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Limestone – Its Processing and Application in Iron and Steel Industry

• satyendra
• July 7, 2017
• 1 Comment
• calcining, endothermic reaction, exothermic reaction, hydrated lime, hydration, lime, limestone, quicklime, slaking. kiln,

Limestone – Its Processing and Application in Iron and Steel Industry

Limestone is a naturally occurring and abundant sedimentary rock consisting of high levels of calcium carbonate (CaCO3) in the form of the mineral calcite. Some limestones may contain small percentage of magnesium carbonate (MgCO3). These limestones are known as dolomitic limestones.

Limestone is also a very important industrial mineral. Its chemical properties make it a valuable mineral for a wide range of industrial/manufacturing uses. Limestone is also one of the vital raw materials used in production of iron and steel.

Limestone, by definition, is a rock that contains at least 50 % of CaCO3 in the form of calcite by weight. There can be small particles of quartz (silica), feldspar (alumino-silicates), clay minerals, pyrite (iron sulphide), siderite (iron carbonate), and other minerals associated with the limestone. All limestones contain at least a few percent other materials. The Impurities in limestone can consists of silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3), sulphur (as sulphides or sulphates), phosphorus (P2O5), potash (K2O), and soda (Na2O). Silica and alumina are the main impurities of limestone. The limestone which is used in ironmaking is required to contain at least 85 % of calcium carbonate and a low percentage of alumina. Similarly limestone which is used for steelmaking is required to contain at least 92 % of calcium carbonate and a very low percent of impurities especially the silica percentage.

The main uses of limestone in iron and steel industry are (i) as a fluxing material, and (ii) other usage which consists of desulphurizing agent, coating of moulds of pig casting machine, neutralizing of acidic water, water treatment, waste water(effluent) treatment, flue gas treatment, and sludge and sewage treatment. It is also a component of synthetic slag. Limestone is normally used in three forms. These are (i) raw limestone which is also the natural form of limestone, (ii) calcined limestone or quicklime, or simply lime, and (iii) as hydrated lime. When limestone is used as a fluxing material then it is used as either raw limestone or as calcined limestone. Hydrated lime is generally produced from high calcium quicklime and contains 72 % to 74 % calcium oxide with 23 % to 24 % chemically combined water.

Processing of limestone

Limestone after its mining has to undergo several processing before it can be used in various processes. The basic processes in the production of limestone are (i) quarrying of raw limestone, (ii) preparing mined limestone for its use by crushing and sizing, (iii) calcining of raw limestone, (iv) processing the calcined limestone further by hydrating to produce hydrated lime if required for use, and (v) miscellaneous transfer, storage, and handling operations. All these processes may not be necessary to be present in every plant.

Basically three types of limestone products are used in iron and steel plants. They are (i) raw limestone products, (ii) calcined limestone or quicklime products, and (iii) hydrated lime products. The processing for limestone for these products is described below.

The first process takes place at limestone mines where the mined ore undergo crushing and screening for the separation of the different size fractions of the ore. In the iron and steel plant, in some of the places, raw limestone is further processed. Example is sinter plant where the raw limestone is crushed in hammer mills to – 3 mm size (generally in the range of 85 % to 90 %).

Calcination of limestone is a thermal treatment process for carrying out the thermal decomposition of the raw limestone and removal of LOI (loss on ignition) or carbon di-oxide (CO2) part of its composition. Calcination process consists of an endothermic reaction which is carried out in the solid state. High quality lime which is used in steelmaking generally contains around 90 % to 95 % CaO. Theoretically 56 kg of CaO is produced from 100 kg of CaCO3 during complete calcination. However, in actual practice it varies because of several factors. During the calcination of limestone, since CO2 is removed, the lime (i) is porous (ii) has higher surface area, (iii) has high reactivity, and (iv) is hygroscopic. Around 1.8 t of limestone is required to produce 1 t of quicklime.

Calcination of limestone, since an endothermic chemical reaction, requires considerable input of energy. When limestone is heated, the calcium carbonate is decomposed as per the following equation.

CaCO3 + around 42.5 kcal of heat = CaO + CO2

The calcination of limestone is a simple single stage firing process which is carried out in a shaft kiln or a rotary kiln. It consists of five process steps. These are (i) heat transfer from ambient to particle surface, (ii) heat conduction from surface to reaction front, (iii) chemical kinetics at front, (iv) diffusion of the CO2 through the porous oxide layer to surface, and (v) then transfer into ambient. The limestone is charged into the kiln and, while progressing through the kiln, is being decomposed or calcined. The decomposition of CaCO3 starts at 810 deg C.

