, (xvi) sewage sludge disposal, and (xvii) waste wood recycling etc.

Rotary kiln can also be used as a rotary dryer to remove water and moisture content from solid substances by introducing hot gases into a drying chamber. Kiln shell is to be structurally strong with non-conductor lining and designed to withstand high temperature and prevent the thermal losses of the kiln. Rotary kilns have become the backbone of several new industrial processes which make the world a more efficient and sustainable place. As the new applications for rotary kilns are continued to be developed, much experimental work is being done, prompting several questions and the need for further study and development.

The wide spread usage of the rotary kilns  can be attributed to such factors as (i) the ability to handle varied feedstock, (ii) spanning slurries to granular materials having large variations in particle size, and (iii) the ability to maintain distinct environments, for example, reducing conditions within the bed coexisting with an oxidizing freeboard (a unique feature of the rotary kilns which is not easily achieved in other reactors). The nature of the rotary kiln, which allows flame residence times of the order of 2 seconds to 5 seconds and temperatures of over 1,750 deg C, makes such kilns a competitive alternative to commercial incinerators of organic wastes and solvents.

The operation of rotary kilns, however, is not without problems. Dust generation, low thermal efficiency, and non-uniform product quality are some of the difficulties which still plague rotary kiln operations. Although the normally long residence time of the material within the kiln (typically higher than one hour) aids in achieving an acceptably uniform product as the early users had intended, there is considerable scope for improving this aspect of kiln performance. In order to achieve this improvement, a more quantitative understanding of transport phenomena within the bed material is needed, specifically of momentum transport, which determines particle motion and energy transport, which, in turn, determines the heating rate for the individual particles.

Fundamentally, rotary kilns are heat exchangers in which energy from a hot gas phase is extracted by the bed material. During its passage along the kiln, the bed material undergoes different transformations. Fig 1 shows a schematic diagram of a direct fired rotary kiln. A typical sequence of processes taking place in long kilns are drying, heating, and chemical reactions which cover a broad range of temperatures. Although non-contact (i.e., externally heated) rotary kilns are being used for specialized work, majority of the kilns allow direct contact between the free board gas and bed material. The most common configuration is counter-current flow whereby the bed and gas flows are in opposite directions although co-current flow can be utilized in some instances, for example, rotary driers.

Fig 1 Schematic diagram of a direct fired rotary kiln

Historical aspects

The first rotary kiln was manufactured around 1885, and not until 1895, the rotary type kiln was considered a success. The plants of several countries utilized vertical or stationary kilns, which are much more economical from the point of fuel consumption, but very costly on account of the manual work necessary. By 1885, an English engineer, F. Ransome, had patented a slightly tilted horizontal kiln which could be rotated so that material could move gradually from one end to the other. The underlying principle of this invention constitutes the rotary kiln transport phenomenon as being known today. About a year later, Alphonse de Navarro purchased the American rights to this patent, and built the first rotary kiln.

The first kiln was 5 feet (1.52 metres) in diameter by 40 feet (12.2 metres) long. The first fuel tried was wood, but a sufficient temperature for ‘clinkering’ could not be obtained, and petroleum was utilized. The cost of the petroleum became excessive, and in 1895 pulverized coal was first tried as a fuel. The development of the kiln from its original size of 5 feet (1.52 metres) in diameter by 40 feet (12.2 metres) long to one 12 feet (3.66 metres) in diameter by 200 feet (60.96 metres) long indicates a remarkable growth. The main idea in developing the rotary kiln appears to be, increase in output, decrease in fuel consumption, and decrease in the quantity of operating equipment. The design and operation of rotary kilns have undergone a systematic evolution over time. Improvements include reduced manpower, increased productivity, mixing, heat transfer, and product quality.

Features of rotary kiln system

Rotary kiln system for material processing consists of unit operation equipment and other components which are added together. Perhaps the most important is the rotary reactor, which forms the heart of the process and hence warrants special attention. The rotary reactor is normally a long horizontal cylinder tilted on its axis. In majority of the rotary kiln process applications, the objective is to drive the specific bed reactions, which, for either kinetic or thermodynamic reasons, need frequently high bed temperatures.

Several rotary kiln designs have evolved, each specific to the process application it is intended for. The rotary kilns also come in several forms and shapes. Although the majority consist of straight, cylindrical vessels, dumb bell-shaped designs take advantage of the benefits that variable drum sizes can bring to process application. With regard to internal kiln fixtures, majority of the direct fired kilns are lined with refractory materials for several reasons, but the primary purposes are to insulate and protect the outer shell, in high temperature applications, from thermal damage and to save energy. Kilns can also be equipped with dams to increase the material residence time or with lifters and tumblers for aiding the materials to flow axially and in some cases to improve particle mixing achieved through surface renewal.

Depending on the rotational speed of the rotary kiln, the bed motion in the transverse plane can be characterized as (i) slipping, (ii) slumping, (iii) rolling, (iv) cascading, (v) cataracting, and (vi) centrifuging. Centrifuging bed motion occurs at critical and high speeds. This is an extreme condition in which all the bed material rotates with the drum wall. Cascading motion occurs at relatively high rates of rotation, and it is a condition in which the height of the leading edge (shear wedge) of the material rises above the bed surface and particles cascade or shower down on the free surface. However, operating the rotary kiln in either of these conditions is rare because of attrition and dusting issues.

Drying applications take advantage of the high particle-to-heat transfer fluid exposure associated with the cascading mode and the separation effect is caused by the centrifugal force component. For example, starting at the other extreme, that is, at very low rates of rotation and moving progressively to higher rates, the bed typically move from slipping, in which the bulk of the bed material, en-masse, slips against the wall, to slumping, whereby a segment of the bulk material at the shear wedge becomes unstable, yields and empties down the incline, to rolling, which involves a steady discharge onto the bed surface. In the slumping mode, the dynamic angle of repose varies in a cyclical manner while in the rolling mode the angle of repose remains constant.

In the rolling mode, where rotary drum mixing is maximized, two distinct regions can be discerned, the shearing region, called the active layer, formed by particles near the free surface, and the passive or plug flow region at the bottom where the shear rate is zero. The particular mode chosen for an operation is dependent upon the intent of the application. Fig 2 shows types of bed motion in rotary kilns in cross sectional plane.

Fig 2 Types of bed motion in rotary kilns in cross sectional plane

Owing to the poor thermal efficiency of earlier long kilns and the need for fuel efficiency, majority of the designs are aimed at maximizing mixing and heat transfer. For accomplishing this, kilns are frequently equipped with heat recuperators, such as pre-heaters, in which part of the energy in the exhaust gas is recovered to preheat the feed before it enters the kiln. Although coolers are frequently used to cool the product for safe material handling, they are also used to recuperate the energy, which otherwise go to waste, as in the earlier-day kilns, to preheat the combustion air and / or to provide other energy needs.

