Chemicals from Limestone

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1 Chapter 5 Chemicals from Limestone The next major raw material for which we discuss the derived chemicals is calcium carbonate, common limestone. It is the source of some carbon dioxide, but, more importantly, it is used to make lime (calcium oxide) and slaked lime (calcium hydroxide). Limestone, together with salt and ammonia, are the ingredients for the Solvay manufacture of sodium carbonate, soda ash. Soda ash is also mined directly from trona ore. The Solvay process manufactures calcium chloride as an important by-product. Soda ash in turn is combined with sand to produce sodium silicates to complete the chemicals in the top 50 that are derived from limestone. Since lime is the highest-ranking derivative of limestone in terms of total amount produced, we discuss it first. Refer to Fig. 2.1, Chapter 2, Section 1, for a diagram of limestone derivatives. 1. LIME CaO Before going further, let us clarify the various common names of limestone and lime. The following is a summary of the nomenclature and the chemicals referred to. Industrial chemists quite often use the common names for these substances rather than the chemically descriptive names. CaCOs: CaO: Limestone, calcite, calcium carbonate, marble chips, chalk Lime, quicklime, unslaked lime, calcium oxide

2 Ca(OH) 2 : Slaked lime, hydrated lime, calcium hydroxide. A saturated solution in water is called limewater. A suspension in water is called milk of lime. 1.1 Manufacture Lime is one of the oldest materials known to humankind. It was used by Romans, Greeks, and Egyptians for the production of cement and was employed in agriculture as well. One of the first things done by American settlers was to set up a lime kiln for the "calcining" or heating of limestone Reaction CaCO 3 ^ CaO + CO 2 CaO + H 2 O ^ Ca(OH) 2 Common temperatures used in converting limestone into lime are O 0 C. For this reason lime is a very energy-intensive product. It takes the energy from a third of a ton of coal to produce 1 ton of lime Description Fig 5.1 outlines lime production. The limestone is crushed and screened to a size of approximately 4-8 in. There are different heating techniques and kiln styles. The one diagrammed is a vertical Dorrco Fluo Solids system. The limestone enters the top. Air entering the bottom "fluidizes" the solids to get better circulation and reaction. Approximately 98% decarbonation is typical. When a kiln is used in conjunction with the Solvay process and the manufacture of soda ash, coke can be fired in the kiln along with limestone to give the larger percentages of carbon dioxide needed for efficient soda ash production by the reaction of the carbon with oxygen to give carbon dioxide. If a purer lime product is desired, the fine lime can be taken from area 4. A less pure product is obtained from the bottom kiln section. Another kind of kiln is the rotating, nearly horizontal type. These kilns can be as much as 12 ft. in diameter and 450 ft. long. Limestone enters one end. It is heated, rotated, and slowly moves at a slight decline to the other end of the kiln, where lime is obtained.

3 CaCO 3 Limestone Crusher Dryer Screening Ca(OH) 2 Slaked lime Slaker Fine lime CaO Lime Cooler Cooler Kiln Fluidizing air Figure 5.1 Lime manufacture. 1.2 Uses For most applications slaked lime is sold. The hydration of lime is very exothermic and could ignite paper or wood containers of the unslaked material. Slaked lime is slightly soluble in water to give a weakly basic solution. Table 5.1 summarizes the uses of lime. Lime is used as a basic flux in the manufacture of steel. Silicon dioxide is a common impurity in iron ore that cannot be melted unless it combines with another substance first to convert it to a more fluid lava called slag. Silicon dioxide is a Lewis acid and therefore it reacts with the Lewis base lime. The molten silicate slag is less dense than the molten iron and collects at the top of the reactor, where it can be drawn off. Over 100 Ib of lime must be used to manufacture a ton of steel. CaO + SiO 2 ^ CaSiO 3 Table 5.1 Uses of Lime Metallurgy 40% Pollution control 15 Water treatment 10 Chemical manufacture 10 Pulp paper 5 Construction 5 Miscellaneous 15 Source: Key Chemicals

