Calcination of Limestone
Calcination of Limestone
Calcination or calcining is a thermal treatment process to bring about a thermal decomposition. The process takes place below the melting point of the product. The name calcination is derived from the Latin word ‘Calcinare’ which mean to burn lime. Lime (CaO) is one of the oldest chemicals known to man and the process of lime production is one of the oldest chemical industries. Quicklime was produced in USA as early as 1635 in Rhode Island. Technical progress which was non-existing in centuries past has rapidly advanced the lime industry during the last fifty years in the area of process methods and design.
Limestone is one of the most basic raw materials employed in the steel industry and is used both in ironmaking, steelmaking, and auxiliary processes. Most of the lime used in the iron and steel industry is for fluxing impurities in the steelmaking furnace and in many of the secondary steelmaking processes. Lime is also used in different quantities in the sintering process for the preparation of iron ore, in the desulphurization of pig iron, for acid neutralization, and in water treatment facilities.
Limestone is a naturally occurring mineral. Limestone deposits have wide distribution. The limestone from the various deposits differs in physical and chemical properties. The chemical composition can also vary greatly from region to region as well as between different deposits in the same region. Limestone can be classified according to their chemical composition, texture and geological formation. Typically, limestone contains more than 90 % CaCO3 (calcium carbonate) and a few % MgCO3 (magnesium carbonate). Dolomite is a double carbonate containing 54 % to 58 % CaCO3 and 40 % to 44 % MgCO3.
Limestone is generally classified into the two types namely (i) high calcium in which the carbonate content is composed mainly of CaCO3 with a MgCO3 content not more than 5 % (usually less), (ii) magnesium or dolomitic limestones which refers to a limestone containing MgCO3 at a higher level than limestone but less than dolomite and which contains MgCO3 in the range of around 5 % to 20 %. Limestone is usually associated with impurities like silica (SiO2), alumina (Al2O3), iron (Fe), sulphur (S) and other trace elements.
The chemical reactivity of various limestones also shows a large variation due to the difference in crystalline structure and the nature of impurities such as SiO2, Al2O3, and Fe etc. The varying properties of the limestone have a big influence on the processing method. Hence it is necessary to know comprehensive information of the limestone such as physical and chemical properties, the burning characteristics and kinetic parameters for the calcination of the limestone. This aids optimal design and operation at lime kilns.
Calcination reactions usually take place at or above the thermal decomposition temperature. This temperature is usually defined as the temperature at which the standard Gibbs free energy is equal to zero. The decomposition reaction of the limestone is CaCO3= CaO + CO2 (g). The activation energy of the calcination reaction is generally between 37 kcal/mol to 60 kcal/mol, with values predominantly nearer to 50 kcal/mol. These values are compared with the theoretical value (at equilibrium) being between 39 kcal/mol to 41 kcal/mol. The uncertainty derives from the inherent complexity of the calcination process which, assuming a shrinking core model, involves a seven step mechanism. Heat Is to be transferred (i) to the particle outer surface, then (ii) conducted through the calcinated outer shell to the internal reaction interface, where (iii) a chemical reaction occurs and the CO2 (carbon di oxide) evolved is to either (iv) react at the interface, or (v) diffuse from the interface to the outer surface and it then (vi) diffuses away from the surface to the surrounding atmosphere, and (vii) CO2 from the surrounding atmosphere also diffuses to the reaction interface. The rate of calcination is governed by any one or any combination of these steps.
Calcination is an equilibrium reaction. In principle, CaCO3 decomposes to lime if the ambient partial pressure of CO2, is below the equilibrium value of the partial pressure at a given temperature. On the other hand, any lime formed is transformed back to carbonate if the partial pressure of CO2, exceeds this equilibrium value. The rate of the decomposition reaction is thus governed by the partial pressure of CO2, the reaction temperature and the particle size. At 700 deg C and atmospheric pressure the rate of the reaction becomes exceedingly slow, even in the absence of CO2. The chemical reactivity is known to vary between limestone sources, not only because of the differences in crystalline structure but also depending on the nature of the impurities.
