To conserve natural resources and comply with responsible care initiatives, the soda ash industry has concentrated on optimal and effective utilization of lime in the ammonia recovery section of the soda ash process.  The soda ash industry consumes around 1.39 to 1.5 tons of limestone per tons of soda ash.  Therefore, it is desired to evaluate the business proposition right from the mining stage to the ammonia recovery section of the process.
 
Apart from the lime utilization in the ammonia soda process, high quality lime will reduce the suspended solids level in liquid effluent. 

General

Limestone as a naturally occurring mineral exists nearly all over the world including in Saurastra in Gujarat. The chemical composition of limestone varies greatly from region to region as well as between different deposits in the same region. Therefore, the end product from each natural deposit is different. For a stone to be classified as limestone suitable for calcium processing, it should contain a minimum of 90% calcium carbonate.
In general, all limestone contains a mixture of minerals such as CaCO₃, MgCO₃, CaO, Iron, Silica, Alumina and other trace components. To review the effect of all of these constituents in the conversion of limestone to Quicklime (CaO) is beyond the scope of this article; however, we will concentrate on the main mineral – Calcium carbonate (CaCO₃).
 
Natural Limestone

Impurities

As discussed above, impurities in the limestone would affect the quality of the final CaO. Typical limestone is composed of the following minerals:
·         Calcium carbonate
·         Magnesium carbonate
·         Silica
·         Alumina
·         Iron
·         Sulfur and other trace minerals
 
Of the above minerals, only calcium carbonate and magnesium carbonate are of interest. These two minerals constitute 89% to 94% weight of the total composition of limestone. There are two basic types of lime produced from these limestones: calcium lime and magnesium lime. The high calcium limestone, when calcined, will have between 90% and 95% CaO and 1% to 2% MgO.

Crystalline structure of stone

Crystalline structure affects the rate of calcination, internal strength of limestone, as well as resultant CaO crystal size. The smaller crystals coagulate during calcination, forming larger crystals, thus causing shrinkage and volume reduction. The higher the kiln temperature, the more coagulation.  The more coagulation, the more shrinkage of volume.


Density of limestone and crystalline structure

The density of limestone and its crystal structure are somewhat related.The shape of the crystals determines the void space between crystals, and thus, the density of the limestone.  Larger voids will allow easy passage for CO₂ gases during calcination, but they also result in a reduction of volume during calcination. Some limestone, due to its crystalline structure, will fall apart in the calcination process. This type of limestone is not of any value for calcining. Other limestone will act the opposite and become so dense during calcination that they will prevent the escape of CO₂ and become non-porous. Again, this type of limestone is not suitable for calcination.
 
Calcination process

Chemical composition of limestone

The chemical composition of the limestone cannot be controlled without a major cost impact on manufacturing, therefore, variation is generally accepted. The calcination temperature should be controlled very closely. To heat the limestone in the kiln uniformly, the particle size of the feed must be relatively uniform. In addition, to avoid long residence time in the kiln, the particle size of the limestone must be small, typically about 1.5 inches. However, due to the nature of the crushing operation, there is a range in size from 0.5 to 2 inches. Since the residence time and temperature in the kiln is constant, the heat penetration in the particles of limestone is different due to variation of the size of the limestone.  A larger size of stone heat does not quite penetrate to the core; therefore the center of these pieces remains as calcium carbonate while the outside is converted to CaO. These center cores are referred to as grit. For medium size stones, the heat penetration is complete and the entire stone is converted to CaO.  For the smaller stones, the heat reaches the core rapidly and the outside layer is overheated forming a hard outer shell where water cannot penetrate, therefore, the slaking process is greatly retarded or prevented.  Here, the large and medium size particles are highly reactive, soft burned, quicklime and the smaller particles are called hard-burned quicklime.

