REFRACTORIES IN CEMENT INDUSTRIES:-

Plant in cement industry

PRE-HEATERS:

A device for preliminary heating of a material, substance, or fluid that will undergo further use or treatment by heating.The above mention definition of pre-heaters gives that some material is to be heated before undergoing another heating process at elevated temperatures to increase the thermal efficiency & minimize the heat losses & minimize the cost loss of the company. The history tells us that in the early fifties there was a technical revolution. KHD built the first Preheater (Germany, Bomke & Blackman), a starting point of a new area of reducing the heat consumption of the clinker pyroprocessing system by nearly 50% at this time.

Today KHD is one of the technical leaders worldwide in this field. The raw meal processed in the kiln first passes through the preheater absorbing the heat content of the kiln and PYROCLON gases minimizing the heat consumption of the system. KHD Preheater with modern designed cyclones produce low pressure drops and is high in separation efficiency. The resulting low waste gas temperatures lead to reduced heat losses, waste gas quantities, CO2 emissions, dust emissions and electrical energy consumption.REFRACTORIES IN PRE-HEATERS:As we know that in pre-heaters there are some temperatures so to minimize the heat losses due to that temperatures we use refractories….Cyclones and connecting ducts The operational requirements of most modern cyclones include increasingly larger diameters, which challenge refractory stability, and higher operating temperatures, which increase the risk of alkali & chloride penetration. Build-ups are also a problem area, related both to chemical attack and to the cyclone’s increasingly complex design.Large cyclone diametersIncreasingly large cyclone diameters make it difficult to keep bricks in place in the vertical sections. To meet this challenge, Höganäs Bjuf developed the CYbrick, which has an interlocking profile that allows adjacent bricks to support each other.Higher operating temperaturesHigher operating temperatures, particularly in the lower cyclones, require use of refractories with high refractoriness and high strength. With higher temperatures corrosive vapors can penetrate higher into the cyclones systems so alkali & chloride resistant refractory should be use.Build-upsUsually caused by chemical attacks in combination with the venture effect with the cyclone’s complex design, build-ups can also be caused by rapid temp. Drops the result of this for example: air leakage.Build ups reduce the cyclone efficiency, and there removal ultimately requires the production stop.Alkali & chloride penetration

Chemical attack – in the form of alkali penetration – is unavoidable in cement production. The worst damage occurs in lower cyclone stages and riser ducts, kiln inlets and even recliners. Alkali- and acid vapors infiltrate the refractory linings and attack the binding phase at temperatures as low as 600-700°C, thus endangering the lining. When these gases penetrate behind the refractories the effect is even worse. Chloride & sulphur dioxide combine with condensing steam to form acids that corrode anchors. Left unchecked lift the lining collapse. The safest most cost effective method to deal with this? Reduce the amount of insulation used, in order to move the vapors dew point outside the shell of the cyclone. If refractory & cyclone external temp. Remain above 100℃ ambient moisture & chemical vapors will not condense.Lining your cyclone Following is Höganäs Bjuf’s expert suggestion for lining your cyclone system, both for standard and alternative fuels. Cyclone roof linings usually depend on the type of cyclone construction. Brick or monolithic?It is commonly believed that casting, gunning and shot-gunning are faster, and therefore less expensive than installing bricks.The simple truth is quite the opposite.Brick is simple to install & ready to use immediately - unlike castables, gunnables and monolithic. It is a finished product pressed, fired & quality controlled.We therefore suggest you use brick or pre cast where possible. Unless a monolithic solutions offers the most important refractory characteristics.

CALCINER:Re-cycling heated air delivered from the grate cooler via the tertiary duct, the calciner dramatically improves thermal efficiency in the cyclone system.It is also used for the same purpose as the pre-heaters to reduce the thermal efficiency and minimize the fuel cost...Because of high operating temperatures, chemical attack is more severe problem in the calciner than in the cyclones. High refractoriness and good thermal shock are also necessary, especially around the burner blocks...Lining your calcinerExpert suggestion for lining your calciner, both for standard and alternative fuels. The three key areas – cone, walls and roof –are specified individually.