The calcinations process can be explained using a partially decomposed piece of limestone, whose profiles of CO2 partial pressure and temperature are shown in Fig 1. The sample comprises a dense carbonate core surrounded by a porous layer. In the calcining kiln at a temperature Tgas heat is transferred by radiation and convection (symbolized by ‘h’) to the solid surface at a temperature of Tsurface. By means of thermal conduction (A) heat penetrates through oxide layer to reach the reaction front, where the temperature is Trc. As the reaction enthalpy is many times greater than the internal energy, the heat flowing further into the core is negligible during the reaction. Therefore the core temperature is only slightly lower than the front temperature. Once heat is supplied, the chemical reaction constant (k) then takes place for which the driving force is the deviation of CO2 partial pressure from the equilibrium (p-eq – p-f). The released CO2 diffuses (Dp) through the porous oxide layer to the surface and finally passes by convection (B) to the surroundings where the CO2 partial pressure p-surface exists.  The chemical and physical properties of lime are influenced by the calcination which in turn is influenced by the conductivity, mass transfer coefficient, and diffusivity of the lime layer.

Fig 1 Profile of sample of limestone during calcining

For the full calcination of limestone and to have no residual core of uncalcined limestone, it is necessary that the heat supplied to the surface of limestone must penetrate via conductive heat transfer to the core. A temperature of 900 deg C is to be reached in the core at least for a short period of time since the atmosphere inside the material is pure CO2. The limestone surface is to be heated to greater than 900 deg C to maintain the required temperature gradient and overcome the insulating effect of the calcined material in the limestone surface. However, when producing quicklime, the surface temperature must not exceed 1,100 deg C to 1,150 deg C as otherwise re –crystallization of the CaO occurs and results in lower reactivity and thus reduces the slaking properties of the quicklime.

A certain retention or residence time is required to transfer the heat from the combustion gases to the surface of the limestone and then from the surface to the core of the limestone. Larger pieces of limestone require longer time to calcinate than smaller pieces. In principal, calcining at higher temperatures reduces the retention time needed. However, too high temperatures adversely affect the reactivity of the product. The relation between calcining temperature and retention time needed for different sizes of limestone is shown in Tab 1.

Several different types of kilns are used for the calcination process. These kilns can be rotary kilns or shaft kilns. The type of the kiln to be selected strongly depend on the characteristics of the limestone, anticipated production rate, cost of fuel,  investment costs, available fuel, local conditions, infrastructure and other things. In general, all data including laboratory tests are to be evaluated prior to selecting the calcining kiln.

Rotary kilns, with or without preheaters, usually process limestone with material size between 10 mm and 50 mm. The heat balance of this type of kilns is categorized somewhat by the high losses with the off-gases and through the kiln shell. Typical values for the off-gas losses are in the range of around 25 % and for the kiln shell losses are in the range of around 20 % of the total heat requirement. Only around 60 % of the fuel energy introduced into the kiln with preheater is used for the process of calcining.

In case of vertical single shaft kilns, there exists an imbalance between the heat available from the calcining zone and heat required in the preheating zone. Even with the ideal calcination process the temperature of the waste gases may be higher than 100 deg C. In case of parallel flow regenerative (PFR) type of kilns, there is better utilization of the heat of the calcining zone and minimization of the loss of heat in the waste gases, resulting into lower heat consumption per ton of lime.

Schematic diagram of a vertical single shaft kiln showing material flow and gas flow, main components of a rotary lime kiln, and mechanism of heat transfer in a rotary kiln are shown in Fig 2.

Fig 2 Schematic diagram of a vertical single shaft kiln, components of rotary kiln and heat transfer mechanism in a rotary kiln

Comparison of data for different kilns typically used for limestone calcination with important consumption figures and typical raw material size is given in Tab 2.

The following are the factors and operating parameters which have a bearing on the calcination of limestone.