Calcination of CaCO3 is a highly endothermic reaction, requiring around 755 Mcal of heat input to produce a ton of lime (CaO). The reaction only begins when the temperature is above the dissociation temperature of the carbonates in the limestone. This typically is between 780 deg C and 1340 deg C. Once the reaction starts the temperature is to be maintained above the dissociation temperature and CO2 evolved in the reaction is to be removed. Dissociation of the CaCO3 proceeds gradually from the outer surface of the particle inward, and a porous layer of CaO, the desired product, remains. Hence, the process depends on an adequate firing temperature of at least more than 800 deg C in order to ensure decomposition and a good residence time, i.e. ensuring that the lime/limestone is held for a sufficiently long period at temperatures of 1,000 deg C to 1,200 deg C to control its reactivity.
The factors affect the calcination are crystalline structure affects the rate of calcination, internal strength of limestone and resultant crystal size of lime after calcination. The smaller crystals agglomerate during calcination and forms larger crystals which in turn cause shrinkage and volume reduction.
Calcination at higher temperature means higher agglomeration and more shrinkage. Also the density of limestone is related to the crystal structure. The shape of crystals determines the void space between crystals, and hence the density of the limestone. Larger voids allow easy passage for CO2 gases during calcination and it results in a reduction of volume during calcination. Some limestone, due to its crystalline structure, disintegrates during the calcination process. This type of limestone is not useful for calcining. There is some other limestone whose behaviour is the opposite. This type of limestone become so dense during calcination that it prevents the escape of CO2 and become non porous. This type of limestone is also not suitable for calcination
The reactivity of lime is a measure of the rate at which the lime reacts in the presence of water. The test method to measure the reactivity of ground lime is carried out by slaking the lime in water. The reactivity of lime depends on different parameters related to the raw material and the process. These parameters are namely (i) burning temperature and time, (ii) crystalline structure of the limestone, (iii) impurities of the limestone, and (iv) kiln type and fuel. The classification of lime is often seen in terms of its reactivity, such as (i) dead burned, (ii) hard, (iii) medium, and (iv) soft.
Lime with a lower reactivity is often referred to as medium, hard and dead burned. The decrease of reactivity is accompanied by a reduction of the surface and the porosity of the lime, which is called sintering. Each specific type of lime has a particular reactivity which, in turn, is governed by the requirements of the application and the specific process. As mentioned above, the characteristics of lime also depend on the limestone feed material, the type of kiln and the fuel used. For example, coke-fired shaft kilns generally produce lime with a medium to low reactivity, whereas gas-fired parallel flow regenerative kilns usually produce a high reactivity lime. The chemistry and reactivity of lime are the main parameters which drive its use.
The decomposition of dolomite and dolomitic limestone is much more complex. Decomposition can occur via a single or two discrete stages or even via intermediate stages. The reactions involved in these stages are CaCO3.MgCO3 + heat = CaCO3.MgO + CO2, CaCO3.MgO + heat = CaO.MgO + CO2, and CaCO3.MgCO3 + heat = CaO.MgO + 2CO2. The temperature required for the decomposition of dolomite and dolomitic limestone is usually in the range of 500 deg C to 750 deg C.
The smaller size limestone is more suitable for calcination in rotary kilns and it allows optimum residence time. The lower calcining temperature also allows less fuel consumption. In contrast, larger size limestone and low calcining temperature is needed for vertical kilns. If the temperature rise is too rapid, the outer layer of the limestone pieces is calcined very fast. As the temperature rises, the surface of the limestone shrinks and closes the pores created by the escape of CO2. This causes increased internal pressure within the limestone. Since the CO2 gas cannot escape, the limestone explodes and disintegrates producing unwanted ‘fines’ thus reduces the quality of the lime.