ROTARY KILN:A Rotary kiln is a pyroprocessing device used to raise materials to a high temperature (calcinations) in a continuous process. Materials produced using rotary kilns include: Cement Lime Refractories Met kaolin Titanium dioxide Alumina Vermiculite Iron ore pellets

They are also used for roasting a wide variety of sulfide ores prior to metal extraction.

Principle of OperationThe kiln is a cylindrical vessel, inclined slightly to the horizontal, which is rotated slowly about its axis. The material to be processed is fed into the upper end of the cylinder. As the kiln rotates, material gradually moves down towards the lower end, and may undergo a certain amount of stirring and mixing. Hot gases pass along the kiln, sometimes in the same direction as the process material (co-current), but usually in the opposite direction (counter-current). The hot gases may be generated in an external furnace, or may be generated by a flame inside the kiln. Such a flame is projected from a burner-pipe (or "firing pipe") which acts like a large Bunsen burner. The fuel for this may be gas, oil, pulverized petroleum coke or pulverized coal.

ConstructionThe basic components of a rotary kiln are the shell, the refractory lining, support tyres and rollers, drive gear and internal heat exchangers.

Kiln ShellThis is made from rolled mild steel plate, usually between 15 and 30 mm thick, welded to form a cylinder which may be up to 230 m in length and up to 6 m in diameter. This will be usually situated on an east/west axis to prevent eddy currents.Upper limits on diameter are set by the tendency of the shell to deform under its own weight to an oval cross section, with consequent flexure during rotation. Length is not necessarily limited, but it becomes difficult to cope with changes in length on heating and cooling (typically around 0.1 to 0.5% of the length) if the kiln is very long.

Refractory LiningThe purpose of the refractory lining is to insulate the steel shell from the high temperatures inside the kiln, and to protect it from the corrosive properties of the process material. It may consist of refractory bricks or cast refractory concrete, or may be absent in zones of the kiln that are below around 250°C. The refractory selected depends upon the temperature inside the kiln and the chemical nature of the material being processed. In some processes, such as cement, the refractory life is prolonged by maintaining a coating of the processed material on the refractory surface. The thickness of the lining is generally in the range 80 to 300 mm. A typical refractory will be capable of maintaining a temperature drop of 1000°C or more between its hot and cold faces. The shell temperature needs to be maintained below around 350°C in order to protect the steel from damage, and continuous infrared scanners are used to give early warning of "hot-spots" indicative of refractory failure.

Tyres and RollersTyres, sometimes called riding rings, usually consist of a single annular steel casting, machined to a smooth cylindrical surface, which attach loosely to the kiln shell through a variety of "chair" arrangements. These require some ingenuity of design, since the tyre must fit the shell snugly, but also allow thermal movement. The tyre rides on pairs of steel rollers, also machined to a smooth cylindrical surface, and set about half a kiln-diameter apart. The rollers must support the kiln, and allow rotation that is as nearly frictionless as possible. A well-engineered kiln, when the power is cut off, will swing pendulum-like many times before coming to rest. The mass of a typical 6 x 60 m kiln, including refractories and feed, is around 1100 tones, and would be carried on three tyres and sets of rollers, spaced along the length of the kiln. The longest kilns may have 8 sets of rollers, while very short kilns may have only two. Kilns usually rotate at 0.5 to 2 rpm, but sometimes as fast as 5 rpm. The Kilns of most modern cement plants are running at 4 to 5 rpm. The bearings of the rollers must be capable of withstanding the large static and live loads involved, and must be carefully protected from the heat of the kiln and the ingress of dust. In addition to support rollers, there are usually upper and lower "retaining (or thrust) rollers" bearing against the side of tyres, that prevent the kiln from slipping off the support rollers.

Drive GearThe kiln is usually turned by means of a single Girth Gear surrounding a cooler part of the kiln tube, but sometimes it is turned by driven rollers. The gear is connected through a gear train to a variable-speed electric motor. This must have high starting torque in order to start the kiln with a large eccentric load. A 6 x 60 m kiln requires around 800 kW to turn at 3 rpm. The speed of material flow through the kiln is proportional to rotation speed, and so a variable speed drive is needed in order to control this. When driving through rollers, hydraulic drives may be used. These have the advantage of developing extremely high torque. In many processes, it is dangerous to allow a hot kiln to stand still if the drive power fails. Temperature differences between the top and bottom of the kiln may cause the kiln to warp, and refractory is damaged. It is therefore normal to provide an auxiliary drive for use during power cuts. This may be a small electric motor with an independent power supply, or a diesel engine. This turns the kiln very slowly, but enough to prevent damage.