The key geometrical feature is the vessel size, given in terms of the cylinder diameter and kiln length related by the aspect ratio, that is, the length-to-diameter ratio (L/D) and also the slope. Other pertinent features include the internals, such as constriction dams and lifters, which impact the residence time. Since the vessel is partially filled and rotating on its horizontal axis, the free board or open space above the bed depends on the bed depth and, for that matter, on the kiln loading (% fill). The shape of the free surface (the interface between the bed and the free board) is dependent upon the operational requirements, i.e., the feed rate, the drum rotational rate, and the material properties.

As a result, the sizing of the rotary kiln depends on the application, typically, the feed rate (capacity) and related transport properties such as temperature, gas flow rates, and bed material velocities which ultimately determine the residence time. For example, in dry processing applications, cylinder length-to-diameter ratios of the order of 5 to 12 are typical depending on whether the heat exchange is contact or non-contact type. Such L/D ratios can result in residence times in the 20 minutes to 120 minutes range depending upon the kiln rotational speed, the type of internal flights, if any, and the slope in the longitudinal direction, typically in the range of 1 degree to 3 degrees. The movement of a charge in a rotating cylinder can be resolved into two components which are (i) movement in the axial direction, which determines residence time, and (ii) movement in the transverse plane, which influences majority of the primary bed processes such as material mixing, heat transfer, and reaction rate (physical or chemical), as well as the axial progress of the charge.

In majority of the cases, an oil or gas burner fires directly into the discharge end of the unit. The material is heated in three ways namely (i) by radiation from the burner frame, (ii) by conduction from the refractory lining, and (iii) by convection because of the contact with the hot gases.

For direct fired kilns, the energy necessary to raise the bed temperature to the level needed for the intended reactions, and in some cases, for example, the endothermic calcination of limestone, to drive the reactions themselves, originates with the combustion of hydrocarbon fuels in the free board near the heat source or burner. This energy is then transferred by heat exchange between the gas phase (the free board) and the bed. Heat transfer between the free board and the bed is rather complex and occurs by all the paths established by the geometric view factors in radiation exchange. All these manifests themselves into a combined transport phenomenon with the different transport processes coming into play in one application. Fig 3 shows heat transfer mechanisms in a rotary kiln.

Fig 3 Heat transfer mechanisms in a rotary kiln

Types of kilns

The present-day rotary kilns can be distinguished as wet kilns, long dry kilns, short dry kilns, coolers and dryers, and indirect fired kilns. The direct fired rotary kiln is broadly used for physical activation. Construction and position alignment of the kiln is very important for all the processes.

Wet kilns – These kilns are those which are normally fed with slurry materials. Wet kilns are normally long with kiln lengths of the order of 150 metres to 180 metres. The feed end is normally equipped with chains which serve as a heat ‘flywheel’ by recuperating the heat in the exhaust gas for use in preheating the feed to assist the drying. Chains are also used to break up any lumps which the material can form during the transition phase of changing from slurry to solids upon drying. In the cement industry, these kilns are frequently not efficient and are becoming a thing of the past replaced by long dry kilns. However, there are certain applications which are not amenable to the alternative use of long dry kilns, for example, lime mud kilns found in the pulp and paper industry and some food applications.

Long dry kilns – These kilns are shorter than wet kilns with lengths of the order of 90 metres to 120 metres. For long dry kilns, as with wet kilns, the drying, preheating, and calcination all occur in one single vessel. However, these kilns work well when the feed particles are large. The reason for the relatively shorter length is that the feed is dry with a moisture content the same as granular solids rather than slurry. Applications include lime kilns and lightweight aggregate kilns where the mined stones are crushed to around 120 mm to 400 mm before feeding them into the kiln.

Short dry kilns – These kilns are normally accompanied by an external pre-heater or pre-calciner in which the feed is dried, preheated, or even partially calcined prior to entering the main reactor (kiln). As a result, the thermal load on the kiln proper is reduced. Hence kilns equipped with pre-heaters or pre-calciners tend to be short, of the order of 15 metres to 75 meters depending on the process. The shorter kilns are those in which the entering feed material is almost calcined. Applications include cement and some lime kilns. Because of the large feed particle size encountered in limestone calcination, modern lime kilns are equipped with pre-heaters which function as a packed bed of stone with a counter-current flow of kiln exhaust gas rather than the typical cyclone pre-heaters in cement kiln systems.

Coolers and dryers – Some coolers and dryers can be in a form of contactors such as the rotary kiln itself, although some are packed-bed contactors such as grate coolers. Rotary coolers can be either in-line or attached, the number of which is determined by a simple formula N = pi × (D + d +2)/(d +1), where ‘D’ and ‘d’ are the respective diameters of the kiln and the cooler. However, attached coolers place extra mechanical load which is to be accounted for in the design calculations. They also present maintenance challenges. Rotary coolers and dryers are normally to be equipped with tumblers and lifters, which cascade the material well above its angle of repose to take advantage of better solid-gas contact.

Indirect fired kilns – These kilns are those kilns which are heated externally. These kilns are normally designed for applications where direct contact between the material and the gas providing the heat source is undesirable. In this case, the heat source is external to the kiln. Any internally flowing gas which is in the free board is used for purging any volatile or gas which arise from the bed as a result of chemical / physical reactions. Because of their low thermal efficiency, indirect fired kiln which are externally heated, are small, typically up to 1.3 metres in diameter and are used for niche applications such as calcining of specialty materials (Fig 4).

Fig 4 Indirect fired small rotary kiln used for niche applications

A unique feature of indirect fired rotary kilns is multiple and compartmentalized temperature control zones, which can be electrically heated or gas fired individually. Hence, these kilns provide the capability of achieving high temperatures. In some cases, for example graphite furnaces, the kilns can attain temperatures of the order of 2,400 deg C. The zones can also facilitate tightly defined residence times and controlled atmosphere including flammables. Typical applications include calcination, reduction, controlled oxidation, carburization, solid-state reactions and purification, including waste remediation on a small scale, which need extremely high temperatures and tight control. Materials processed in indirectly fired rotary kilns include phosphors, titanates, zinc oxide, quartz ferrites, and so on. These materials are normally small in quantity but with a high margin of commercial materials which are economical to process in small quantities.

Indirect heated rotary kilns are used for pyrolysis and thermolysis processes because of the advantages of continuous process, very good blending of the product unlike batch processing and simple plant layouts.

Working of rotary kilns

Rotary kilns are used to heat solids to the point where a chemical reaction or physical change takes place. They work by holding the material to be processed at a specialized temperature for a precise quantity of time. Temperatures and retention times are determined through creating temperature profiles, based on thorough chemical and thermal analyses of the material. The rotary kiln is comprised of a rotating cylinder (called the drum), sized specifically to meet the temperature and retention time requirements of the material to be processed. The kiln is set at a slight angle, in order to allow gravity to assist in moving material through the rotating cylinder.