Kiln temperature

The theoretical temperature required for calcination is about 900°C; however, in practice, we find this temperature to be much higher about, at 1,350°C. To determine the correct temperature in the kiln is an art rather than a science, and it depends on the limestone size as well as type of kiln and type of fuel used. The kiln operator must experiment to determine the exact temperature for the particular size limestone that is being used. In general, it is best to use the lowest temperature with the shortest possible residence time to achieve full calcination. Higher calcination temperature will cause increased shrinkage and reduction in volume. Higher temperature also will cause recarbonation of the surface of CaO pebbles with the presence of CO₂, which makes the lime non-porous, and thus unsuitable for hydration.

Kiln temperature affects the quality of CaO produced and the resultant hydroxide produced is from slaking this CaO. Very small particle sizes with large specific surfaces are the most desirable end product from calcium oxide.
A soft-burned lime pebble is full of small hair-like cracks where CO₂ has escaped from the limestone during the calcination process. When this lime is exposed to water the water penetrates the cracks in the quicklime pebbles and fills these cavities. The hydration takes place quickly, releasing a lot of heat energy. This heat will boil off the water and generate steam, which makes the particles burst, exposing the inner surfaces to water for further slaking. This process will continue until hydration is complete.

Rate of temperature rise

The temperature rise must be gradual and even. It is particularly important when using larger size limestone (10 to 15 cm or 4 to 6 in.).  When calcining this size limestone, the limestone must remain porous during the process. As the temperature rises, the outer layer of limestone is heated to disassociation temperature, where CO₂ escapes the stone, 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 center of the limestone, preventing any additional CO₂ gas from escaping.  Too long of a residence time will combine the CaO and CO₂ back to CaCO₃ (recarbonation) at temperatures above 1350°C.  A good practical size for limestone in VSK kilns is 5 to 10 cm (2 to 4 in.).  This size will allow for quick heating, short residence time and a minimum amount of cores that create grit.  In conclusion, the smaller size limestone (4 to 5 cm or 1.5 to 2 in.) is most suitable for calcination in rotary kilns and will allow optimum residence time. This lower calcining temperature will also allow less fuel consumption. However, larger size limestone and low calcining temperature are needed for vertical single-shaft and multi-shaft 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 pebbles will shrink, closing the pores created by the escape of CO₂.  This produces increased internal pressure within the limestone. Since the gas cannot escape, the limestone will explode and disintegrate, producing unwanted fines, reducing the quality of the resultant calcium oxide.

Retention in the kiln

Retention time depends on the size of the limestone as well as calcination temperature. The size of the limestone is the most critical element in 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 DelT (temperature of stone vs. temperature of gasses). In addition to DelT, it takes time for heat to penetrate the limestone. The smaller the stone, the shorter the time for heat penetration. In the case of pulverized limestone, this time may be reduced to less than one minute.  If the retention is too short, the core of the limestone will remain calcium carbonate while the outside will convert to calcium oxide. If the retention time is too long, the surface of the pebbles will shrink and the pores created by CO2 gas escape will close, producing an impervious surface. This type of limestone is called “hard burned” or “dead burned” lime. This lime will not slake in standard slakers. In addition, longer retention time means less production and higher costs for manufacturing.

The residence time of CaO in the kiln is very critical during the calcination process. It is important that the resident time be as short as possible.  However, enough time must be allowed for heat to penetrate the particles of CaO and drive the CO2 out of these particles. Calcination is done either with low temperature and high residence time, or high temperature and low residence time. Each lime manufacturer must balance the time of residence and temperature to suit their system.

CO₂ concentration in kiln

As CO₂ is released from limestone during calcination, the concentration of CO₂ in the kiln atmosphere is increased. For proper calcination, the CO₂ must be vented on a continuous basis. If CO₂ is not vented, a combination of high CO₂ concentration and high calcination temperature will recarbonate the lime on the surface of the pebbles and convert CaO back to CaCO₃. In addition, the CO₂ and CO will react with the limestone impurities that are part of the limestone inerts (like silicate, alumina, and irons).  The concentration of CO₂ is around 38-40% V/V for vertical shaft kilns. The concentration of CO₂ affects the conversion efficiency in the carbinating tower of the ammonia soda process. It is desirable to feed the high concentrated CO₂ in carbonating tower for crude bicarbonate production.