The production of good quality lime depends upon the type of kiln, conditions of calcination and the nature of the raw material i.e. limestone. At relatively low calcination temperatures, products formed in the kiln contain both unburnt carbonate and lime and is called ‘under-burnt’ lime. As the temperature increases, ‘soft burnt’ or ‘high reactive lime’ is produced. At still higher temperatures, ‘dead burnt’ or ‘low reactive lime’ is produced. Soft burnt lime is produced when the reaction front reaches the core of the charged limestone and converts all carbonate present to lime. A high productive product is relatively soft, contains small lime crystallites and has open porous structure with an easily assessable interior. Such lime has the optimum properties of high reactivity, high surface area and low bulk density. Increasing the degree of calcination beyond this stage makes formed lime crystallites to grow larger, agglomerate and sinter. This results in a decrease in surface area, porosity and reactivity and an increase in bulk density. This product is known as dead burnt or low reactive lime.
Calcining of limestone in the kiln
Passing limestone (with or without a significant MgCO3 content) through the kiln can be divided into three stages or heat transfer zones (Fig 1) consisting of (i) pre-heating zone, (ii) calcining zone, and (iii) cooling zone.
Preheating zone -Limestone is heated from ambient temperature to around 800 deg C by direct contact with the gases leaving the calcining zone composed mainly of combustion products along with excess air and CO2 from calcinations.
Calcining zone – Fuel is burned in preheated air from the cooling zone and (depending on the design) in additional ‘combustion’ air added with the fuel. In this zone, temperatures of greater than 900 deg C are produced. From 800 deg C to 900 deg C, the surface of the limestone starts to decompose. At temperatures above the decomposition temperature of limestone, i.e. 900 deg C, decomposition takes place below the surface of the limestone pieces. At a temperature of 900 deg C, these pieces leave the calcining zone and are sometimes found as residual limestone which is still trapped inside. If the pieces which are decomposed fully and still reside in the calcining zone, sintering occurs.
Cooling zone – Lime which leaves the calcining zone at temperatures of 900 deg C, is cooled by direct contact with ‘cooling’ air, part or all of the combustion air, which in turn is preheated. Lime leaves this zone at temperatures of less than 100 deg C.
The residence time of the limestone-lime in a kiln varies depending on the type of kiln and type of final product needed. This period is found to be between six hours and two days. Lime is often referred to as light or soft, medium or hard burned depending on the extent to which it has been calcined. The degree of reactivity, i.e. reactivity to water, is found to decrease as the level of porosity increases.
Fig 1 Stages of heat transfer zones and long rotary kiln
The rate of limestone decomposition in the kiln is, hence, found to depend on several factors inherent of the limestone particles themselves, i.e. morphology and composition, and of the process conditions. The main variables are found to be (i) the chemical characteristics of limestone, (ii) the particle size and shape, (iii) the temperature profile of the calcining zone, and (iv) the rate of heat exchange between gases and particles.
Most of the kilns used are based on either the shaft or the rotary design. There are a few other kilns based on different principles. All of these designs incorporate the concept of the three zones. Whereas shaft kilns usually incorporate a preheating zone, some other lime kilns, namely rotary kilns, sometimes operate in connection with separate pre-heaters. Most kiln systems are characterized by the counter-current flow of solids and gases, which has implications for the resulting pollutant releases.
The run-of-kiln (ROK) lime is processed by screening the minus fraction from the lime. The plus fraction of the lime is used for steelmaking while the minus fraction is used in iron ore sintering, water treatment plants and many other small uses in the steel plant.
Types of lime kilns – techniques and design
Calcining kilns are basically comes in two categories. They are i) rotary kilns and ii) vertical kilns. Both the types of kilns can be designed with any of the solid, liquid or gaseous fuels. Rotary kilns can be long kilns with straight rotary coolers while verticals kilns can be several types. Calcining kilns need limestone with proper decrepitation index. Decrepitation index of limestone is a measure of its susceptibility to disintegration during calcination. Low value of decrepitation decreases the porosity of the bed thus impeding the flow of the gases the kiln efficiency. Rotary kilns also need limestone with good tumbling index.