Rotary kilns can be either of the direct-fired type, or the indirect-fired type (sometimes referred to as a calciner). In the direct-fired kiln, a process gas is fed through the drum, processing the material through direct contact. In the indirect-fired kiln, material is processed in an inert environment, and is heated through contact with the shell of the kiln, which is heated from the outside to maintain an inert environment.

Rotary kiln construction

While rotary kilns are custom designed around the material to be processed, in general, there are some standard components which serve as the basis of a rotary kiln. Fig 5 shows standard components of a basic direct fired kiln. Similarly, there are standard components of man indirect fired kiln.

Fig 5 Standard components of a basic direct fired kiln

Drive assembly – The drive assembly is the component which causes the kiln to rotate. A variety of drive assembly arrangements are available such as (i) chain and sprocket, (ii) gear drive, (iii) friction drive, and (iv) direct drive assembly. Unlike majority of other rotary kiln components, there is not a need for further customization in terms of the mechanical components of the drum. The need for one drive type over another is solely dependent on how much drive power is needed.

Chain and sprocket – The chain and sprocket arrangement operate similar to a bicycle. There is a large sprocket wrapping around the rotary drum with a chain on it which goes to the reducer and motor. The spinning motor turns a gear box, which spins a small sprocket which is attached by the chain to the large sprocket wrapping around the rotary drum. Chain and sprocket drive set ups are reserved for small rotary kilns, running up to 55 kilowatts power. This type of arrangement is not suitable for larger kilns running above 55 kilowatts power, but is ideal for smaller jobs, as it is cost-effective, and easy to run.

Gear drive – The gear drive is best for heavy-duty applications running above 55 kilowatts power. Similar to the chain and sprocket drive, instead of a sprocket wrapped around the girth of the drum, this drive has an actual gear around the drum. This gear meshes with a small gear drive, which rotates it. This type of drive is more costly, but wears better in heavy-duty applications.

Friction drive – Friction drive assemblies are reserved for small applications operates needing lower power. This is normally seen with drums with diameter around 1.5 metres and lesser. With a friction drive, two of the four trunnion wheels are connected by one shaft and driven by a shaft mounted reducer and motor arrangement.

Direct drive – Direct drive assemblies are used in small to medium sized drums, with motors up to 55 kilowatts power. In this design, a shaft is mounted to a solid, discharge end plate at the outlet of the kiln. The motor and reducer are either directly connected to this shaft with a coupling, or a shaft mount arrangement.

Riding rings – The riding rings provide a surface for the kiln load to be distributed.

Thrust rollers – Thrust rollers prevent the drum from drifting or moving horizontally by pushing against the riding rings.

Trunnion wheels – The trunnion wheels act as the cradle for the rotating drum shell. They ensure smooth and concentric rotation during operation. They also act as a wear piece, since they are easier and less costly to replace than the riding ring itself. The wheels are mounted to steel support bases with sealed roller bearings. Support rollers bear the weight of the drum.

Discharge breeching – The discharge breeching serves two purposes. One of the purposes is to provide a place for product to exit the kiln, so it can move on to subsequent processing, while the second purpose is to mount the kiln burner in a counter-current system.

Product discharge – The product discharge area is where product exits the kiln. Typically, the product then moves on to cooling or subsequent processing if needed.

Exhaust gas system – The exhaust gas system is typically much larger when working with a direct fired kiln. Here, exhaust gases and any small particulates exit the system and typically go through exhaust gas treatment to remove contaminants before being discharged into the environment. The exhaust gas system of a kiln frequently needs an after-burner and heat exchanger or quench tower to cool the gases before they enter the bag filter.

Refractory – Refractory serves the purpose of insulating and protecting the shell of the drum from the high temperatures within, and also minimizing heat loss. Several types of refractories are available, and refractory layers can be customized to suit the unique application.

Burner – The burner of a rotary kiln supplies the energy needed by the process. Instead of utilizing a combustion chamber, typically the burner of a kiln is mounted on the discharge end housing. Burners can be designed to accommodate a variety of fuel sources, from natural gas, to propane, diesel, pulverized coal, and other common fuel sources. Choosing the appropriate burner for a rotary kiln is integral to ensuring efficient processing.

Raw material feed – The raw material feed, or feed chute, is where the feed-stock enters the drum. This is typically carried out using a feed screw or chute and is frequently made from a more heat-resistant alloy. This area is to be designed to be robust and to lessen the opportunity for build-up to occur.

Air seal – The seal connects the stationary breeching to the rotating drum, and helps to prevent the escape of process gas from the system, as well as prevents air from leaking in. Holding the appropriate temperature within a rotary kiln is what allows the desired chemical reaction to occur. Sustaining that temperature, however, can be difficult if the right seal is not chosen. Various seal options exist.

Shell – Direct fired kilns are typically made out of carbon steel. Indirect-fired kilns, however, are to be more resistant to high temperatures, and are hence made out of a more heat-resistant alloy.

Rotary kiln processes

Since rotary kilns simply serve as a vessel to cause a chemical reaction or phase change, there are several types of processes for which these kilns can be used for. Described below are an overview of some of the processes which are normally carried out in a rotary kiln and how they work.

Calcination – The calcination process needs heating a material to a high temperature with the intent of chemical dissociation (chemical separation). Calcination is normally used in the creation of inorganic materials. One of the most common examples of this process is the dissociation of calcium carbonate (CaCO3) to create calcium oxide (CaO) and carbon dioxide (CO2). The calcination process can also be used in the removal of bound moisture, such as that which is chemically attached in borax {Na2[B4O5(OH)4]·8H₂O}.

Thermal desorption – Thermal desorption uses heat to drive off a volatile component, such as a pesticide, which has mixed with an inorganic mineral like sand. It is important to remember that this is not incineration, which can produce harmful pollutants and needs a more extreme exhaust treatment system. Instead, it is a separation process which uses the different reaction temperatures of absorbent minerals and chemicals. The organic chemical (e.g., pesticide) is vapourized at the increased temperature, causing a separation without combustion. An indirect rotary kiln is best for this application, since the volatile chemicals can be combustible. The indirect kiln supplies the heat for desorption, without the material coming into direct contact with the flame.

Organic combustion – Organic combustion is the thermal treatment of organic waste with the intent of reducing mass and volume. This is normally seen in waste treatment plants to reduce the volume of waste for depositing in landfills. Direct fired rotary kilns are the most common style for this application, since air is needed to combust the organics.