Physical size of limestone/types of kilns

Depending on the type of kiln, either vertical or horizontal rotary, the size of stone charge is different.
On vertical kilns, the limestone moves downward, and the hot gases flow upward through the limestone, therefore the stones must be large enough to provide cavities for combustion gases to move upward.  These kilns usually use limestone sizes between 2 to 4 in.  In this type of kiln, the temperature rise must be slow and therefore the resultant residence time, high. Typically, vertical kilns are operated at 900 to 1000° C. (Note: The temperature range listed here is an average range, and great variations exist in the industry.) Vertical kilns are fuel-efficient, but limited in capacity.
 
Type of fuel used

Most calcining is done by the use of oil, coal or natural gas for fuel.  Typically, vertical kilns use oil or natural gas, hard coke, pet coke for fuel, while horizontal kilns use coal.  However, either type of kiln can use any of these fuels. Coal is generally pulverized and blown into the combustion chamber.

Both oil and coal contain certain percentages of sulfur or sulfur compounds. These vary from 0.5% to 3%. Sulfur will combine with CaO at proper temperatures and produce calcium sulfide or calcium sulfate. This generally happens on the surface of CaO pebbles and renders them non-porous, thus not suitable for slaking. In addition, a high percentage of ash in the coal will result in buildup on the refractories in the kiln, thus interfering with the flow of limestone in the kiln. The kiln must be periodically cooled and the ash build-up removed manually, which is a very costly operation.
Natural gas is the cleanest fuel and mostly used in vertical kilns. To calcine limestone for food-grade lime, natural gas is the fuel of choice. Coal-fired kilns are the cost-effective way to produce the lime for soda ash.

Pre-heating and cooling

Limestone calcination is very energy-intensive and consumes a considerable amount of fuel. Most of the energy waste comes from dumping the kiln gases. To improve the fuel consumption efficiency, the industry has devised the following processes:

·   The hot exhaust gasses are used to preheat the limestone before entering the kiln. This not only recovers substantial heat from the exhaust gases, but will also reduce residence time in the kiln, reducing the size of the kiln.

·   When limestone has been calcined and exits the kiln, it is red hot, about 1200°C. This represents a substantial source of heat. To recover part of this heat, the combustion fresh air is used to cool the quicklime. The resulting heated air is then fed into the kiln. This heated air improves the fuel consumption efficiency by the recovery of part of the wasted heat.

·   Calcining of limestone is done on a continuous basis, thus avoiding heating and cooling of calciner. This continuous operation reduces fuel consumption and minimizes degradation of the kiln’s refractory lining.
 
Kiln atmosphere

In addition to kiln temperature and residence time, kiln atmosphere affects the quality of CaO. As the temperature of the CaCO₃ increases, the CO₂ gas is released and CaO is produced. The CO₂ must be vented out of the kiln. The CaO has an affinity to absorb moisture and CO₂, reverting back to CaCO₃.  The effect of this conversion is more pronounced with small particles of CaO versus larger pebbles due to the specific surface of the pebbles. There is a great difference between the reactivity of these limes, the rate of temperature rise, and the time required to complete the slaking process.
 
Factors affecting the slaking process

The most important single factor that affects the process efficiency of a slaking system is the specific surface area of the particles of calcium hydroxide. The larger the specific surface area of the hydrate, the more surface is available for reaction, therefore, the more efficient the reaction and the lower the consumption of lime. The specific surface of calcium hydroxide varies a great deal based upon variables that are described below. Typical specific surfaces of calcium hydroxide range between 8,000 to 58,000 Cm2/gr. Empirical data indicates that the relationship between the particle size of the hydrate and the specific surface, even though related, is not linear.

