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Guniting Refractory Maintenance and Guniting Refractories


Guniting Refractory Maintenance and Guniting Refractories

Guniting of refractory materials has evolved in importance with modern steelmaking technology, and largely because of it. Guniting was seen by farsighted steelmakers as the only practical means of maintaining the steelmaking furnaces, particularly of countering the grossly uneven wear sustained by the linings. The steel industry has invested deeply in the development of guniting techniques and has been rewarded with successes. Yet the target keeps moving for the betterment of the technology. Steelmaking schedules call for furnace maintenance while the furnace is still hot.

Guniting is a process used for the hot repair of the refractory lining which get damaged during the operation of the furnace. It is an important process which is frequently used for enhancing the refractory lining life of the furnace. Sometimes it is also called shotcreting. It is a process or technique which involves pumping of refractory guniting refractory mixes under high pressure through a hose to a nozzle (spray-gun) and then spraying them at high velocity onto a surface either by dry-mix or by wet-mix process.  The guniting process has grown into an important and widely used technique for the maintenance of refractory lining. Guniting process due to its method of placing results into positive adhesion to any applied surface and, due to its density ensures a good repair of the damaged refractory lining.

There are two processes involved when it comes to spraying guniting refractory mixes pneumatically through a spraying gun (nozzle) at high velocity onto a surface (Fig 1). One is called the dry-mix process while the other wet-mix process. The dry-mix process involves preparation of the mortar mix consisting of fine and coarse particles completely dry in a bin and pumping it through a hose pneumatically (i.e. using compressed air) under high pressure to a nozzle (the spray-gun). Water is introduced to the dry mix only at the nozzle (by means of a water feeding line) just before the mix blasts off the nozzle at a high velocity onto the surface being gunited. On the other hand, the wet-mix process involves spraying of a pre-mixed or ready-mixed (wet) refractory mortar under high pressure and at high velocity eliminating the need of adding any water in the nozzle or the spray-gun. Both processes have their unique advantages and requirements.

Fig 1 Processes for guniting of refractory materials

Historical aspects

The term ‘gunite’ came into existence when a new technique was developed in USA in which concrete was used to be sprayed under pressure through a spray-gun at high velocity onto a surfacing requiring such an application. Since the material was sprayed through a gun, the term gunite seems to have surfaced. All these happened at the start of 20th century.

Later, the term ‘shotcrete’ came into being for the first time in the early 1930s when the American Railway Engineers Association (AREA) started using it to describe concrete (or mortar) nozzle-sprayed pneumatically at high velocity onto a surface either by dry-mix or by wet-mix process.

In the early 1950s, the American Concrete Institute (ACI) too adopted the term ‘shotcrete’ to describe the dry-mix spraying process previously known as guniting. ACI classified the shotcreting process into two types, namely, the dry-mix process and the wet-mix process. They also accepted the term guniting for the dry-mix shotcreting process. In other words, according to ACI, guniting is a type of shotcreting only. In some countries the word gunite is not used, instead the words dry shotcreting process is used for the guniting process.

The evolution from a wet guniting method, which closely resembles that of Portland cement concrete, to the present, where dry and semi-dry mixes have been projected onto hot surfaces, is a start. The use of flame-spraying techniques has also made a promise. But automation has done more than guniting methods, so far, toward successful maintenance of a basic oxygen furnace (BOF). Not many years ago, an operator was using a hand-held the guniting nozzle on the end of a wand and was pointing it where it was thought repair was needed. Visibility of the BOF lining was remote and poor. Today the gun is held robotically inside the vessel, moved and aimed by programmed command. Today the gun is preceded into the vessel by a laser device that scans the interior and topographically maps the thickness of the entire lining. By the time the gun enters, its repair task is digitized and ready for the ‘start’ signal. No waiting for the vessel to cool. No discomfort for the operator. No uncertainty in where to traverse. No oblique trajectories. No guesswork and no mistakes or oversights. It may not be perfect yet, perhaps. But this system is so close to trouble-free and routine realization as to dare the technology to match it with gunning methods and materials.