Internal Heat ExchangersHeat exchange in a rotary kiln may be by conduction, convection and radiation, in descending order of efficiency. In low-temperature processes, and in the cooler parts of long kilns lacking preheater, the kiln is often furnished with internal heat exchangers to encourage heat exchange between the gas and the feed. These may consist of scoops or "lifters" that cascade thefeed through the gas stream, or may be metallic inserts that heat up in the upper part of the kiln, and impart the heat to the feed as they dip below the feed surface as the kiln rotates. The latter are favored where lifters would cause excessive dust pick-up. The most common heat exchanger consists of chains hanging in curtains across the gas stream.

COOLERS:Achieve efficient cooling and heat regeneration.

The aim of any cooling system is to cool the clinker as quickly as possible, to set and maximize C3S content. Simultaneously, it is important to capture and re-use the heat liberated by this process.In the grate cooler, the clinker enters at a temperature of around 1,200°C, rapidly spreading its heat into the grate and surrounding refractories. High refractoriness, high abrasion resistance and resistance to thermal shock are necessary, especially at the clinker downfall and bull nose.Recycling heat from the clinker is important to economical, environmentally friendly cement production. In grate cooler kiln systems, the tertiary air duct helps to recover valuable energy.Refractories that will optimize your grate coolerThe cooler is effectively divided into two zones – the ‘hot’ zone and the ‘cold’ zone.In the ‘hot’ zone, from the clinker downfall to the bypass duct and partition wall, the goal is to rapidly liberate heat and route it back to the preheater via the tertiary duct. For dramatically extended refractory life and the fastest possible installation at the clinker downfall, bull nose, front sidewalls and roof, use Fire bolt refractory precast, which can be quickly bolted into place and just as quickly removed. For brick or monolithic alternatives, refractories should offer high refractoriness, abrasion-resistance and cold crushing strength. Bypass duct wear linings should be lined with Viking 450 bricks and Dens castcastable in order to withstand the abrasive, alkaline effects of the tertiary airstream.In the ‘cold’ zone, when clinker temperature has dropped below 800°C, it is most important to achieve a uniform temperature decline throughout the clinker body. Abrasion is the worst problem here. The optimal refractory alternative is a brick such as Viking 330, complemented by Dens cast 50 A QF castable.To achieve exceptionally long life in the partition wall separating the two zones, use Dens cast 50 A QF refractory precast.Refractories that will optimize your Tertiary air ductIn grate cooler kiln systems, the tertiary air duct helps to recover valuable energy.Air from the grate cooler is filled with highly abrasive clinker dust as well as residual alkali vapors. When it is travelling through the tertiary air duct at a velocity of 25-30 m/s and an initial temperature of about 1050°C, it wears down the lining - particularly at bends and dampers. Abrasion resistance and alkali resistance are both important in tertiary air duct refractories.The refractory solution for the tertiary air duct is straightforward:Alkali- and abrasion-resistant brick in straight sections, Viking 330.Alkali- and abrasion-resistant low-cement castable in curved sections, Dens cast 50 A QF.Refractories that will optimize your kiln hoodLow thermal conductivity and abrasion resistance are important for refractories at the kiln hood and burner lead-in. Waste fuels can make chemical resistance necessary too.An arched brick construction is the best refractory solution. We suggest the high-alumina Victor 60 RK brick.If hood design makes brick installation difficult, a good alternative is Dens cast 50 A QF castable (or, when waste fuels are a problem, Dens cast SIC 30 castable or Fire bolt precast). Anchored anchoring simplifies anchoring as well as subsequent wrecking.REFERENCE LINK:http://cement.hoganasbjuf.com/en/Process-stage/Coolers-TAD/Grate-coolerhttp://cement.hoganasbjuf.com/en/Process-stage/Coolers-TAD/Grate-coolerhttp://www.cementmachinery.com/product.asp?keyno=103