Sintering / induration – Sintering is the process of heating the raw materials to a point just before melting. The objective of this process is to use the high internal temperature of the rotary kiln to increase the strength of the material. The most common use of this process is iron ore pelletizing and in the creation of manufactured proppants, where the sand or ceramic material needs to have high strength.

Heat setting – This is a process of bonding a heat resistant core mineral with another, less heat resistant coating material. Much like other coating processes, there is a core material and a coating material (normally mixed with a binding agent). The difference between this process and a non-heated coating process is that a rotary kiln heats the coating material to just below its liquefaction point. At this heated state, the material can coat the heat resistant core evenly and, since this is a chemical phase change, more securely than a traditional coating process. A common application of this process is in the manufacturing of roofing granules, where a mineral such as granite is coated with a coloured pigment, producing a durable and aesthetically pleasing granule.

Reduction roasting – Reduction roasting is the removal of oxygen from a component of an ore normally by using carbon mono oxide (CO). An example of this is the reduction roasting of a hematite containing material to produce magnetite which can be magnetically separated.

Rotary kiln sizing and design

Every material is different in terms of how it behaves in the kiln and at what temperatures different reactions take place. When designing a process around a rotary kiln, as well as in the design of the kiln itself, the material is to undergo thorough chemical and thermal analyses. Different material characteristics play a part in how the material perform in the kiln, and subsequently, how the kiln is required to be designed around the material to accomplish the process goal. For example, whether the material is going to melt, vapourize, or combust at certain temperatures. Majority of this data can be gathered through testing. Characteristics which affect rotary kiln design are described below.

Particle size distribution and bulk density – The particle size distribution and bulk density of a material has influence on the sizing of some kiln components. For example, a material with a high bulk density is likely to need more power, and hence a more robust drive system is needed. Additionally, a material which has been agglomerated into pellets (or has a larger particle size distribution) does not need as large of a kiln diameter as fines need. This is since when processing fines, a lower air velocity is to be used to minimize entrainment. When processing pellets, however, a higher air velocity can be utilized, and hence, the kiln does not need to be as large.

Abrasiveness and corrosiveness – While the abrasiveness or corrosiveness of a material does not have a direct effect on the sizing aspects of the kiln, it does influence the materials of construction. Working with abrasive or corrosive materials can need parts, or all, of the kiln to be lined or constructed with a corrosion / abrasion-resistant refractory.

Specific heat – The specific heat of a material is another central factor in the design of a rotary kiln. Specific heat determines how resistant a material is to heating. By definition, it is how much energy (i.e., calories) it takes to raise 1 gram of material by 1 deg C. Some materials, such as water, have a very high specific heat, meaning it takes a considerable quantity of energy to raise the temperature. Other materials, such as metals, have a much lower specific heat, meaning it takes much less energy to cause a change in temperature.

Heat of reaction – In several kiln applications, heat is needed for a reaction to take place. For example, in the calcination of lime stone to lime, energy is needed to dissociate calcium carbonate into calcium oxide and carbon di-oxide. In addition to energy, a high temperature is needed for the majority of the reactions to take place, the dissociation of lime stone does not take place at a temperature less than 900 deg C. The temperature and energy needed for a reaction can be found in published data or by running a DTA (differential thermal analysis) test.

Thermal conductivity – Similar to specific heat, the thermal conductivity of a material also plays an important part in the design of a rotary kiln. The way a material transfers heat has a direct effect on how the material behaves in the rotary kiln. When the material transfers its heat easily, it results into even heat distribution and low retention time, or when the material holds onto its heat, it results into cold pockets of material, longer retention time, and possibly the need for additional accessories like dams or bed disturbers.

Temperature profiles – A ‘thermal gravimetric analysis’ or TGA, can be performed on a material to determine changes in mass as a function of temperature. A TGA describes the temperature ranges at which mass loss occurs. This is critical in determining the needed temperature profile in a kiln. As an example, free water shows primary removal at around 100 deg C, where tightly bound chemical water can show a mass loss upwards of 260 deg C.

A TGA also helps to show where a reaction begins, and ends, since frequently, the curve on a TGA starts at a specific temperature, but does not complete until a much higher temperature. Overall, a TGA helps to determine the temperature profiles which is needed in a rotary kiln by showing at what temperature reactions are occurring. Additionally, while the intent of a process can be to convert a material in a specific way, a TGA reveals reactions which can occur between the start and end point, helping to indicate where unpredicted reactions can occur.

A ‘differential scanning calorimeter’ (DSC) or a ‘differential thermal analysis’ (DTA) is also useful at this stage, as it shows the quantity of heat needed to perform the reactions and to heat the material to the final temperature.

Chemical composition – Knowledge of the chemical composition of a material is a valuable asset in rotary kiln design for several reasons. One important reason is that several materials combust inside the rotary kiln at high temperatures, creating more heat than has been put into the rotary kiln. In cases such as these, the rotary kiln is required be designed to withstand those excess quantities to of heat. In other cases, materials can need a particular chemical environment for a reaction to occur, as an example, an environment devoid of oxygen, or rich in carbon di-oxide. Still another reason to understand the chemical make-up of a material, and how those chemicals react together at certain temperatures, is to predict what type of exhaust gases are generated and subsequently, what type of exhaust gas treatment is necessary.

Sizing – After the material has been thermally and chemically analyzed, sizing can begin. Sizing is a complex process which cannot be easily explained in brief. The process of sizing a rotary kiln is one which combines engineering principles with the thermal and chemical analyses, along with experience, to design a kiln which meets its intended processing objectives. The size of a rotary kiln is not only a function of capacity, but also of the quantity of heat which can be generated inside the rotary kiln from the volatizing and / or combustion of the material. The diameter and length of the rotary kiln are calculated based on the maximum feed rate, the needed retention time, and the bed profile (how full of material the rotary kiln is) which is required to look like.

During the design process of a rotary kiln, once the designers have engineered a rough design of the rotary kiln, they use several computer programmes to help predict and model how the material is going to behave in the rotary kiln they have designed. The designers review the combined analyses, and if their design does not meet the appropriate criteria, they make adjustment accordingly. Once the designers have their preliminary rotary kiln size, they start thinking about the details of the rotary kiln internals, such as if there is a need for a dam, or what type of refractory best suits the process. Normally, each material goes through a study and development process at the concept testing facility. The information gained through the proven testing procedures allows the designers to design the most efficient and beneficial thermal processing system for the material requirements.

Increasing efficiency through customization – Rotary kilns are extremely customizable, and can be configured to meet nearly any process needs. There are different ways to customize a kiln in order to attain the most efficient processing possible. Some of the common customizations used to maximize the performance of a rotary kiln are described below.