The guniting process

During the guniting of the refractory material, it is important to follow good guniting practices. This means keeping the nozzle is to be as close to a 90 degree angle to the guniting surface as possible. The operator is to try to maintain a circular guniting pattern and he is not to shoot over rebound. Rebound is expended or improperly hydrated material which bounces off the guniting surface. Guniting over rebound in high temperature applications ultimately leads to failure. Hence care is to be taken to ensure that any rebound which collects is properly cleaned out. This can be easily achieved by shutting off the material at the nozzle and turning up the air pressure air to blow out the corner or area where the rebound has collected.

While guniting of refractory material, it is important to bring the refractory material out the full thickness in one pass rather than in layers. A monolithic lining with no laminations provides a superior result. When the refractory is cold gunited to the full thickness in a furnace, vent holes are to be poked in the green material with a straight steel rod to the full depth of the lining, on around at 300 mm centres. Venting the refractory is extremely important since it provides a place for the steam to escape during the heat up. During a heat up, it is not uncommon to see steam coming out of all vent holes. Entrapped steam has a great amount of power and if the unit is heated too quickly, it can cause an explosion.

As a furnace is heated, the chemically combined moisture in the new refractory material migrates away from the hot face toward the steel shell, and when it reaches the steel, the steam escapes back toward the heat source through the vent holes. Hence, taking the time to properly vent the gunited refractory is essential. Quick fire fibres are now common in refractory installations. The quick fire fibres are typically made of polypropylene and burn out quickly when the temperature increases, creating fissures and avenues for the steam to escape. Even with quick fire fibres, it remains a good practice to vent the gunited refractory.

Dry mix and wet mix processes

The dry mix process consists of five steps namely (i) all dry ingredients, except water, are thoroughly mixed (usually done by the supplier of the guniting refractories), (ii) the guniting refractory material is fed into a special mechanical feeder or gun which is called the delivery equipment, (iii) the mixture is normally introduced into the delivery hose either through a metering device or air pressure alone (orifice feed) for delivering of the material into the hose, (iv) the guniting material is carried by the compressed air through the delivery hose to a nozzle body which is fitted inside with a water ring through which water is introduced under pressure and thoroughly mixes with the guniting material, (v) the material is jetted from the nozzle at high velocity on the damaged refractory surface to be gunited.

Since in the dry mix process an intimate mixing of the water and dry materials does not occur at the nozzle, dry-process relies heavily on the skills of the guniting operator who manipulates the nozzle in order to produce an effective mixing on the application surface. The amount of water added at the nozzle is critical, as insufficient water increases both the material rebound and dust, while excessive water causes the mix to slough off. Only fine refractory materials are used in dry guniting process, since higher particle size of the guniting material tend to rebound from the application surface. The guniting refractory material is required to generally have high effective cohesiveness to reduce rebound and produce a certain refractory build-up of the damaged surface.

The wet mix process also consists of five steps namely (i) all the ingredients of the guniting material and mixing water is thoroughly mix, (ii) the wet guniting mixture is introduced into the delivery of the equipment, (iii) the mixture is metered into the delivery hose and moved by positive displacement, (iv) compressed air is injected at the nozzle to increase the velocity and improve the shooting pattern, and (v) the wet guniting mixture is jetted from the nozzle at high velocity on the damaged refractory surface to be gunited.

Wet guniting process consists of a mixing chamber fed by a pneumatically-driven stream of dry solid particles and by jets of high pressure water. After in-line mixing, the wet solids pass out through a nozzle and the air stream eventually disengages. Nozzle design largely controls the dispersion of the spray. The liquid feed rate, hence ratio of liquid to solids, is controlled by the liquid pressure and by valves.

In the wet mix process, due to the reduced possibility of rebound, higher size refractory materials can be used in the mix. Since the guniting operator has no control over the mixture proportioning of the final product, the quality of wet-mix guniting is less dependent upon the skills of the operator.