Type of limestone

Calcium carbonate deposits are generally not pure. They contain many other elements, such as magnesium, aluminum, and compounds that affect the quality of hydrate produced from their limestone. Manufacturers of lime have no control over the impurities that are interspersed in a vein of limestone.

Calcination process

Proper temperature and residence time during calcination have a great deal of influence on the quality of hydroxide produced. The most common problem associated with the calcination process is hard-burned lime. When a lime is hard-burned, an impervious layer forms on the outside of the CaO making it difficult for water to penetrate to start the slaking process. To slake a hard-burned lime, the outer layer of the particle must wear off to open up the pores for water to penetrate. This is done by vigorous agitation that abrades the outer layer of CaO. This type of lime generally requires more retention time in the slaker. In practice, using hard-burned lime, the slaker capacity must be derated by 50% to minimize CaO carry over.

It should be noted that when using slaker grit separators that rely on gravity for settling the grit, some of the hard-burned lime which did not slake will float in the grit separation chamber due to air entrapment inside the CaO pores. These particles will end up in the process, which may cause blockage of lines, sprayers, atomizers, etc.
 
Slacking temperature

Slaking temperature is the most important factor that affects particle size and specific surface of hydrate particles. The closer the slaking temperature is to 210°F, the finer the particle sizes and the greater the specific surface of the particles. However, the relationship between temperature and particle size and specific surface is not linear.

In some instances when slaking at high temperatures around the boiling point of water, hot spots can develop within the slurry, which will cause hydrate particles to crystallize and agglomerate forming larger flat particles with reduced specific surface. This problem is more likely to happen in paste slakers, since they operate at higher temperatures, and in the areas where mixing is not vigorous.

Even though from a theoretical point of view temperatures around 212°F is desirable, from a practical point of view it is very difficult to slake successfully at these high temperatures without safety problems or adverse affects due to agglomeration.
In practice, slaking temperatures between 160°F to 185°F are more practical for optimal operation. The heat released due to exothermic reaction is different for different quality limes. A high reactive soft-burned lime will produce 490 BTUs of heat per pound of quicklime.   A low reactive lime will produce about 380 BTUs per pound of quicklime.

These BTUs will bring the slurry temperature to a certain degree based on temperature of the dry lime, temperature of incoming water, and heat losses from the slaker vessel.

As stated before, the optimum temperature for slaking varies from job to job, depending on equipment and site conditions. Since temperature is the most important factor affecting specific surface, temperature control is essential for a uniform quality product. Controlling a slaking process by lime-to-water ratio or slurry consistency is not the best way because of variables such as lime reactivity, incoming water and lime temperature, which result in variation in hydrate quality. The optimum way to control a slaking process is by controlling slaking temperature by varying the lime-to-water ratio as necessary.

Lime-to-water ratio

The lime-to-water ratio also affects slaking time by affecting the slaking temperature. The higher the temperature, the shorter the slaking time. Controlling a constant lime-to-water ratio in a slaking process does not guarantee a constant temperature. Temperature will vary due to variation in water temperature, lime reactivity, and quality of water, thus requiring frequent operator adjustment. As stated before, a better way to maintain a correct lime-to-water ratio is to control the slaking temperature. Slaking tests performed on the same lime with different lime-to-water ratios showed a significant difference in settling rate. In both cases, the samples were allowed to settle to 50% of their volume. Lime-to-water ratio settling time to 50% of volume is minimal.  This clearly indicates that an excess amount of water used in slaking will result in smaller particles, assuming that slaking temperature was the same. 

Degree of agitation

Degree of agitation during the slaking process has an impact on the end product. Too little agitation will result in uneven temperature within the slaking chamber, resulting in hot and cold spots. The hot spots will result when slaking temperatures are greater than 212°F. Slaking at these temperatures will result in large hexagonal crystals with reduced surface area, an agglomeration of particles, and cold spots will result in either drowning or unhydrated particles of CaO.