Dams – For different reasons, it is frequently desirable to increase retention time or bed depth in the rotary kiln. This is done by adding what is called a ‘dam’. A dam in a rotary kiln works much like a dam in a river i.e., material builds up behind the dam, forcing retention time and bed depth to increase. Material then spills over the dam, and discharges from the rotary kiln. Since majority of the kilns utilize a counter current air flow, end dams are the most commonly used. End dams efficiently hold the material where the air is the warmest (at the discharge end in a counter current kiln). Internal dams can also be used if a discharge end dam is not sufficient.

Dams are put in place when retention time needs to be increased using the same size rotary kiln. Dams allow the loading to be increased, which increases retention time and bed depth by forcing the material to build up in the rotary kiln. Unfortunately, not all materials tumble well, which results in a slipping bed with poor mixing and large temperature variation.

Bed disturbers – Indirect rotary kilns create heat transfer by conduction through the shell of the rotary kiln, rather than by means of contact with a process gas. Since all of this heat transfer is occurring through the shell, it is essential that the bed rolls rather than slides in order to expose fresh material, allowing for even heat distribution throughout the bed of material. This assures that the transfer of heat is as efficient as possible. For this reason, when processing material in an indirect fired kiln, it is frequently desirable to use a bed disturber. Bed disturbers are also normally used in a direct fired kiln for the same reason where the bed disturber helps to prevent the bed from sliding, as well as promotes more uniform heating. A bed disturber, frequently custom designed to create maximum material specific efficiency, is essentially anything affixed to the inside of the rotary kiln which helps to mix the bed of material. Ideally, the bed is to tumble, turning over and minimizing dead spots, or temperature variations within the bed. Fig 6 shows the gradations which can occur in a bed of material which is poorly rotated (left). The addition of a bed disturber helps to rotate the bed, ensuring even distribution of heat throughout the bed (right).

Fig 6 Functions of bed disturber and leaf seal

Bed disturbers can be attached to the interior of the rotary kiln in order to disturb the bed and turn it over. However, what seems like a simple task can get complicated quickly, as thermal stresses are to be accounted for. A common bed disturber used in an indirectly heated kiln is a bar which runs the length of the interior of the rotary kiln. Material pushes up against the bar, building up and rolling over it, so material which has been on the top of the bed now gets redistributed to the bottom of the bed. The disadvantage to using a bar bed disturber is that they can sometimes bend and break with the thermal stresses of the rotary kiln.

A rotary kiln naturally has gradients of temperature, normally cooler on the ends of the rotary kiln, and hotter in the middle. This gradation in temperature causes differential thermal expansion on the rotary kiln shell. Because of this, the bar, welded to the shell, is pulled in different directions, which can cause the weld to break. When this kind of thermal expansion is at work, it is normally best to look at alternative bed disturbers.

Flights – Flights or lifters are most commonly seen in rotary dryers. They are, however, sometimes utilized in low temperature kilns in order to shower the material and increase heat transfer efficiency.

Flights can also be used as a bed disturber. In this case, lights are welded with one weld point each to the inside of the rotary kiln. This method of disturbing the bed is designed to accept the different thermal expansion stressors, making it ideal for drums with temperature gradations.

Seals – Almost all rotary kilns run at a negative pressure, meaning gas does not leak out, but rather, air leaks in. Since rotary kilns are always running at a higher temperature than ambient, any ambient air leaking into the rotary kiln causes the temperature inside of the rotary kiln to drop. Not only this results in an unnecessary quantity of energy being used and wasted, but if the leak is severe enough, it can also potentially disrupt the process. This is why it is crucial to have a quality seal.

Sealing the ends of a rotary kiln can be a difficult task, because of the fact that it amounts to trying to seal something moving to something stationary. The stationary part, referred to as the breeching, is typically where leakage occurs. One option is a leaf seal (Fig 6). Leaf seals are the standard seal used on both rotary kilns and rotary dryers. Leaf seals are similar to a fanned-out deck of cards. The ‘cards’, or leaves, are made out of spring steel. These fanned out leaves are bolted to the breeching, and the springy leaves are forced to push against the seal / wear plate of the rotating kiln, naturally keeping pressure on the rotary kiln to create a good seal.

The purged double leaf seal is a variation of the leaf seal, and is typically used in situations where maintenance of the environment inside the rotary kiln is very critical. For example, in cases where the environment inside the rotary kiln cannot tolerate oxygen from ambient air leakage, a leaf seal may not be sufficient, and a purged double leaf seal provides a better seal. The purged double leaf seal is made up of two components. The first is two sets of seals, which consist of two layers of ‘leaves’ on top of each other. The second component is an inert purge gas, such as nitrogen, which is introduced between the two sets of seals. This purge gas pushes outward to ambient, so that there is a flow of gas going out, and hence, no oxygen can flow in.

Refractory – Perhaps one of the most critical components of a direct fired rotary kiln, the refractory not only protects the shell from the high temperatures within, but also maintains heat retention. A quality refractory is of the utmost importance, and several options are available, depending on the needs of the rotary kiln. Refractory is specific to the direct fired kilns, while the addition of refractory to an indirect fired kiln decreases its efficiency, since it adds another layer for heat to pass through before it can reach the material.

Typically, there are two kinds of refractories for lining a rotary kiln. These are castable, and brick. Each kind of refractory has its advantages and disadvantages. The choice of refractory is dependent on the rotary kiln temperature, material chemistry, and the abrasiveness of the material. Castable and brick refractory are comparably priced for similar refractory compositions. However, the installation cost for brick is higher, since it is more labour intensive. Brick is normally used for abrasive materials, since it is more wear resistant.

The way in which refractory is layered is also customizable. When higher efficiency is desired, or very high temperatures are involved, it is frequently desirable to use multiple layers of refractory consisting of a working lining and an insulating layer. The working lining is in direct contact with the material being processed. Because of this, this working lining is a dense lining which can withstand the high temperatures within the rotary kiln and the constant abrasion from the material. However, when it comes to refractory, the denser it is, the less insulating capabilities it has. This means that even though there can be a tough, durable, thick working layer in place, the heat can easily pass through to the shell of the rotary kiln. For this reason, an insulating layer is needed beneath the working lining. The insulating layer does just that. It insulates the shell of the rotary kiln so the high temperatures cannot reach the shell and damage it.

Typically, the working lining and the insulating layer are made of the same form of material (i.e., brick or castable), with varying chemistries. The working lining tends to be a higher density, stronger material which is more conductive. The insulating layer does not need these qualities, and tends to be softer, lighter, and less conductive, and hence more insulating. These two lining layers vary frequently in thicknesses, and these are determined by the needs of the rotary kiln and the type of material being processed.