In the dry mix process (i) there is instantaneous control is available over mixture water and consistency of the mixture at the nozzle to meet variable field conditions, (ii) the process is better suited for finer refractory guniting materials, (iii) the delivery hoses are easy to handle, (iv) the process is well suited to condition where the timing of pacing the guniting material cannot be predicted or is intermittent, and (v) there is lower volume to be handled per hose size. In the wet mix process (i) the mixture water is controlled at the mixing equipment and hence can be accurately measured, (ii) the process provides better assurance that the mixture water is thoroughly mixed with the refractory guniting materials, (iii) there is lesser dust and lesser loss of refractory guniting material during the shooting operation, (iv) there is normally lesser rebound resulting in lesser waste, and (v) there is higher volume to be handled per hose size.

While comparing the two processes, it can be seen that either of the process can be suitable for the guniting repairs of the damaged refractories surface. However, differences in the capital and the maintenance cost of the equipment, operational features, suitability of the available guniting refractory materials, and placement characteristics, can make one or other method more attractive for a particular application.

Guniting equipment

The main equipment for guniting of the refractory material is a guniting machine. The guniting machines are mounted on wheels for portability. A valve is provided at the nozzle for the regulated flow of water which ensures very efficient and controlled hydration of the guniting refractory mix. For avoiding manual work, one lever operation is generally provided for charging the guniting material into working chamber. This also makes the operation easy and safe. Auxiliary equipment upstream includes the necessary pumps and pressure regulators, pre-wetting equipment, and the solid feed regulating system where the solids flow rate is determined.

Manifolds are provided in airline to ensure minimum air pressure loss for the optimum utilization of air. This results into saving in air consumption. The upper and bottom vessels in the guniting machine can be unbolted for inspection and cleaning. A check valve is normally provided before manifold to prevent back flow of material into valves and air motor. The guniting machines are simple and safe to operate and can be operated even by an inexperienced person.

 Some accessories are needed for the guniting machines. These are standard hoses for air, water and material discharge with end connections. For nozzle body the accessories needed are light weight mixing nozzle complete with water regulating valve, water ring and nozzle tip with wear resistant liner. The nozzle assemblies are for the guniting of the refractory materials.

The guniting machine can be with manual or automatic operation. The automatic machine has several advantages namely (i) it places complete on/off control of flow of guniting refractory material in the hand of operator at nozzle point, (ii) guniting starts just by pressing handle and stops by releasing it, (iii) it saves guniting material when the operator changes position or performing patch-up repairing work, since the operator can start or stop guniting operation at will, (iv) it provides added safety to operator and other personnel in the area since if the operator for any reason lose the control of hose, guniting stops instantly and automatically.

Guniting refractories

Guniting refractories are granular refractory mixtures designed for application with air placement guns. A variety of air guns are used to spray the mixes at high velocity and pressure to form homogeneous compact lining essentially free from lamination and cracks.

Guniting refractories are either air-setting or heat-setting and some allow repairs to furnace linings without greatly reducing the furnace temperature. Light weight guniting refractories are used for insulation, while the denser guniting refractories are used in the more severe applications. Some compositions combine relatively low heat losses with good strength.

Although guniting refractories require more skills than pouring castables, it can place a higher volume of material in less time than any other method. Guniting also makes it possible to repair refractory linings in horizontal, vertical, and overhead positions or of irregular shapes. It does not need any forms.

The properties of guniting refractories vary considerably, and hence encompass a wide range of applications. The guniting refractory mixes perform very well in both original linings and maintenance applications within a service range of 850 deg C to 1900 deg C.

In the beginning, refractory wet guniting mixes were developed in and for the type of equipment available during that period. Advances in the equipment itself have not altered the principles of guniting mix formulation. Most modern commercial mixes have been fine-tuned or improved more than once over their predecessors, while those introduced I the later periods have had the benefit of the same informed viewpoints. Thus while there are several of wet mixes available, they all perform well in guniting.

Guniting refractories are monolithic refractories which are installed by guniting process. Guniting refractories consist of graded refractory aggregate and a bonding compound, and can contain plasticizing agent to increase their stickiness when pneumatically placed onto a furnace wall.  Typically guniting refractories are supplied dry. To use, they are pre-damped in a batch mixer, then continuously fed into a guniting machine. Water is added to the guniting mix as required by the guniting process to reach the proper consistency.