Viscosity of slurry

The viscosity of hydroxide slurry can vary greatly from lime to lime as well as different process conditions. Certain changes in the hydration conditions or impurities in the lime will increase the viscosity of the slurry, thus affecting settling time.  Often times the viscosity increases at slaking temperatures of 180°F and above. The relationship of viscosity size, particle size, specific surface, and settling rate has not been completely researched. In general, it is presumed that higher viscosity means smaller particle size of hydrate and greater specific surface and settling rate. Variations of viscosity of hydrated lime slurry have been reported between a range of 45 to 700 centipoises.

Slaking time

Slaking time is the time required to complete hydration. This time varies from lime to lime. A high reactive lime will hydrate completely in 2 to 3 minutes.  Medium reactive limes will hydrate completely in 5 to 10 minutes. Low reactive limes, hard-burned limes, and magnesium limes will hydrate in 15 to 30 minutes. The field results vary greatly depending on the field conditions.

Water chemistry

Water chemistry is a major factor in the slaking process. Presence of certain chemicals in the slaking water will accelerate or hinder the slaking process.  Water with high dissolved solids generally causes excessive foaming, which results in operational problems. Waters containing more than 500 mg/l of sulfates or sulfites are unsuitable for slaking. This is true for paste- and slurry-type lime slakers. Ball mill slakers, because of their ability to grind the particles of lime, are not affected as much by the presence of sulfates or sulfites in the slaking water.

Seawater can effectively be used for slaking. However, the material of construction for the slaker must take into consideration corrosion caused by chlorides.

Water temperature

Slaking water temperature has a great influence on the slaking process and specific surface of the hydrate particles. The incoming water temperature and the lime-to-water ratio inversely affect the slaking time.

Cool slaking water should not contact the dry lime in the slaker. The water and lime must enter the slaker apart from each other so that by the time water comes in contact with the lime its temperature is raised to more than 150°F. If cool water and lime come in contact, a condition called “drowning” takes place. Particles of hydrate formed under drowning conditions are very coarse and not very reactive.
 
Lime slaking and factors that affect the process

Since lime slaking is an integral part of treatment systems in water, wastewater, air pollution, and process industries, its performance will influence the overall effectiveness of the process as well as operation costs.

(Limestone) CaCO₃ + HEAT-------> (Calcium Oxide) CaO + CO₂
However, CaO is unstable in the presence of moisture and CO₂. A more stable form of lime is calcium hydroxide Ca(OH)₂.
(Calcium Oxide) CaO + (Water) H₂O -----> Calcium Hydroxide Ca(OH)₂ + HEAT 3
The atomic weight of the above formula is:
Ca= 40 O= 16 H= 1
(40+16) + (2+16) = 74
56 + 18 = 74
 
Therefore, 56 units of CaO plus 18 units of H₂O results in 74 units of Ca(OH)₂. The ratio of hydroxide to CaO is 74/56= 1.32. This means that 1 kg of CaO and 0.32 kg of water will produce 1.32 kg of Ca(OH)₂, the minimum water required for chemical reaction, so calcium hydroxide contains 75.7% CaO and 24.3% H₂O. The process of adding water to calcium oxide to produce calcium hydroxide is referred to as hydration process or lime slaking. The hydration of CaO, commercially referred to as quick lime, is an exothermic process releasing a great quantity of heat.  This hydration process when done with just the right amount of water is called “dry hydration.” In this case the hydrate material is a dry powder. If excess water is used for hydration, the process is called “slaking.” In this case the resultant hydrate is in a slurry form. Lime manufacturers generally use the dry hydration process for producing powdered hydrated lime. The slaking process is normally done with considerable excess water ranging from 2.5 parts water to 1 part CaO to 6 parts water to 1 part CaO.
 