A large types of techniques and kiln designs have been used, though presently lime kilns are dominated by a relatively small number of designs, many alternatives are available, which are particularly suitable for specific applications. Limestone properties, such as strength before and after burning, type of available fuel and product quality, are to be considered when choosing a kiln technique. There are in general six general types of kilns used for the calcination of limestone. The main important factors for the selection of a kiln include (i) nature of the limestone deposit, (ii) characteristics, availability and quality of the limestone, (iii) input granulometry consisting of mechanical properties of the kiln feed, and fines in the feed, (iv) requirement of the lime properties for its major use, (v) kiln capacity, (vi) type of the fuel available, (vii) environmental impact, and (viii) capital and the operating cost. The physico-chemical properties of lime are inherently linked to the type of kilns used for the calcination. Tab 1 gives different types of kilns.
|Tab 1 Types of lime kilns|
|Sl. No.||Kiln type||Capacity range in tons/day||Kiln feed size range in mm|
|1||Long rotary kiln||160-1,500||2-60|
|2||Rotary kiln with pre-heater||150 -1,500||10-60|
|3||Parallel flow regenerative kiln||100-600||10-200|
|4||Annular shaft kiln||80-300||10-150|
|5||Mixed feed shaft kiln||60-200||20-200|
There are two types of rotary kilns which are normally used for the production of lime namely (i) long rotary kiln (LRK), and (ii) rotary kiln with pre-heater (PRK).
Long rotary kilns (LRK) – The LRK (Fig 1) consists of a rotating cylinder upto 150 meters (m) long and inclined at an angle of 1 degree to 4 degrees to the horizontal with a diameter of around 2 m to 4.5 m. Limestone is fed into the upper end and fuel plus combustion air is fired from the lower end. Lime is discharged from the kiln into a lime cooler, where it is used to preheat the combustion air. Different designs of lime coolers are used including planetary units mounted around the kiln shell, travelling grates, and various types of counter-flow shaft coolers.
In LRK, there is no pre-heater and the fuel burners are at the lime discharge end. Type of fuel can be gas, liquid, pulverized solid fossil fuels, waste fuels, or biomass. Heat requirement is 1,430 Mcal/t of lime to 2,200 Mcal/t of lime. Electricity requirement is 18 kWh/t of lime to 25 kWh/t of lime. The structure of the kiln is inclined rotating cylinder with refractory lining and ‘mixers’ to improve the heat exchange. Types of cooler can be (i) planetary around kiln shell, (ii) travelling grate, (iii) rotating cylinder, or (iv) static shaft cooler. The combustion air injection is through cooling air at the extremity of the cooler and primary air with the fuel. Flue gas extraction is by an induced draft (ID) fan at the end of the rotating cylinder at the limestone feeding side through a duct. The gas is cooled and dedusted before discharge. Drawing of lime is at the extremity of the cooler. Important points are the quality of the refractory and fine grinding of coal to ensure good combustion and reduction of the build-up (ring formation) in the kiln.
Continuous measurement of CO and O2 is necessary for good combustion and safety. LRK has flexibility of production. There is very quick reaction for modification of parameters. Wide range of feed limestone sizes can be used. Very low residual CO2 is achievable. There is flexibility of reactivity from soft to hard-burned. There is possibility to produce dead burned dolomite. There exists flexibility with regards to usage of fuel. Soft limestone can be used, but generates a lot of fines during calcination. The disadvantages of LRK include high energy requirements, and formation of rings (coal ashes, calcium sulphates, and clay etc.)
The design of a burner is important for the efficient and reliable operation of the LRK kiln. The flame is to be adjustable for different type of fuels. Because of the fact that process conditions can be easily and quickly varied, LRKs can produce a wider range of lime reactivity and lower residual CO2 levels than shaft kilns. Relatively weak feed limestones which break up and are unsuitable as feed for shaft kilns are suitable for rotary kilns.
Rotary kilns can be fired with a wide range of fuels. As heat transfer in the calcining zone is largely influenced by radiation and, as the infrared emissivities increase in the sequence gas, oil and solid fuels, the choice of fuel can have a significant effect on heat usage. Radiation and convection losses are highly relative to other designs of lime kilns which result in generally higher energy consumption compared to other types of kilns.
