|LIME AS A CHEMICAL:|
|1.||Lime is only slightly soluble in water.|
|2.||The solubility of lime decreases with temperature.|
|3.||The fineness of hydrated lime is not due to grinding.|
|4.||What "Calcium Oxide Equivalent" in hydrated lime means.|
|5.||The difference between "Total Lime" and "Available Lime."|
|6.||Lime is a very strong base.|
|7.||Why limestone cannot be "perfectly" calcined.|
|8.||Limestone is not "burned" to make lime.|
|9.||Quicklime melts but Hydrated Lime doesn't; (It decomposes).|
|LIME MARKET FACTORS:|
|10.||Lime kilns determine lime markets, not limestone deposits.|
|11.||"Quicklime Recovery" is a hidden factor in quicklime demand.|
Quicklime (calcium oxide) and water chemically react to form hydrated lime (calcium hydroxide), a compound that is only slightly soluble in water. For a solubility comparison, table salt (NaCl) has a solubility in water of 36.0 grams per 100 grams of saturated solution at 20 degrees C, as compared to hydrated lime, which has a solubility in water of 0.165 grams at the same temperature and volume of water. The two factors that enable lime to be so effective a base, despite its low solubility in water, are: (1) The smallness of the hydrated lime particle size and (2) the double hydroxyl groups that result from each molecule of lime that does go into solution (dissociates in water).
The hydrated lime particle is so small that, when the lime/water mixture is agitated, the lime particles stay in suspension for a relatively long time, even if the agitation is stopped. This is due to "brownian motion" (the constant vibration of water molecules) which constantly buffet the suspended lime particles. If the solution is constantly agitated (mixed) the particles will remain in suspension indefinitely. The suspended particles have a very high total surface area which means that, as the lime in solution is used up in reactions, more lime quickly dissolves into the solution ( Le Chatelier's Principle -" ... if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.") Each molecule of calcium hydroxide that ionizes produces two hydroxyl ions (OH-), thus providing a plentiful supply of neutralizing power. A good analogy of the effect of particles with a high surface area is that of a granary explosion. Grain, in a pile, is not all that combustible. However, if you pulverize it, and then suspend it as a fine dust in the air, a spark can result in an explosion.
Most of us are familiar, to some degree, with a basic concept of solubility. As a general rule, we find that most compounds dissolve more quickly, and to a greater extent, as the temperature of the solvent (liquid) increases. Quicklime (calcium oxide) and hydrated lime (calcium hydroxide), both considered to be Lime products, are members of a relatively small group of compounds that are an exception to this general rule. In fact, the solubility of lime in water, at various temperatures, is the exact reverse of what we would expect to find. The highest solubility of lime (calcium hydroxide) occurs at 0 degrees C (freezing) and the lowest is at 100 degrees C (boiling). The reaction of calcium oxide with water to form calcium hydroxide is a highly exothermic reaction (gives off heat). Once the hydrated lime has been produced from the reaction, and is in a slurry form, the highest levels of solubility are reached at the lowest temperatures.
The process of a substance dissolving into an aqueous solution (water) can be viewed as a reaction of the substance (solute) with the water molecules (solvent) to form a solution. Water is a "polar" molecule. The angle between the H-0-H is 104.5 degrees. Because oxygen is much more electronegative (attracts electrons more), the electrons tend to stay more around the oxygen atom than the hydrogen atoms, even though the water molecule is overall electrically neutral. The shape of the molecule results in each water molecule having a partially positive end (oxygen) and partially negative (hydrogens). Though the partial charge is less than a full charge, the water molecules can cluster around an ion, or molecule, in solution. In the case of NaCl, the partially positive ends of the polar water molecules cluster around the Cl- ions and the partially negative ends cluster around the Na+ ions, resulting in the formation of Na+ and Cl- ions in solution. This can be either an endothermic (requires heat) or exothermic (gives off heat).
|Endothermic:||Solute + Solvent + Heat||---->||Solution|
|Exothermic:||Solute + Solvent||---->||Solution + Heat|
The dissolving (dissociation) of the solute in the solution reaches an equilibrium point, as with any reaction. In some cases, all of the solute completely dissolves and the reaction goes to completion. In other cases, as with lime, the solute may only be partially soluble. Once the equilibrium has been established, any change in the concentration, pressure or temperature can disturb this equilibrium: (Le Chatelier's Principle -"... if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.") Adding heat to an endothermic reaction will force the reaction to the right of the equation, whereas, adding heat to an exothermic reaction will force it to the left.