Types of kilns – There are two types of kilns which are used for calcining of limestone. They are either vertical shaft kilns or horizontal rotary kilns. Depending on the type of kiln, the size of limestone charge is different. In vertical kilns, the limestone moves downward, and the hot gases flows upward through the limestone, therefore the stones must be large enough to provide passage for combustion gases to move upward. These kilns usually use limestone sizes ranging from 130 mm to 200 mm. In these types of kiln, the temperature rise must be slow and therefore the resultant residence time is long. Vertical kilns are fuel-efficient, but limited in capacity. In case of horizontal kilns, the kiln body rotates, allowing the limestone to tumble and exposing all of the surfaces to hot gas. Typical size of limestone for these types of kilns ranges from 25 mm to 40 mm. Uniformity of limestone size for charging the kiln is very important for a uniform calcining process. But, from a practical point of view, tight sizing is expensive due to the multiple screening required. Small size limestone such as 6 mm and smaller with a certain percentage of fine in a horizontal kiln, tends to flop over in mass, thus reducing the exposure of particles to hot gases. This process results in uneven exposure, thus reducing the quality of the quicklime. In vertical kilns, the presence of very small size of limestone block the voids between limestone, interfering with gas passage, and thus with the heat transfer, causing uneven calcination. In addition, small limestone particles (less than 3 mm) tend to degrade and cause generation of fines which need to be removed by dust collectors.

Size and gradation of limestone -During the process of calcination, dissociation of limestone normally progress gradually from the surface into the inside of the limestone. The larger is the size of the limestone, the more difficult it is to calcine and it also requires more time. A wide range of particle size distribution in kiln feed also interferes with heat distribution in the kiln. The small stones accumulate between the voids formed by large stones in shaft kilns, thus blocking the draft and the flow of combustion flame and gases. Also, during the calcining of a wide range of limestone sizes, temperatures which calcine the smaller sizes adequately without over calcining, only calcine the outer shell of the limestone with larger dimensions. Hence, limestones with restricted gradations, independent of the size, are much easier to calcine. The size of the limestone is also critical element in the process of calcination. As the limestone enters the kiln, it is exposed to the hot gases within the kiln. The rate of heat penetration is dependent on the temperature of the limestone and that of the surrounding gases. In addition, it takes time for heat to penetrate the limestone. The smaller is the size of the limestone, the shorter is the time of heat penetration. In the case of pulverized limestone, this time can be a fraction of a minute.

Crystalline structure and density of limestone – Crystalline structure of limestone affects the rate of calcination, internal strength of limestone, as well as resultant crystal size of CaO in the lime. The smaller crystals agglomerate during calcination, forming larger crystals, thus causing shrinkage and volume reduction. Higher kiln temperatures assist in the process of agglomeration. The more is the agglomeration, the more is the shrinkage of volume of the final product. The density of limestone and its crystal structure are somewhat related. The shape of crystals determines the void space between crystals, and thus, the density of the limestone. Larger voids allow easy passage for CO2 gases during calcination, but it also results in a reduction of volume during calcination. Some types of limestone, due to their crystalline structure, fall apart in the calcination process. These types of limestone are not suitable for calcining. There are some other types of limestone which behave the opposite way and become so dense during calcination that they prevent the escape of CO2 and become non-porous. Again, these types of limestone are not suitable for calcination.

Calcination temperature – The calcination of limestone starts at around 810 deg C at atmospheric pressure and in an atmosphere consisting of 100 % CO2. Dissociation proceeds progressively from the outside surface towards inner surface. For dissociation to penetrate the interior of the limestone, higher temperatures are necessary. The temperatures require further increase so that dissociation of the core of the limestone can take place. The higher the size of the limestone, the higher the temperature which  is needed for the dissociation of the core because of the increasing internal pressure as the CO2 gas forces its escape. An increase in temperature exerts a much greater influence on the dissociation rate than the temperature retention. Further, the theoretical temperature required for calcination is around 1,000 deg C. However, in practice, the temperatures maintained in the kiln are much higher (around 1350 deg C). The correct temperature in the kiln depends on the limestone size as well as type of kiln and type of fuel used. The kiln operator normally experiments to determine the exact temperature for the particular size and quality of limestone being used. In general, it is better to use the lowest temperature with the shortest possible residence time to achieve full calcination. Higher calcination temperatures cause increased shrinkage and reduction in volume. Higher calcination temperatures  also cause carbonation of the surface of calcined limestone, with the presence of CO2 released from the stone as well as by product of combustion, which makes the lime non-porous, and thus lesser reactive.