An advantage of the rotary kiln is that sulphur (S) from the fuel, and to a lesser extent from the limestone, can be expelled from the kiln in the kiln gases by a combination of controlling the temperature and the percentage of CO in the calcining zone. Thus, low S lime can be produced using high S fuels, subject to the emission limits for SO2 in the exhaust gases. LRKs are flexible kilns regarding the use of fuels and different feed sizes of limestone particularly the finer fractions.
Rotary kilns with pre-heaters (PRK) – Rotary kilns can be fitted with pre-heaters. PRKs (Fig 2) are generally considerably shorter than the conventional LRKs (e.g. 40 m to 90 m). The heat use decreases because of reduced radiation and convection losses as well as the increased heat recovery from the exhaust gases.
A number of pre-heater designs have been developed, including vertical shafts and travelling grates. The pre-heater is to be selected on the basis of the size and properties of the feed limestone. Most can accept a lower size of 10 mm while some have used limestones down to 6 mm, and some cannot tolerate weak limestones or limestone which is prone to break-up.
While the elimination of S is more difficult with PRKs, there are a number of ways in which it can be achieved such as (i) operating the kiln under reducing conditions and introducing additional air at the back-end (only works with certain designs of the pre-heater), and at the burner, combustion air, pre-heater, kiln, and cooler, and (ii) adding sufficient finely divided limestone to the feed for it to preferentially absorb SO2 and so that it can be either collected in the back-end dust collector, or is screened out of the lime discharged from the cooler.
In PRK, the fuel burners are at the lime discharge end. Type of fuel can be gas, liquid, pulverized solid fossil fuels, waste fuels, or biomass. Heat requirement is 1,220 Mcal/t lime to 1,860 Mcal/t lime. Electricity requirement is 17 kWh/t of lime to 45 kWh/t of lime. The structure of the kiln is inclined rotating cylinder with refractory lining and ‘mixers’ to improve the heat exchange. PRKs are having 2 m to 4.5 m diameter and a length of maximum 90 m. Types of cooler can be (i) planetary around kiln shell, (ii) travelling grate, or (iii) rotating cylinder. The combustion air injection is through cooling air at the extremity of the cooler. Flue gas is passed through the pre-heater and the extraction is by an ID fan. The gas is cooled when it exchanges heat with the limestone feed. Drawing of lime is at the extremity of the cooler. Important points are the quality of the refractory, fine grinding of coal to ensure good combustion and reduction of the build-up in the kiln, and air tightness of the joint between pre-heater and kiln. A high content of fines in the limestone feed can block the pre-heater. Continuous measurement of CO and O2 is necessary for good combustion and safety.
PRKs have flexibility of production. There is very quick reaction for modification of parameters. They can use wide range of feed limestone sizes. Very low residual CO2 is achievable. There is flexibility of reactivity from soft to hard burned, with possibility to produce dead burned dolomite. There is fuel flexibility since PRKs can use gas, liquid, or pulverized solid fossil fuels as well as waste fuels and biomass. Soft limestone feeds can be used, but they produce a lot of fines during calcination. Lower fuel requirements in PRKs is due to better heat exchange in the preheater (beginning of de-carbonization). The disadvantages include formation of rings (coal ashes, calcium sulphates, clay), and pre-heater is an additional piece of equipment to maintain.
Fig 2 Rotary kiln with pre-heater
Shaft kilns constitute majority of all the kilns presently being used for the production of lime. The types of shaft kilns are (i) mixed feed shaft kiln (MFSK), (ii) parallel flow regenerative kiln (PFRK), (iii) annular shaft kiln (ASK), and (iv) other kilns. Shaft kilns are vertical in design, upto 30 m (metres) in height and with a diameter of upto 6 m. For this type of kiln, the limestone is fed in at the top section of the kiln which progressively makes its way down through the different stages of the kiln until it is discharged at the bottom as lime. The performance of traditional shaft kilns has been limited by the difficulty in obtaining a uniform heat distribution over the kiln cross-section and uniform material movement through the kiln.