Hydrated lime appears to have a fineness (particle size) not unlike that of flour. Limestone (calcium carbonate), which is used to produce quicklime (calcium oxide), has a rhombohedral crystal structure. The conversion of calcium carbonate to calcium oxide (quicklime) results in a cubic crystal structure. (Carbon dioxide is driven out of the carbonate portion of the crystal structure by intense heat energy, and a Ca-O bond remains.) When quicklime reacts with water, the resultant hydrated lime (calcium hydroxide) has a hexagonal crystal structure which occupies about 25% more volume than the original cubic structure of quicklime. As the quicklime-water reaction takes place, you can see actually see the quicklime pebbles swell and occupy more volume (if the ratio of water to quicklime is low). Mechanical means are used to remove the normal grit that results from lime being produced from a naturally occurring material, however, the particle size of the hydrated lime is generally the result of the formation of lime from the reaction of calcium oxide and water and the resultant shift in crystal structure from the formation of calcium hydroxide.
A high purity limestone (calcium carbonate) is used as kiln feed to produce "high calcium" quicklime (calcium oxide). From an ideal standpoint, and based on atomic and molecular weight ratios, 100 tons of pure calcium carbonate would produce 56 tons of quicklime. When the 56 tons of calcium oxide are reacted with 18 tons of water, the hydrated lime produced (calcium hydroxide) would ideally have a weight of 74 tons. As a result of this, the Calcium Oxide Equivalent (CaO) in the hydrated lime would be 75.7% (56 tons/74 tons). This means that 75.7% is the maximum Calcium Oxide Equivalent possible in any "high calcium" hydrated lime.
Lime is produced from limestone, a naturally occurring material in nature. The deposits of 100% pure calcium carbonate are rare, so non-lime components are a normal part of lime that is produced from quarried limestone and is frequently referred to as "grit". In determining the purity of the lime produced there are two chemical concepts to be aware of. One is "Total Lime" and the other is "Available Lime". Any unconverted ("unburned") limestone that is in the grit will react with the acid solution used in the titration process to determine the amount of lime in a sample, however, all of this lime would not be readily available in most production processes. The term "Available Lime" refers to the lime that is in the form of calcium oxide/CaO, as opposed to be being in the form of calcium carbonate. Since lime has a very low solubility in water, simply titrating a "slurry" of lime with an acid solution can produce inaccurate results. The addition of sugar to the lime solution results in an enormous increase in the solubility of lime through the formation of an intermediate product, calcium succrate. Titrating lime, using one of the types of standard "Rapid Sugar" tests (ASTM C-25 or AWWA B-202) produces a calcium oxide percentage that more accurately reflects the lime that is "available" to react as calcium hydroxide. Typically, the "Available Lime" percent is always less than the "Total Lime". Most users of lime generally prefer to use the "Available Lime" percent, since this represents the lime that is chemically available to them for most reactions.