Rate of temperature rise – The temperature rise must be gradual and even. It is particularly important when using large size limestone (100 mm to 150 mm). When calcining large size limestone, the limestone is required to remain porous during the process. As the temperature rises, the outer layer of limestone is heated to disassociation temperature, where CO2 escapes the limestone, leaving capillary passages making the lime porous. As the gas escapes, the limestone shrinks in volume by as much as 40 %. This shrinkage in volume restricts the passage of gas from the centre of the limestone, preventing any additional CO2 gas from escaping. Too long of a residence time facilitates combining of the CaO and CO2 back to CaCO3 (carbonation) at temperatures above 1350 deg C. If the temperature rise is very fast, the outer layer of the limestone pieces are calcined very fast. As the temperature rises, the surface of limestone pieces shrinks and the pores created by the escape of CO2 are closed. This produces increased internal pressure within the limestone. Since the gas cannot escape, it causes the limestone to explode and disintegrate. This results into production of unwanted fines reducing the quality of the resultant quicklime.

Time of calcination – It is noticed that irrespective of the type of the limestone and its quality, a higher burning temperature and longer calcination period yields a harder-burned quicklime which has high shrinkage, high density, low porosity and low chemical reactivity. The opposite takes place at lower burning temperatures with shorter duration of calcination resulting into production of the desirable, soft-burned highly reactive limes of low shrinkage and density and high porosity. The calcination of small and large stones in terms of their relative heating and calcination times is directly proportional to the square of the thickness (or average diameter for irregularly shaped stone. Retention time in the kiln depends on the size of the limestone as well as calcination temperature. The size of the limestone is the most critical element for the calcination. As the limestone enters the kiln, it is exposed to the hot gasses within the kiln. The rate of heat penetration is based on difference in temperature of the limestone and the temperature of the gasses. In addition to this difference in temperature, it takes time for heat to penetrate the limestone. The smaller is the size of the limestone, the shorter the time for heat penetration. In case of pulverized limestone, this time can reduce to less than 1 minute. If the retention is too short, the core of the limestone remains uncalcined, while the outside surface gets calcined. If the retention time is too long, then the surface of the limestone pieces shrink and the pores created by CO2 gas escape close, producing an impervious surface. This type of lime is called hard burned or dead burned lime. This lime has very low reactivity and does not slake well. In addition, longer retention time means less production and higher costs of production.

CO2 concentration in kiln – CO2 concentration in the kiln atmosphere increases with its release from limestone during calcination. For proper calcination, the CO2 is to be removed on a continuous basis. If CO2 is not removed then a combination of high CO2 concentration and high calcination temperature causes carbonation of the calcined limestone pieces and convert CaO back to CaCO3. In addition, the CO2 also react with the limestone impurities.

Chemical reactivity – There exist an interrelation between porosity, density and pore size distribution. These factors exert a major influence on such standard measurable properties of quicklime such as reactivity, available lime and the particle size distribution and the surface area. It has been noticed that a retention time of 1 hour to 4 hours has very little or no effect on the porosity, surface area, or reactivity in the calcination temperature range of 950 deg C to 1070 deg C and the bulk density of quicklime remain constant. Excessive calcination temperatures and prolonged periods of calcinations lead to hard burning of limestone and this results into the production of lime with low reactivity.

Shrinkage characteristic – The shrinkage of quicklime can be calculated from the densities of the limestone and the lime after allowing for the loss on ignition (LOI). It is calculated by the formula S = 100*{[100/Ds-(100-L)/Dl]/100*Ds} Where S is shrinkage in percentage, Ds is the density of limestone in grams/cc, Dl is the density of quicklime in grams/cc and L is loss of ignition of limestone in percentage. Shrinkage of the limestone has major influence on the bulk density of the limestone charge in the kiln. The higher is the shrinkage the lesser are the voids in the limestone charge. This results into the packing of the limestone in the kiln which leads to high pressure drop in the kiln bed with the attendant influence on the draft of flue gases from the kiln.

Quality and type of fuel – The quality and type of fuel has major influence on the efficiency of the kiln and the quality of lime produced. Solid crude fuels such as wood, charcoal and coal are being used since very early days. Pulverized coal, producer gas, natural gas and fuel oil are being used in the kilns. Natural gas is the most convenient fuel and producer gas is the most troublesome. The final choice of fuel is also determined by environmental considerations since some fuels have higher tendencies to pollute the environment through harmful emissions. Typically, vertical kilns use oil or natural gas for fuel, whereas horizontal rotary kilns use coal. However, either type of kiln can use any of these fuels. Both oil and coal contain certain percentages of sulphur compounds which can vary from 0.5 % to 3 %. Sulphur usually combines with CaO at appropriate temperatures and produce calcium sulphide or calcium sulphate. This generally happens on the surface of calcined material and makes the material non porous, thus reducing its reactivity. In addition, a high percentage of ash in the coal results in the build-up on the refractories in the rotary kiln, thus interfering with the flow of limestone charge in the kiln. The kiln must be periodically cooled and the ash build up removed manually which is a very tedious and costly operation. Natural gas is the cleanest fuel and mostly used in vertical kilns.