Mixed feed shaft kiln (MFSK) – MFSK (Fig 3) uses limestone with a top size in the range of 20 mm to 200 mm and a size ratio of around 2:1. The most widely used fuel is a dense grade of coke with low ash content. The coke size is only slightly smaller than that of the limestone. Hence, it moves down with the limestone rather than trickling through the interstices. The limestone and the coke are mixed and charged into the kiln in such a way as to minimize segregation. Anthracite is used more and more these days due to the price and lesser availability of metallurgical grade coke.
In MFSK, fuel feed is mixed with limestone. Heat requirement is 810 Mcal/t of lime to 1,120 Mcal/t of lime. Electricity requirement is 5 kWh/t of lime to 15 kWh/t of lime. The structure of the kiln is either vertical cylinder or rectangular shaft with refractory lining. The combustion air consists of cooling air injected from the bottom. Drawing of lime is by rotating eccentric plate. The important point is that it requires uniform mixing of stone and fuel and requires even distribution of limestone over the cross-section. There is high retention of S from fuel in the lime.
The lime produced from the kiln has low reactivity. Cooling air is used as combustion air. The kiln works on low excess air. The uniform fuel/air mixing is difficult to achieve in the kiln, producing variations in air / fuel ratio. The process conditions lead to CO emissions. The kiln needs large feed size of the limestone. There is low reaction to modify parameters (24 hours), so great inertia.
Fig 3 Mixed feed shaft kiln
Parallel flow regenerative kiln (PFRK) – The main feature of standard PFRK (Fig 4) is that it has two circular shafts connected by a cross-over channel, although some early designs had three shafts while others had rectangular shafts. The method of operation of PFRK incorporates the following two key principles.
- The preheating zone in each shaft acts as a regenerative heat exchanger, in addition to preheating the limestone to the calcining temperature. The surplus heat in the gases is transferred to the limestone of the other shaft during the first stage of the process. It is then recovered from the limestone by the combustion air, which is pre-heated to around 800 deg C. As a result, the kiln has very low specific heat consumption.
- The calcination of the limestone takes place at a relatively moderate temperature, typically around 900 deg C to 1100 deg C. This makes the kilns ideally suited for producing moderate and high reactivity lime with a low residual CO2 level.
In practice, batches of limestone are charged alternatively to each shaft and passed downwards through the pre-heating zone, around the fuel lances and then into the calcining zone. From the calcining zone, they pass finally to the cooling zone. The operation of the kiln consists of two equal periods, which last from 8 minutes to 15 minutes at full output. During the first period, fuel is injected through the lances at the first shaft and burns with the combustion air blown down in this shaft. The heat emitted is partly absorbed by the calcination of the limestone in this first shaft. Cooling air is blown into the base of each shaft to cool the lime. The cooling air in shaft number one, together with the combustion gases and the CO2 from calcination, pass through the inter-connecting cross-duct into shaft number two at a temperature of around 1050 deg C. In shaft number two, the gases coming from shaft number one are mixed with the cooling air blown into the base of shaft number two and flow upwards. Hence, they heat the limestone in the pre-heating zone of shaft number two.
If the above mode of operation is to continue, the exhaust gas temperature rises to well over 500 deg C. However, after a period of 8 minutes to 15 minutes, the fuel and air flows in the first shaft are stopped and a ‘reversal’ occurs. After charging the limestone to shaft number one, fuel and air are injected into shaft number two and the exhaust gases are vented from the top of shaft number one.
The kiln can be fired with gas, liquid or solid pulverized fuels as well as waste fuels or biomass. The kiln also has a high turn down ratio, although at lower production rates there can be some loss of energy efficiency. Once a kiln has been lit, it is undesirable to shut it down as this can result in a shorter life of the refractory. The campaign life of the refractory in the burning and cross-over channel is around 4 years to 8 years. The standard PFRK needs clean limestone, ideally with a limestone ratio not greater than 2:1. The minimum limestone size is 30 mm, although a modified design called the PFRK fine lime kiln can operate on sizes as small as 10 mm to 30 mm on clean limestone.