Guniting refractories include siliceous, fireclay, high alumina, and dead burned magnesite and chrome types. Many castables, ramming mixes and specially designed plastics can also be applied successfully with pneumatic guniting nozzles. Acid guniting refractories are normally pre-damped and fed through a continuous dual chamber or rotary gun. Magnesite and hot guniting refractories are not pre-damped and are placed in a batch pressure gun. Guniting refractories are needed to wet up well, have as wide a water range as possible, and provide excellent coverage in a variety of applications.

Examples of guniting refractories include fireclay guniting refractories of multipurpose hard fired fireclay and standard calcium aluminate (CA) cement compositions, fire clay guniting refractories with high purity CA bonding system, guniting refractories based on vitreous silica and a special combination of calcined fireclay and high purity CA cement binder, high purity alumina guniting refractories which combine high fired alumina aggregate and high purity CA binder, basic  refractory guniting refractories with magnesia content ranging from 60 % to 95 % with or without a phosphate bond etc.

Two central concerns about every guniting mix are (i) rebound, and (ii) the character of that which does not rebound, including its interface with the substrate. A third concern is the water-solids ratio, which influences both. The mixing of the liquid-solids in the chamber is far from uniform, and slaking of particle surfaces is barely begun before deposit. Hence on the whole, wetter deposits have to be made than are otherwise best. The water-solids ratio is optimized empirically for each mix, but iteratively with the size distribution of the solids.

Rebound is influenced by several interactive factors. The first is particle sizing. An important component of the rebound consists of coarse-sized particles. Minimizing of rebound consistent with performance of the gunited deposit calls for a top size of around 3.3 mm. Since other factors interact, this inertial criterion is somewhat flexible. A number of guniting mixes are top-sized at upto 4 mm or 4.7 mm. Fines, on the other hand, are deflected by the general air deflection in front of the substrate. Yet they are needed, and pretreatment can influence their trajectories. Moistening of the fines before feeding is practiced for other purposes, for example, and this diminishes their loss. Fines sized substantially below 44 micro-meters are increasingly prone to deflection. Between the top size and bottom size limits, a log-linear distribution is reasonable. 

All modern wet guniting mixes suffer less than about 10 % rebound, most of them around 5 % or less. The final factor in controlling rebound is adhesion. The wet or green particles are required to adhere to each other and to the substrate on arrival. Thus a major concern in wet mix formulation is the inclusion of binders. The only way to deliver a binder directly to the larger particles is by feeding the binder in the water. Dry but quickly-wetted and hence activated binders can be included among the fines. Such dry agents are very often surface-active inorganics than organics. They are pre-dampened as mentioned above to increase their adhesive quality. Many have been investigated, and a few in-uses can be identified, but special binders are among the most guarded proprietary aspects of guniting mix formulation.

The remaining major feature of formulation is the bond. It is generally not publicly advertised. The bond can often be inferred from the published chemical analysis. The commonest bond is clay, followed by CA cement. Silicate chemicals are used in some cases. Phosphate bonding chemicals are found principally among the higher-performance basic guniting mixes, and chrome ore is used in a few. Since some bonding chemicals serve also as green binders, in a number of mixes there is no necessary added proprietary binder. Clay-bonded mixes are an example.

An alumina-silica (Al2O3-SiO2) guniting mix product line is evident. There are several of these, ranging in percent Al2O3 (fired basis) at least from 30 % to 95 % and in service temperature limit from around 850 deg C to around 1900 deg C. A few alumina-chrome compositions are included. These materials are used not only for repair but also to build up original wall or roof linings on steel substrates. They are found in stacks, ducts, and hot dust collectors such as steam boilers and incinerators, in a wide variety of petroleum and petrochemical processing vessels, in heat treatment, reheat and annealing furnaces and elsewhere in foundries and nonferrous metal plants, in rotary and ceramic kiln applications, and in pouring-pit and other areas of iron and steel manufacture. Special formulations have been addressed to special problems such as erosion resistance and molten aluminum contact. Many are insulating formulations. On the whole they feature quick lining construction and repair and a minimum of required forms. They are generally not prescribed for highest-temperature slag line and metal-containment duty, e.g., in molten iron, steel, copper and nickel furnaces and ladles, or in glass melting.