4 The uses of lime in chemical manufacture are too numerous to discuss since over 150 important chemicals are made with this basic material. In fact, only five other raw materials are used more frequently than lime for chemical manufacture: salt, coal, sulfur, air, and water. The most important chemical derivative of lime is soda ash, although the synthetic product has been a small percentage of all soda ash in recent years. A growing use of lime is in pollution control, where lime scrubbers placed in combustion stacks remove sulfur dioxide present in combustion gases from the burning of high sulfur coal. SO 2 + H 2 O ^ H 2 SO 3 Ca(OH) 2 + H 2 SO 3 * CaSO 3 + 2H 2 O Lime is used in water treatment to remove calcium and bicarbonate ions. Ca(OH) 2 + Ca HCO 3 " +> 2CaCO 3 + 2H 2 O Lime is employed in the kraft pulping process to be discussed in detail in Chapter 22, Section 3.1. Most of it is recycled. Without this recycling the pulp and paper industry would be the largest lime user. The main reaction of lime in the kraft process is for the purpose of regenerating caustic soda (sodium hydroxide). Na 2 CO 3 + CaO + H 2 O ^ CaCO 3 + 2NaOH The caustic soda is then used in the digestion of wood. The lime is regenerated from the limestone by heating in a lime kiln. A large part of Portland cement is lime-based. Sand, alumina, and iron ore are mixed and heated with limestone to 150O 0 C. Average percentages of the final materials in the cement and their structures are given here. 21% 2CaO^SiO 2 Dicalcium silicate 52 3CaO*SiO 2 Tricalcium silicate 11 3CaO^Al 2 O 3 Tricalcium aluminate 9 4CaO Al 2 O 3 Fe 2 O 3 Tetracalcium aluminoferrite 3 MgO Magnesium oxide The percentage of dicalcium silicate, sometimes abbreviated as C 2 S in the industry, determines the final strength of the cement. The amount of tricalcium silicate, C 3 S, is related to the early strength (7-8 days) required of the cement. Tricalcium aluminate, C 3 A, relates to the set in the cement.

5 Tetracalcium aluminoferrite, C 4 AF, reduces the heat necessary in manufacture. 1.3 Economics The production history of lime is given with other chemicals in Fig. 2.2, Chapter 2, Section 1. Production dropped more for lime than most other chemicals in the 1980s, 2.4% per year. Lime production is very dependent on the steel industry, which in turn fluctuates directly with automobile and housing demand. But production rose again in the 1990s. Lime, being an energy intensive chemical because of the high temperatures required to make it from limestone, fluctuates more with energy prices than most other inorganic chemicals. From the price rose from $12/ton to $28/ton, mainly because the oil embargo increased energy costs. Presently it sells for $57/ton or about 2.90/lb and the commercial value of its 45.2 billion Ib is $1.3 billion. 2. SODA ASH (SODIUM CARBONATE) Na 2 CO Manufacture The LeBlanc process for the manufacture of soda ash was discovered in 1773 and was used universally for many years in Europe. Salt cake (sodium sulfate) reacts with limestone to give soda ash and a troublesome side product gypsum (calcium sulfate). The process is no longer used. Na 2 SO 4 + CaCO 3 -»»Na 2 CO 3 + CaSO 4 In 1864 Ernest Solvay, a Belgian chemist, invented his ammonia-soda process. A few years later the soda ash price was reduced one third. The Solvay process had completely replaced the LeBlanc method by The Solvay method is still very popular worldwide. However, in this country large deposits of natural trona ore were found in the 1940s in Green River, Wyoming. In the last few years there has been a tremendous conversion from synthetic to natural soda ash. The first and last Solvay plant in the U.S. closed in 1986 (a large Allied Chemical plant in Solvay, NY). Trona ore is found about 50Om below the surface. It is called sodium sesquicarbonate