In PFRK fuel feed is through lances in the limestone bed. Heat requirement is 765 Mcal/t of lime to 1,000 Mcal/t of lime. Electricity requirement is 20 kWh/t of lime to 41 kWh/t of lime. The structure of the kiln is two or three vertical cylinders or rectangular shafts with refractory lining connected by a channel for circulation of hot gases. The combustion air injection is at the top (main) and lances (10 %). Drawing of lime is by rotating eccentric plate. The important point is the quality of the refractory works. PFRK has the flexibility of production. The reactivity of produced lime is high with reasonable flexibility of reactivity from high to medium, when the limestone allows. The consumption of fuel and energy is low. PFRK has limited stop/start flexibility. It is not suited to limestone with high decrepitation. The refractory lining is more expensive than for other types of kilns.
Fig 4 Parallel flow regenerative kiln
Annular shaft kilns (ASK) – The major feature of ASKs (Fig 5) is a central cylinder which restricts the width of the annulus, and together with arches for combustion gas distribution ensures good heat distribution. The central column also enables part of the combustion gases from the lower burners to be drawn down the shaft and to be injected back into the lower chamber.
This recycling moderates the temperature at the lower burners and ensures that the final stages of calcination occur at a low temperature. Both effects help to ensure a product with a low residual CO2 level and a high reactivity. The ASK can be fired with gas, oil or solid fuel. The exhaust gases have a high CO2 concentration.
In ASK, the fuel feed is both at the upper and lower part of the burning chamber sometimes mixed with limestone. Central cylinder restricts the width of the annulus. Heat requirement is from 790 Mcal/t of lime to 1,170 Mcal/t of lime. Electricity requirement is 18 kWh/t of lime to 35 kWh/t of lime (upto 50 kWh/t for feed sizes of below 40 mm). The structure of the kiln is vertical cylinder shaft with refractory lining. ASK has external chambers and burners. Type of fuel which is used can be gas, liquid, pulverized solid fuels, waste fuels, or biomass. The combustion air injection is at the top of the calcining chamber. Drawing of lime is by rotating eccentric plate. The important point is that it requires very accurate process control.
The lime produced from the kiln has low residual CO2 and high to medium reactivity. Fuel saving is through heat recovery. The kiln has good heat distribution. ASK has maintenance of heat recuperator and outer chambers. It has relatively high construction cost due to its conception.
Fig 5 Annular shaft kiln
Other types shaft kilns
This group of kilns includes a number of designs. In these designs, fuel is introduced through the walls of the kiln, and is burned in the calcining zone, with the combustion products moving upwards counter-current to the lime and limestone. In some designs, the fuel is partially combusted in external gasifiers. In others, it is introduced through devices such as a central burner, beam burner or injected below internal arches. Some of these kilns are described below.
Double inclined shaft kilns – This type of kiln (Fig 6) can produce a reactive low carbonate product. It is essentially rectangular in cross-section but incorporates two inclined sections in the calcining zone. Opposite each inclined section, offset arches create spaces into which fuel and preheated combustion air are fired through the combustion chambers. Cooling air is drawn into the base of the kiln where it is preheated, withdrawn and re-injected through the combustion chambers. The circuitous paths for both the gases and the burden, coupled with firing from both sides, ensure an efficient distribution of heat. A range of solid, liquid and gaseous fuels can be used, although they are to be selected with care to avoid excessive build-ups caused by fuel ash and calcium sulphate deposits.
Fig 6 Double inclined shaft kiln and gas suspension calcination kiln
Gas suspension calcination (GSC) kilns – Gas suspension calcination (GSC) kilns are a technique for minerals processing, such as the calcination of limestone, dolomite and magnesite from pulverized raw materials to produce highly reactive and uniform products. Most of the processes in the GSC kiln plant, such as drying, preheating, calcination and cooling, are performed in gas suspension. Hence, the plant consists of stationary equipment and a few moving components (Fig 6). The amount of material present in the system is negligible, which means that after a few minutes of operation, the product conforms to specifications. There is no loss of material or quality during start-up and shut-down so there is no sub-grade product. The GSC process produces a product with high reactivity, even when calcined to a high degree. The material to be processed in the gas suspension is required to have a suitable fineness. The practical experience has shown that 2 mm particle size is not to be exceeded. Some performance figures for the balanced operation of GSC kiln plant are fuel consumption of around 1,150 Mcal/ton of product and power consumption of 33 kWh/ton of product.