Lime can be considered a relatively inexpensive, strong base that can readily produce pH values of above 12 in an aqueous solution. The ionization of hydrated lime (calcium hydroxide) in an aqueous solution produces calcium (Ca+2) ions and hydroxyl (OH-1) ions. It is the hydroxyl ions that make this a "base" according to the traditional definition of a base (Arrhenius). For each molecule of hydrated lime that ionizes you get two hydroxyl (OH-1) ions. From a practical standpoint, you get twice as many hydroxyl ions as you would with a compound such as sodium hydroxide, NaOH (caustic).
|Calcium Hydroxide:||Ca(OH)2 --> Ca+2 + OH-1 + OH-1|
|Sodium Hydroxide:||NaOH --> Na+1 +OH-1|
The term "burning" generally refers to an oxidation process that involves the "chemical union of oxygen with any substance". In the lime kiln, the temperature of the limestone (calcium carbonate) is increased to a point above its dissociation temperature. This is called calcination and the process to accomplish this requires a minimum temperature of 1,648 degrees F (898 degrees C). At that point the carbon to oxygen bonds in the carbonate group become unstable. A carbon atom, and two of the three oxygen atoms, form carbon dioxide, which leaves the reaction environment. The remaining oxygen atom quickly forms a strong, high energy, ionic bond with the calcium atom. This is the formation of calcium oxide, commonly known as quicklime.
|Calcium Carbonate||1,648oF||Calcium Oxide||Carbon Dioxide|
This reaction would be reversible except that the carbon dioxide is removed. When calcium oxide reacts with water, the heat that results from the exorthermic reaction can be viewed as a "release of the stored heat from the kiln".
|Calcium Oxide||Calcium Hydroxide|
Referring to the industrial process to produce quicklime as "burning limestone" will probably always be used. It is useful, however, to recognize that we are actually simply heating the limestone to a point above its dissociation temperature.
When enough heat is applied to quicklime it will eventually reach a melting point at 4658 degrees F (2570 degrees C). Continued heating will result in quicklime reaching its boiling point at 5162 degrees F (2850 degrees C). Hydrated lime (calcium hydroxide), on the other hand, doesn't have a melting point. It was formed from the reaction of quicklime (calcium oxide) and water, and begins to decompose at 1076 degrees F (580 degrees C) returning to the original reactants; quicklime and water (as water vapor). At that point the hydrated lime has now become quicklime again and, with enough heat, will eventually reach a melting point (and boiling point) as quicklime.
"High calcium" quicklime (calcium oxide) is produced in large kilns (rotary and/or vertical) from "high calcium" limestone quarried from large deposits. Lime kilns involve large capital investments, as well as environmental impact studies and regulatory requirements. Consequently, the addition of a new lime kiln to the market involves both expenses and time. Lime is also a very freight intensive product, so lime markets are generally very regional and customers are usually relatively close to the supply source. As with any product, the law of supply and demand directly affects the lime market. However, because of the cost and time to introduce a new kiln to the market there can be a delay in the lime industry's response to market demands for lime. Quite often, people equate the available limestone deposits with the availability of lime in the market. It's important to keep in mind that all limestone destined for lime production must go through a lime kiln, which means that the total number of lime kilns in the regional market has a major impact on lime availability and pricing.
There are a number of industrial users of quicklime (calcium oxide) who have lime kilns of their own. In their industrial process, or as a result of a recovery reaction, they generate significant amounts of calcium carbonate (sludge) which they then de-water and calcine in their own kiln(s) to produce quicklime. Most of the companies that have this capability will recover approximately 90% of the quicklime they use. In addition, they will buy about 10% of fresh lime to combine with their recovered lime to help maintain a high purity. Many of these industries will annually purchase thousands of tons of quicklime from lime producers, so the amount of quicklime they use in their plant turns out be about ten times the amount of quicklime they purchase. Occasionally, one of these user's kilns will have a problem that results in the kiln being temporarily shut down for repairs. When this occurs, they usually must purchase all of their quicklime from lime suppliers.
From a lime market standpoint, this situation has about the same effect as that of adding nine additional major quicklime users to the lime market during the period that the user's kiln is down. If more than one user's kiln is down at the same time, the effect of the demand on the lime market can be tremendous and the demand for pneumatic trucks used to transport the quicklime can increase dramatically as well.
Because of this hidden factor in quicklime demand, the lime user may find it advisable to have at least two lime suppliers.