The third type of processing of limestone consists of the production of hydrated lime, which is a dry powder obtained by treating quicklime with sufficient water to satisfy its chemical affinity for water, thereby converting the oxides to hydroxides. Hydrated lime is also sometimes called slaked lime. For the flue gas desulphurization, the characteristic of hydrated lime requires enhanced average fineness, higher surface area, and higher volume of the pores. The hydration reaction is chemically simple but it is strongly exothermic, with a heat generation of around 276 kcal/kg of CaO. The reaction is given below.

CaO + H2O = Ca(OH)2 + heat

For the sake of comparison, the exothermic heat evolved from the hydrating of 1,000 tons of high calcium quicklime is equal to the total heat value of around 35 tons of coal. The terms hydration and slaking are used quite often interchangeably. However, there is a definite and distinct difference. Hydration is normally defined as a process whereby around stoichiometric amounts of water and lime react to form a product, hydrate, which is a dry powder. It contains less than 1 % of free moisture and is handled as a powder. Slaking on the other hand is defined as a process whereby lime is reacted with an excess amount of water to form a lime slurry which is handled as a liquid.

Rapid hydration is prone to produce the finer particles, since hydrate crystals are given lesser chances to agglomerate. However, the most rapid reaction is not necessarily the best condition. In principle a typical high reactive lime reaction develops in three different phases (Fig 3). These phases are termed as (i) kinetic, (ii) transitory, and (iii) diffusion. The kinetic phase is normally very short (less than 10 seconds), and shows a sharp temperature increase, which can be as high as 50 % of the total increase in the temperature. The length of the transitory phase (often less than one minute) can change due to the size of the lime lumps feed to the hydrator. It shows a visible bend of the temperature increase. In the diffusion phase, the temperature again rises sharply until it flattens quickly to show the end of the reaction.

Fig 3 Hydration reaction phases and hydration mechanism

Hydration mechanism is also shown in Fig 3. The hydration mechanism of lime particles shows that after the initial contact with water the reaction starts very strongly in a few seconds due to the unobstructed contact between lime and water. After the first layer of partly hydrated lime is generated on the surface, it acts as a shield to the quick lime layers underneath since it tends to remain near the particles surface. Hence, the layer of partly hydrated lime delays the water penetration. When Ca(OH)2 crystals are gradually formed to their final shape, they start to separate. This improves water penetration which resumes the reaction trend.  The other factor which is important is that the development of the reaction is the function of the lump size of the quicklime.

Application of limestone and lime in iron and steel plant

During the production of iron by blast furnace (BF) route, limestone is added either in the process of sintering or as a direct feed in the blast furnace. Limestone is normally added during ironmaking for obtaining either neutral or slight basic BF slag. Addition of limestone through sinter is more preferred route since CO2 of limestone is driven out during the sintering process. In case of sintering, limestone is crushed to -3 mm (in the range of 85 % to 90 %) in hammer crushers before mixing it in the sinter mix. In case of direct feed to blast furnace, limestone lumps of 10 mm to 40 mm size are used.

In case of limestone addition through sintering, these days calcined limestone (lime) is also directly being used. Use of quicklime in the sintering process has the advantages of (i) Improvement in the binding characteristics of sinter mix, (ii) improvement in the productivity of sintering machine, (iii) improvement in the sinter strength, and (iv) reduction in the volume of exhaust gases with associated advantages.

High silica (SiO2) content in the limestone used for sintering purpose is preferred since SiO2 counter the effect of alumina (Al2O3) in the blast furnace.

The second major use of limestone is in steelmaking. It is being used for maintaining slag basicity (CaO/SiO2) of around 3. In steelmaking, limestone is used in calcined form. For use in steelmaking, the SiO2 (because of its acidic nature) content in the limestone is to be very low preferably less than 1 %. Also, the reactivity of the lime is to be very good, because of lesser time available since process of steelmaking is a very fast process. The entire steelmaking process takes less than 20 minutes to complete.

Minor uses of lime (either calcined or hydrated) consists of desulphurizing agent, coating of moulds of pig casting machine, neutralizing of acidic water, water treatment, waste water(effluent) treatment, flue gas treatment, and sludge and sewage treatment etc. Lime is also sometime added as a component of synthetic slag.