Multi-chamber shaft kilns – This is another type of double inclined kiln. It consists of four or six alternately inclined sections in the calcining zone, and opposite of each is an offset arch. The arches serve the same purpose as in the double-inclined kiln. Cooling air is preheated by lime in the cooling zone and is withdrawn, de-dusted and re-injected through the combustion chambers. A feature of the kiln is that the temperature of the lower combustion chambers can be varied to control the reactivity of the lime over a wide range. The kiln can be fired with solid, liquid and gaseous fuels or a mixture of different types of fuels.
Travelling grate kilns – For limestone feed with a size range of 15 mm to 45 mm, an option is the ‘travelling grate’ or CID kiln. It consists of a rectangular shaft preheating zone, which feeds the limestone into a calcining zone. In the calcining zone, the limestone slowly cascades over five oscillating plates, opposite of which are a series of burners. The lime passes to a rectangular cooling zone. The kiln can burn gaseous, liquid or pulverized fuels and is reported to produce a soft burned lime with a residual CaCO3 content of less than 2.3 %.
Top-shaped kilns – The ‘top-shaped’ lime kiln is a relatively new development, which accepts feed limestone in the range of 10 mm to 25 mm. This consists of an annular preheating zone from which the limestone is displaced by pushing rods into a cylindrical calcining zone. Combustion gases from a central, downward facing burner, fired with oil and positioned in the centre of the preheating zone are drawn down into the calcining zone by an ejector. The lime then passes down into a conical cooling zone. The kiln is reported to produce high quality lime, suitable for steelmaking. The kiln capacities are upto 100 tons/day of lime. It is reported that, because of its relatively low height, the kiln can accept limestone with low strengths.
Rotating hearth kilns – This type of kiln, now almost obsolete, was designed to produce small sized lime. It consists of an annular travelling hearth carrying the limestone charge. The limestone is calcined by multiple burners as it rotates on the annular hearth. The combustion air is preheated by surplus heat in the exhaust gases and/or by using it to cool the lime. Due to the reduced abrasion compared to rotary and shaft kilns, rotating hearth kilns produce a high proportion of small sized lime.
Storage and handling
Storage of lime – Lime is preferably stored in dry conditions and free from drafts to limit air slaking. High care is to be exercised to ensure that water is excluded from the lime, as hydration liberates heat and causes expansion, both of which can be dangerous.
Air pressure discharge vehicles are able to blow directly into the storage bunker, which is fitted with a filter to remove dust from the conveying air. The filter is to be weather-proof and water-tight. The collected dust can be discharged back into the bunker. A pressure/vacuum relief device fitted to the bunker is a precautionary measure to enable maintenance work to be done on the discharge mechanism. Where the amount of lime is insufficient to justify storage bunkers the product can be stored on a concrete base, preferably in a separate bay within a building to prevent excessive air slaking.
Many types of equipment are suitable for transferring the lime and new ones are continually being developed. The following techniques have been used successfully, but may not be suitable for all applications.
Skip hoists can be used for all granular and lump grades but are more suitable for particles greater than 100 mm. Elevators (both belt-and-bucket and chain-and-bucket elevators) have been used for all grades of lime. Drag-link conveyors are suitable for granular and fine lime. They are generally used for horizontal or inclined transfer. Conveyor belts are widely used for transferring lump and granular grades horizontally and on an upward slope. Screw conveyors are widely used for fine lime. Vibrating trough conveyors have been used for particle sizes upto 40 mm. They operate more successfully when there is a slight downward slope from the feed to the discharge point. Pneumatic conveying can be used for products with a maximum size of upto 20 mm and often has a lower capital cost than alternatives, but the operating costs are higher. The product is fed into a rotary blowing seal connected to a blower. The pipeline bore, and volume/pressure of the blowing air, is designed taking into account the size of lime being conveyed, the transfer rate and the length/route of the pipeline. The receiving silo is equipped with an air filter and a pressure relief valve.