In some unique cases, where processing temperatures are low, or efficiency is of less concern, it is only necessary to use one working lining. For these reasons, refractory in a rotary kiln is frequently a very custom part of the design. When insulation is extremely critical, an optional third layer of ceramic fibre backing is used. Though there are different types of this backing, this thin, but very efficient layer is similar to fibreglass insulation found in a house, but it is much more compressed. Fig 7 shows brick and castable lining of a rotary kiln showing both the working lining and the insulating layer. The castable lining also shows optional ceramic fibre backing layer.

Fig 7 Brick and castable refractory lining of a rotary kiln

Thermal testing

Testing plays an integral part in the development of several industrial processes, and is especially critical in the thermal processing industry when working with kilns. Testing gathers important process data, and lays the groundwork for developing a safe, efficient, and effective process which meets the desired processing capacity and product quality.

Testing is useful in a multitude of processes. The processes which are normally tested include (i) thermal desorption of organic / hazardous wastes, (ii) sintering / induration, (iii) heat setting, (iv) organic combustion, (v) metal recovery, (vi) calcination, (vii) mineral processing, (viii) iron ore reduction, and (ix) reduction roasting.

There are several reasons why it is desirable to conduct testing with a rotary kiln. Perhaps the most common reason for thermal testing with a kiln is to gather the data necessary to size and design a commercial size kiln for an intended application. In this setting, the desired set of product specifications have typically been determined, but the people installing the kiln need to know what the kiln and surrounding process looks like to reach those parameters. It helps in aiding in the product development since frequently people installing the kiln are looking to develop a new or improved product. This is normally seen in the proppant industry, where ceramic proppants are processed in a rotary kiln to develop the ideal characteristics needed for the hydraulic fracturing process.

Testing can be carried out on small samples of material and used for field trials to evaluate the product properties. For confirming the viability of an intended process, testing is also useful in determining if a particular process holds potential for a commercial-scale operation. Frequently people have an idea and a sample of material, and trials are required to be run to determine if the process is technically and economically viable. While this is seen throughout a variety of industries, one common example is in the reclamation of valuable materials, such as the recovery of metals from wastes. Several waste materials have previously been land filled even though they contained a valuable component, since the component was not accessible or recoverable in its present form. Advanced thermal processing techniques have opened the door to the recovery of these valuable components. In cases such as these, an organization can get a material tested to see if the valuable component can be recovered from the waste material in an economically viable way.

Another common reason for thermal testing is to study and develop different processing conditions. Some organizations can have an existing thermal system, but are looking to adjust the process, or feel that their present process can be improved upon. Testing allows them to try out different process conditions in a test setting, without disrupting production in their existing commercial kiln.

Testing is normally conducted first at batch scale, and then at pilot scale. Batch testing, also referred to as feasibility testing, is a cost-effective way to test small sample sizes and gather initial process data, such as time and temperature profiles. Batch testing also helps to define the process parameters needed for continuous pilot-scale testing. Pilot testing is done on a much larger scale than batch testing, allowing for a continuous process, including exhaust gas treatment, to be tested, and commercial process conditions to be simulated. During both batch and pilot testing, solid samples can be regularly withdrawn in order to determine the material chemistry and physical properties of the material at various intervals.

Material characteristics which are normally analyzed to ensure that a product is meeting desired specifications are (i) flowability, (ii) compression strength, (iii) bulk density, (iv) crushing strength, (v) chemical analysis, and (vi) gas sampling and monitoring. Gathered process data can then be used to produce the desired product specifications and aid in the process scale-up. These data points can include (i) residence time, (ii) kiln slope, (iii) temperature requirements, (iv) kiln rotating speed, (v) emissions, and (vi) feed rate.

Direct fired and indirect fired kilns can be tested both at the batch scale and the pilot scale. Co-current and counter current air-low configurations can also be tested, with a variety of additional testing equipment to accommodate the process. These additional testing equipments include (i) kiln combustion chamber, (ii) thermal oxidizer, (iii) bag house, (iv) wet scrubber, (v) removable lights, dams, and bed disturbers, and (vi) automation with an extensive programmable logic control system. The testing equipment is to allow for a variety of data points to be tracked and adjusted from a single interface, in real time. This includes (i) current (amperes), (ii) feed rate, (iii) flow rates, (iv) fuel usage, (v) gas sampling and analysis, (vi) electric power, (vii) speed, (viii) system pressure, (ix) temperature, and (x) torque. In addition, the testing equipments need to have capabilities for the data points to be selected, trended, and reported on, allowing users to select only the data they need, from the exact time frame they need.

Direct fired and indirect fired kilns

When designing a thermal processing operation, confusion can frequently result on whether a direct fired or indirect fired kiln is the better option. And while there can be some overlap in applications, in general, each type of kiln is better suited for different processes. A brief overview on these two types of kilns is described below.

Direct fired rotary kilns rely on direct contact between the process gas and the material in order to heat the material to the specified temperature. Direct fired kilns can be either of the co-current design, or counter current design, referring to the direction which the process gas flows through the drum in relation to the material. Direct fired rotary kilns are most frequently the equipment of choice in thermal processing, since they are more efficient than their indirect counterparts. There can be disadvantages to a direct fired kiln, however. For example, since a process gas is used to treat the material, direct fired kilns subsequently produce more off-gases which need treatment.

Further, some materials are to be processed in an inert environment, so as not to be exposed to oxygen or nitrogen. In such applications, a direct fired kiln is not an option. Materials which are normally processed in a direct fired kiln include (i) proppants, (ii) minerals, (iii) specialty ceramics and clays, (iv) limestone, (v) cement, and (vi) iron ore.

Indirect fired kilns can process material in an inert environment, where the material never comes into contact with the process gas. Here, the kiln is heated from the outside, using a heat shroud, and the material is heated through contact with the hot kiln shell. While this method is significantly less efficient than a direct fired kiln, it is necessary in some processes which need more tightly controlled environment. This can include examples where an undesirable oxide compound gets formed in the presence of oxygen at high temperatures. Similarly, some materials can form an undesirable compound with nitrogen at high temperatures. In situations such as these, the use of an indirect fired kiln provides the necessary inert environment for effective processing.

Indirect fired kilns also allow for precise temperature control along the length of the kiln. This is advantageous in settings where a material is required to be brought up to temperature, and then held there for a specific period of time as it moves through the kiln. Indirect fired rotary kilns can also be beneficial when the material to be processed consists of finely divided solids. In a direct fired rotary kiln, the heat source is hot gas (products of combustion and air), which flows with an inherent velocity. These gases can carry particles through form drag. The degree of entrainment depends on a variety of factors, such as gas velocity, gas density, particle density, and shape. Because of the entrainment potential, direct fired rotary kilns processing fine materials need the design to be centered on permissible gas velocities as opposed to heat transfer requirements. Examples of fine materials normally processed in an indirect fired kiln include (i) carbon black, (ii) chemical precipitates, (iii) filter cakes, and (iv) finely ground solids.