6 and is mostly 2Na 2 CO 3 *NaHCO 3 *2H 2 O (45% Na 2 CO 3, 36% NaHCO 3, 15% water + impurities). Heating this ore gives soda ash. The conversion from the Solvay process to natural soda ash has been called one of the most successful chemical industry transformations of the late 1970s and early 1980s. The ratio of production for selected years certainly proves this point. Year Solvay O Natural Despite the fact that no new Solvay plants have been started since 1934 in this country, it is still an important method worldwide. There is some fascinating chemistry in this involved process and we will discuss it Solvay Reactions FCaCO 3 +> CaO + CO 2 ~~ source of CO 2 [CaO + H 2 O *> Ca(OH) 2 J 2NH 3 + 2H 2 O ^ 2NH 4 OH source OfNH 4 OH "2NH 4 OH + 2CO 2 * 2NH 4 HCO 3 2NH 4 HCO 3 + 2NaCl * 2NaHCO 3 + 2NH 4 Cl _2NaHCO 3 * Na 2 CO 3 + CO 2 + H 2 O mam S lvay reactions 2NH 4 Cl + Ca(OH) 2 ^ 2NH 3 + CaCl 2 + 2H 2 O recycle of NH 3 CaCO 3 + 2NaCl ** Na 2 CO 3 + CaCl 2 overall reaction Description A detailed description of salt mining will be postponed until the next chapter, but it is important to note that soda ash is made from both limestone and salt, the two major raw materials. As outlined in Fig. 5.2, the brine (salt solution) is mixed with ammonia in a large ammonia absorber. A lime kiln, using technology similar to that discussed earlier, serves as the source of carbon dioxide, which is mixed with the salt and ammonia in carbonation towers to form ammonium bicarbonate and finally sodium bicarbonate and ammonium chloride. Filtration separates the less soluble sodium bicarbonate from the ammonium chloride in solution.

7 NH 3 (recycle) CO 2 (recycle) CO 2 (recycle) NaCl Brine Ammonia absorber Carbonating tower Carbonating tower NaHCO 3 NH 4 Cl CO 2, H 2 O (recycle) CaCO 3 _ Limestone Lime kiln + slaker Ammonia still NH 4 Cl solution Vacuum filter NaHCO 3 solid Rotary dryer Light Na 2 CO 3 Dense Na 2 CO 3 Densifler Mill Figure 5.2 Manufacture of soda ash. The sodium bicarbonate is heated to C in rotary dryers to give light soda ash. The carbon dioxide is recycled. Light soda ash is less dense than the natural material because holes are left in the crystals of sodium bicarbonate as the carbon dioxide is liberated. Dense soda ash, used by the glass industry, is manufactured from light ash by adding water and drying. The ammonium chloride solution goes to an ammonia still where the ammonia is recovered and recycled. The remaining calcium chloride solution is an important by-product of this process, although in large amounts it is difficult to sell and causes a disposal problem. 2.2 Uses Table 5.2 outlines the uses of soda ash. Glass is the biggest industry using soda ash. The 49% used by this industry is divided into 44% bottles and containers, 38% flat glass, 9% fiberglass, and 9% other. The glass industry is very complex and would take some time to discuss at length. There are about 500 different kinds of glass. However, 90% of all glass made is soda-lime-silica glass, which incorporates ingredients to be heated to give an approximate weight ratio of 70-74% SiO 2, 10-13% CaO, and 13-16% Na 2 O. These glasses can be used for windows, containers, and many transparent fixtures. The sand must be nearly pure quartz, a crystalline form of silicon dioxide. These deposits often determine the location of glass

8 Table 5.2 Uses of Soda Ash Glass 49% Chemical manufacture 27 Soaps/detergents 11 Flue gas desulfurization 3 Pulp and paper 2 Water treatment 2 Miscellaneous 6 Source: Chemical Profiles factories. Sodium oxide is principally supplied from dense soda ash, but other sources of the oxide include sodium bicarbonate, sodium sulfate, and sodium nitrate. Some nitrate is generally used because it will oxidize iron impurities and avoid coloration of the glass. Limestone is the source of lime. When these substances are heated the following reactions occur. Na 2 CO 3 + 0SiO 2 ^ Na 2 O - asio 2 + CO 2 CaCO 3 + ^SiO 2 ^ CaO 6SiO 2 + CO 2 For common window glass the mole ratio may be 2 mol Na 2 O, 1 mol CaO, and 5 mol SiO 2. Glass is essentially an amorphous, multicomponent solid mixture. Specific CaO-SiO 2 or Na 2 O-SiO 2 compounds do not exist. The addition of borax increases the glass resistance to acids and thermal shock. This is called Pyrex glass. In many other uses soda ash competes directly with caustic soda as an alkali. The chemical of choice is then dependent on price and availability of the two. 3. CALCIUM CHLORIDE CaCl 2 Calcium chloride is no longer in the top 50, but it is very high in the second 50 chemicals (see Chapter 13). Because it is an important by-product of the Solvay process, we will mention it here. Besides being a Solvay byproduct it is also obtained from natural brines (especially in Michigan). A typical brine contains 14% NaCl, 9% CaCl 2, and 3% MgCl 2. Evaporation