The basic guniting mix product line is comparably numerous, though dedicated to a lesser number of different applications. The composition range is at least from 50 % MgO to 98 % MgO, including some burnt dolomite, magnesia-chrome, and a few magnesia-spinel types. These materials are used for taphole maintenance and other local high-wear repair applications in iron, steel and copper. The most corrosion-resistant formulations are used in EAF maintenance (banks, hearths, slag lines, and tapholes). And of course the largest tonnage goes to maintenance and repair of oxygen-blown steelmaking vessels, typified by the BOF. In this connection a few graphite containing basic guniting mixes are available but there is no indication of their exceptional popularity.

The line is rounded out with gunned zirconia-alumina, zircon magnesia, and others selected from the A-Z-S composition triangle and nearby, also gunited chrome ore and a few other selected minerals. The aggregate usage of these is minor compared with that of the two principal families.

A number of commercial wet guniting mixes are claimed to be adherent on either hot or cold substrates. Applied to a cold substrate, a gunited deposit is subsequently to be dried. The usual cautions about drying rate apply, but gunited deposits are a little more porous than most other monolithics and hence a little less sensitive to internal steam pressure. As regards to guniting on hot substrates (e.g., for furnace repair), there is increasingly hot means diminishing adherence to the substrate on account of flash boiling of water at the interface. The most critical circumstance is in the steel melting shop, since there guniting occurs on an almost daily basis. The BOF, for example, may not have to cool completely for gunited lining repair, but a severe limit is placed on the lining temperature by otherwise poor adhesion and poor material utilization in guniting. That temperature limit has not been codified. But it is sufficiently irritating to BOF operating schedules that dry and semi-dry gunning techniques have been sought.

In case of dry and semi-dry guniting mixes, a least amount of water can be employed in a conventional wet-mixing gunite, or the solids can be only moistened before feeding or not at all. In the latter two cases the wet mixing gunite is replaceable by a simple tube and nozzle. The move to semi-dry guniting in Japanese steel plants is said to have started in the 1970s.

The use of substrate temperature in place of water as the medium of binding and adhesion is appealing, but extremely difficult to carry out without downgrading the service temperature of the gunited deposit. Very few dry or semi-dry compositions have been found suitable. A family of dry MgO-CaO and MgO-CaO + graphite mixes has been used as well as phos-bonded and silicate-bonded versions of the same types. An alumina + SiC (silico carbide) + graphite composite has been investigated for blast furnace trough repair.

Only mediocre performance has been realized, on the whole. Among commercial products, dry basic guniting mixes have hardly been featured. Recognizing that even higher guniting temperatures is to yield qualities of binding and adhesion by out-and-out melting of particles in flight, technologists tried to the evaluation of flame and plasma spraying.

Flame and plasma guniting mixes – Oxy-gas, oxy-acetylene, and oxy-hydrogen torches have long been familiar devices for flame spraying and flame-plating of refractory compounds in other contexts. The plasma torch can double to triple the gas temperature of fuel torches, but at such increased cost that it might be economically impractical for refurbishing a hot BOF. Because of this, the plasma torch in or out of the picture. Hence, the confined to combustion torches are discussed where there is some experience has already been accumulated.

There is no doubt that flame guniting mixes of interest can be torch-melted, at least partially. One starts with the phase diagrams and then begins to inquire about flame heat-transfer kinetics and also about the cost of providing various flame temperatures. An interestingly economical torch has been devised and used, in which most of the fuel is provided as coke particles accompanying MgO particles entrained in the oxygen stream. Progress has been made in flame guniting in the BOF. An MgO-NaPO3 has been investigated, while the use of MgO-SiO2 compositions has been reduced to commercial practice.

Evaluations of the flame-gunited deposit are few, but there are good indications from actual use in BOF that the method is superior to semi-dry guniting and that it is cost-effective for converter maintenance. Practical oxide particle sizing guidelines shows particle size lying between around 50 micro-meters and 3 mm. However, the initial optimism of flame-guniting maintenance of BOFs could not be sustained. 

 

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