Since the heat is being transferred through the shell, an indirect rotary kiln is not lined, in order to maximize the heat transfer through the shell. Hence, an indirect rotary kiln is normally made out of a temperature resistant alloy, instead of carbon steel. While direct fired kilns offer maximum efficiency, they are not always appropriate for the intended process. In settings such as these, an indirect fired kiln offers the best processing solution. In some process situations, a combination of a direct and indirect rotary kilns is needed, such as the direct fired rotary kiln is used to burn off the organic fraction of the material being processed, and further polishing of the resultant ash material is conducted in a specialty indirect kiln. Fig 8 shows heat transfer paths of direct and indirect fired rotary kilns.

Fig 8 Heat transfer paths of direct and indirect fired rotary kilns

Air flow options

Direct rotary kilns are available in two types of air flow configurations namely (i) co-current, and (ii) counter current. Both options have been developed through extensive study and development in order to maximize the thermal efficiency of the process. Majority of the rotary kilns are of the counter current configuration, since this option is much more energy efficient. However, in some cases, the co-current configuration is more appropriate. Additionally, indirect kilns use a different air flow altogether i.e., cross flow. This is since combustion gases are not flowing through the kiln, but rather, are perpendicular to the material.

During the design process of a direct fired kiln, the selection of which air flow configuration (co-current or counter current) best suits the application is based on the material’s properties, as well as overall process requirements. Because of this, it is important to understand how each air flow option functions to fully understand the benefits each has to offer.

Co-current air flow, also being referred to as parallel flow, is when the products of combustion flow in the same direction as the material. This immediately puts the coldest material in contact with the hottest gas in the kiln, resulting in a rapid initial temperature change. Co-current air flow kilns work best with materials which do not need a gradual temperature increase for a controlled transformation. An organic combustion process normally uses this air flow configuration, since it does not need a very specific end product. In this example, a waste material (e.g., spent catalyst) containing both organic and inorganic material is introduced into the kiln. These materials can come into immediate contact with the high heat and the volatile components are vapourized soon after feeding. The organic material is burned off with the high heat and what is left is a dry ash.

Counter current air flow is when the air flows in the opposite direction of the material flow. In this design, the material is heated gradually while travelling through the kiln. Here, material comes in contact with the hottest products of combustion just before discharge. The main benefit of this air flow configuration is the thermal efficiency, with the burner being mounted at the end of the thermal processing cycle, less heat is needed, resulting in decreased fuel consumption.

The co-current configuration needs a much higher initial temperature in order to heat the process material from its initial temperature and get the desired phase or chemical change. In contrast, in a counter current configuration, the material and the process gas temperature are directly correlated. For the example, the air flow (process gas) temperature only needs to be slightly higher than the needed temperature for the material transformation. The result is a lower exhaust gas temperature and lower operating costs. Additionally, the counter current design is normally used for a more controlled physical or chemical change, where the material temperature needs to be gradually increased to achieve the desired end result. Sintering is a common process which utilizes the counter current air flow to maintain a controlled phase change. The gradual, yet extreme heating process allows for a material such as a proppant, to transform into a much harder material.

The cross-flow configuration is specific to indirect kilns. In co-current or counter current flow, the gas and solid streams low parallel to each other. In a cross-flow configuration, however, the gas and solid streams are perpendicular to one another. One advantage of cross-flow heat transfer is that the solids can be held at a constant temperature for an extended time. This is very difficult to achieve in a co-current or counter current kiln. Understanding how each air flow system works is one of the several considerations in designing the most efficient and effective rotary kiln for the job. Each air flow configuration has its unique and varying benefits for material transformation. Fig 9 shows different types of air flows in the rotary kilns.

Fig 9 Types of air flows in the rotary kilns

Although rotary kilns are designed to be used for driving a chemical reaction, an issue which frequently comes up in the processing of a material is when to remove the excess moisture from the feed-stock. Several times, there is surface moisture which needs to be removed from the material before it is processed in a rotary kiln, and one is faced with a decision whether to have another piece of equipment, or use the rotary kiln itself to do this work. There are costs and benefits for each approach. While rotary kilns have the ability to remove moisture from a material, this tends to be a less efficient process. In a rotary kiln, material is typically not showered like in a rotary dryer, but rather, slides along the interior of the rotary kiln. This results in lower heat transfer between the material and the gas. Because of this, drying material in a rotary kiln takes much longer than drying in a rotary dryer.

The alternative to drying material in a rotary kiln is to add a rotary dryer into the process prior to the material going into the rotary kiln. Taking this approach, efficient drying of the material is done before it enters the rotary kiln, leaving the rotary kiln the sole job of converting the material. With a rotary dryer, flights lift the material and drop it through the stream of hot gas, creating a showering effect called the curtain. This showering effect allows for a maximum heat transfer between the material and the gas, drying the material in an efficient manner. In this situation, the rotary kiln becomes shorter. The disadvantage of this method is that an additional piece of equipment is needed. Another option is to use the kiln exhaust in a rotary dryer.

Kiln installation

A rotary kiln is a major investment and integral part of several industrial processing systems. The installation of a new rotary kiln is important and it needs planning and attention to ensure proper installation, optimal performance, and equipment longevity. A properly installed rotary kiln is the first step in prolonging equipment life and reducing potential downtime and maintenance. Problems which begin during the installation can quickly result in serious damage and downtime.

A poorly installed rotary kiln can result in a variety of problems, which include (i) damage to wheels / tyres from poor alignment, (ii) damage to drum shell since it has not been handled properly, and (iii) re-work needed and / or voided warranties since critical hold points / inspections have not been done. However, there are a few simple steps that one can take which help to achieve a smooth and successful installation and avoiding the problems later.

The kiln installation is to be done under supervision of the installation personnel of the kiln manufacturer. It is beneficial since the installation personnel of the manufacturer are well trained in the exact specifications needed for efficient installation and operation of the kiln. These personnel know what to look for and any potential places for error to occur and can oversee installation, assuring that things are done right, and no warranties are voided in the process. In addition, installation personnel of the manufacturer are a valuable source of knowledge for answering installation and operation questions on the spot. Also, they can train maintenance personnel on the ins and outs associated with the equipment during their presence on-site.

Planning is important for carrying out a seamless rotary kiln installation. Ideally, the kiln owner, kiln manufacturer, and installation contractor are to be in contact with one another prior to installation so that everyone knows what needs to be done before installation. This helps to ensure that the installation personnel of the manufacturer and the supporting manpower have everything which they need on-site, and do not waste valuable time waiting on things which can be prepared in advance. The items described below are to be considered during the planning stages of installation.

Appropriate equipment staging – In cases where the kiln is a replacement and needs to be into its place, by ensuring all ancillary equipment, such as feed chutes and / or discharge chutes are in place and pre-positioned prevent wasted time during installation. This is less of a concern when putting together a new kiln where equipment around the kiln can be prepared for during the kiln installation.