Equipment used for the slaking process

There are basically two types of lime slakers available on the market. They are:
·   Slurry Detention Slakers
·   Ball Mill Slakers
 
A slaker must mix the correct amount of quick lime (CaO) and water, hydrate the quicklime, and separate the impurities and grit from resultant calcium hydroxide slurry.

Slurry slakers

A slurry slaker generally uses an initial lime-to-water ratio of 1 to 3.3 to 1 to 5 depending on the make of equipment and quality of CaO and water. Typically a slurry slaker, sometimes called a detention type slaker, is composed of two chambers.  The first chamber is called the slaking chamber where lime and water are mixed. The second chamber is usually used as a grit removal chamber. The lime slurry flows by gravity from the first chamber to the grit chamber. The slurry viscosity is reduced in the second chamber by the addition of cold water to allow the heavier grit to settle to the bottom of the second chamber where the grit is elevated and discharged by a screw.
The slurry slakers are generally designed for a retention time of 10 minutes at full rated capacity. This means that a particle of CaO from the time it enters the slaker until it exits into the grit remover takes an average of 10 minutes.

Ball mill slakers

Ball mill slakers are an adaptation of ball mills, which originally were designed for wet and dry grinding, to the job of slaking. Two types of ball mills are used for slaking, horizontal and vertical. Ball mill slakers are generally used where:

·         Capacity required is too large for other types of slakers.
·         Due to zero discharge conditions at the site no grit discharge is allowed.
·         When water available is too high in sulfates or sulfites for regular slakers. 
 
The ball mill slakers are much more expensive than paste or slurry slakers. They are available in sizes ranging from 1,000 lb/hr to 50 tons/hr. Schematic one shows an attritor type vertical ball mill lime slaker.
 
The ball mill slakers are equipped with an external classifier, which separates slurry from the oversized grit and impurities. The oversize grit is recycled back into the mill for regrind.


 

Lime slaking is a critical process and there is not enough known about the process by people who generally operate the equipment. The method and type of control used for slaking greatly affects the efficiency of the process. In addition to the quality of quicklime, the temperature at which slaking is done affects the quality of hydrated lime produced. Proper instrumentation is essential in maintaining proper slaking temperature and lime-to-water rations within a certain range.

Factors affecting ammonia recovery plant-lime path

In the ammonia recovery process, the role of slaked lime is to supply one of the reactants (milk of lime) for pre-limer reaction to remove the fixed ammonia (Ammonium chloride) from filter liquor of the soda ash plant.  Reaction between the lime and fixed ammonia is an endothermic reaction.  The reaction occurs at the reaction plane and it is controlled by the diffusion of milk of lime to interface.  The availability of milk of lime is determined by the solubility of milk of lime in the water. The reaction of Ca(OH)₂ and NH₄Cl at reaction plane creates the necessary driving force for dissolution of Ca(OH)₂ and diffusion into the interface and its process goes on as long as both the reactants are present.  Schematic two shows the recovery of ammonia.  The overall reaction in the ammonia recovery process is:
 
Ca(OH)₂  +  2 NH₄Cl  ------------------> Ca Cl₂ +2 NH₃ + H₂0
 
1.      Lime concentration :
The lime concentration is desired around 220 to 280 Titer. Higher concentration of lime leads to low pressure stream saving
 
2.      Calcium carbonate and inert materials
Milk of lime contains around 30 to 40% calcium carbonate as suspended solids which are not taking part in the reaction.  The suspended solids along with inerts  and sulfate reduces the working life of ammonia stills.
 
3.      Particle Size
Pre-limer reaction is the endothermic reaction and is controlled by mass transfer of Ca(OH)₂ particles from bulk of the liquid-to-solid interface. Smaller particle size will reduce the reaction time in the reactor.
 

 

By: Mohamad Hassibi, www.chemcosystems.net, modified for publication with permission