9 Table 5.3 Uses of Calcium Chloride Road Deicing 40% Road dust control 20 Industrial processing 20 Oil and gas wells 10 Concrete 5 Miscellaneous 5 Source: Chemical Profiles precipitates the sodium chloride. The magnesium chloride is removed by adding slaked lime to precipitate magnesium hydroxide. MgCl 2 + Ca(OH) 2 ** Mg(OH) 2 + CaCl 2 The uses of calcium chloride are given in Table 5.3. A large amount of calcium chloride is used on roads for dust control in the summer and deicing in the winter. The dust control use percentage includes some for roadway base stabilization. It is less corrosive to concrete than is sodium chloride. A debate on which is worse environmentally on local plant life because of high salt concentrations remains to be resolved. The home ice-melt market for calcium chloride has grown recently. Local governments are also using more calcium chloride. Another recent competitor in the market is calcium magnesium acetate, made by reaction of high-magnesium content lime with acetic acid. This salt could prove to be a noncorrosive alternative to the chlorides. Calcium chloride is used for some industrial refrigeration applications. Saturated calcium chloride does not freeze until - 5O 0 C, whereas saturated sodium chloride has a freezing point of- 2O 0 C. 4. SODIUM SILICATE (SILICA GEL) Soda ash is heated with sand at O 0 C to form various sodium silicates (over 40 of them), which collectively are produced at levels sufficient to rank in the top 50. Some common ones are listed here. Na 2 CO 3 + ^SiO 2 *» Na 2 O ^SiO 2 + CO 2

10 Table 5.4 Uses of Sodium Silicate Soaps and detergents 38% Silica gel and catalysts 15 Pulp and paper 12 Rubber and elastomers 7 Food and health care 5 Agriculture 3 Paints and coatings 3 Miscellaneous 17 Source: Chemical Profiles Ratio of SiO 2 TNa 2 O Sodium tetrasilicate Na 2 Si^ 4 Sodium metasilicate Na 2 SiOs 1 Sodium sesquisilicate Na3HSiO 4 5H 2 O 0.67 Sodium orthosilicate Na 4 SiO Table 5.4 gives the uses of sodium silicate. It is a partial replacement for phosphates in detergents as a builder (see Chapter 24, Section 6) that does not pollute rivers and lakes. This is a growing use of sodium silicate. As a fine silica gel with a large surface area it is used for catalysis and column chromatography. In the pulp and paper industry it is used as a hydrogen peroxide stabilizer. Hydrogen peroxide is a pulp bleaching agent that is growing in use, replacing chlorine. In rubber tires it is replacing some of the carbon as a reinforcing agent, yielding the so-called "green tire." Suggested Readings Austin, Shreve 's Chemical Process Industries, pp Chemical Profiles in Chemical Marketing Reporter, , , and Kent, Riegel's Handbook of Industrial Chemistry, pp Thompson, Industrial Inorganic Chemicals: Production and Uses, pp

Soda Ash ( Sodium carbonate) Manufacture

Soda Ash ( Sodium carbonate) Manufacture Pertinent properties Mol. Wt. 106 M.P. 851deg.C. B.P. Decomposes Soluble in water 8.9 gm/100gm at 20 deg.cel. Grade s: 99% sodium carbonate washing soda ( Na 2