Materials and equipment – Having the right materials and equipment on hand can mean the difference between a smooth installation and days wasted. Materials such as grout needed for pouring under the bases, or shimming materials used in the alignment process are to be available and prepared for installation. Also, proper tools and equipment for the installation are to be available on-site as well. This also prevents wasted time. As an example, an inadequate crane, for example, can mean that the installation has to wait for a new crane to arrive and be mobilized before work can begin.

Pre-alignment – The installing contractor is to install and pre-align the drum bases prior to kiln installation. Having the drum bases installed and pre-aligned allow the installation personnel of the manufacturer to begin their work right away on installation day, instead of waiting a day or two for the pre-alignment to be completed.

All of the items above are to be planned properly since planning for these items helps in ensuring that no time is wasted during the kiln installation and progress moves as per the plan.

Properly aligning the kiln bases and shell is an important part of installation, and can set the tone for operational efficiency and equipment longevity. Rotary drum misalignment is one of the most common causes of drum damage and premature equipment failure, making proper alignment during installation a key to the operational success. One way to ensure proper alignment is achieved is through the use of a laser tracking system. While traditional alignment techniques can offer a reliable alignment option, they leave much room for error. New laser tracking systems, however, provide a more efficient, and accurate solution to alignment needs, offering fast, precise alignment. In a typical setting, laser alignment can get the bases to within +/- 0.13 mm. Additionally, while traditional alignment methods rely on manual measurements and mathematical equations to determine and execute proper alignment, advanced laser tracking systems eliminate the opportunity for human error by utilizing a laser beam to measure 3D coordinates, and recording and analyzing data on a software programme, resulting in faster alignment and extreme precision.

Rotary kiln maintenance

The reliable maintenance of rotary kilns needs an understanding of the prerequisites for the mechanical stability of the various interacting components. Timely and proper maintenance procedures consistent with that understanding assures long and trouble-free operation.

Rotary kilns are a valuable component in several industrial process systems. Protecting them is of utmost importance in maintaining process efficiency, prolonging equipment life, and avoiding costly repairs. If properly maintained and serviced, a high-quality rotary kiln results into very little downtime. Because of the high heat and process reaction which occurs within a rotary kiln, there are certain wear points to monitor. The main focus points are (i) refractory degradation or damage, (ii) burner maintenance or upgrade, (iii) worn out breeching seals, and (iv) drum misalignment.

A rotary kiln relies on its refractory lining in order to operate efficiently and maintain the desired temperature within. Refractory is also what protects the shell of the kiln from the high temperatures within. Unfortunately, refractory lining begins to degrade over time, causing a loss in kiln efficiency. Also in some cases, an object, such as hard material build-up, can find its way into the kiln and cause damage to the refractory lining. The damage can seem minimal, but can cause a material trap or cold spot, resulting in process inconsistencies. Also, since the refractory is meant to absorb the heat before it can come into contact with the drum shell, any thin or damaged areas can result in heat distortion. A distorted drum shell can cause serious damage to several components which are required to be replaced as opposed to be repaired. Since the refractories plays an important role in protecting the kiln, routine scheduled inspections of the refractory lining are to be carried out.

The biggest source of refractory failure is what is called cycling. Cycling is the heating up and cooling down of a rotary kiln. Each time the rotary kiln is heated, the refractory expands with the drum. As the rotary kiln is cooled, the refractory also contracts. If a kiln is regularly being turned on and shut down, the refractory can easily get stressed, causing cracks. Cracks can also occur from heating or cooling the kiln too quickly. It is important to try to reduce cycling as much as possible, keeping shut downs to a minimum.

Another source of refractory failure is the chemical incompatibility. Refractory is not designed to withstand certain chemicals. A big offender of this is chlorides. Chlorides can aggressively attack refractory lining, causing excessive wear of the lining because of their corrosive nature. When these chemicals are identified up front, refractory lining can be designed to handle such aggressive corrosion. Similarly, this failure can also happen when a rotary kiln is used for something the refractory lining has not been designed for. Sometimes there are unknown components in a material, and when a feedstock is changed, these unknown components can attack the refractory, again, causing excessive wear.

Aside from regular inspections, one easy way to help extend the life of the rotary kiln is to check for hot spots regularly. This can be done by picking a spot on the rotary kiln shell, and holding a temperature gun in place. As the rotary kiln rotates, that spot is to have the same temperature for the entire circumference of the shell. There is indication of hot spot if the temperature readings are 200 deg C, 200 deg C, 250 deg C, and 200 deg C.  A hot spot on the shell of the rotary kiln indicates a failure in refractory. Left unnoticed, it can lead to severe damage to the rotary kiln shell. In addition to circumference temperature being the same in a given location, there is to be a gradual shift in temperatures from one end of the kiln to the other, not a drastic change, which can indicate a problem.

While a rotary kiln has a high quality and reliable burner, it is still possible for burner related issues to occur. Parts such as the burner nozzle, burner cone, and burner sensors can need replacement for the kiln to continue operating as designed. In the case of an older rotary kiln, it is beneficial to upgrade the burner. Burner technology is constantly progressing and a new burner can result in higher energy efficiency and material output, making for a cost-effective upgrade.

The main function of the breeching seals in a rotary kiln, no matter the style, is to prevent outside air from entering the kiln. Several kilns are specifically designed to funnel exhaust air along with dust to a controlled area. Similarly, the design is configured to discharge the maximum quantity of process material, while still maintaining the desired internal temperature. This also means keeping cool, ambient air from entering the drum. Breeching seals prevent outside air from mixing with the controlled combustion air inside of the kiln. Worn out seals can alter this precise system, resulting in poor dust control and varying kiln temperatures. This can lead to added plant clean-up and process inconsistencies.

Similar in all rotary drum equipment, proper alignment preserves all components from premature wear. Major damage to the inlet or discharge breechings from a misaligned drum can result in extensive downtime. Also, a drum which is out of alignment can accelerate wear on the tyres, trunnion wheels, and thrust rollers, all of which are a necessity for rotary kiln operation. Misalignment can occur naturally over time, or as a result of improper installation. No matter how the misalignment happened, it is to be re-aligned before operation continues in order to prevent further damage.

Since a rotary kiln is, in several ways, similar to other rotary drum equipment, many of the same preventive maintenance procedures apply, and are to be routinely carried out by the maintenance personnel. These procedures include (i) lubrication of bearings, (ii) changing gear box oil, and (iii) rechecking / defining backlash. More extensive maintenance such as (i) tyre and wheel grinding, (ii) gear replacement, and (iii) changing of damaged parts etc. is to be carried out by trained and experienced maintenance personnel.

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