Chemical Uses of Lime
Lime is a basic industrial chemical that is used as a component in many other chemical processes.
Alkali plants with access to natural soda ash use the “lime-soda process to manufacture caustic soda (sodium hydroxide). Sodium carbonate (soda ash) reacts with lime slurry to form caustic soda and precipitated calcium carbonate as co-products.
Calcium Carbide and Cyanimide
Calcium carbide, the oldest source of acetylene, is formed by mixing quicklime and coke and heating to a temperature of 2000 degrees C. Molten carbide is continuously removed from the furnace and, upon solidifying, is crushed and ground to the desired size. Calcium cyanamide, a nitrogen fertilizer, is made by heating calcium carbide in the presence of nitrogen. Acetylene gas is generated from carbide by adding water, yielding a waste hydrated lime.
Lime is used in the purification of citric acid. Hydrated lime is added to a solution of citric acid that has previously been filtered to remove suspended solids. Calcium citrate is precipitated and removed. Citric acid can later be regenerated from the calcium citrate by reacting it with sulfuric acid to precipitate calcium sulfate.
Lime is used in the two major processes to manufacture magnesia (magnesium oxides): the seawater process and production from brines. In the seawater process, milk of lime precipitates magnesium hydroxide (Mg(OH)2) from seawater. Where dolomitic lime is used, up to half of the Mg(OH)2 derives from the lime. So too, in the production of Mg(OH)2 from brine, lime acts as a precipitant.
Calcium hypochlorite bleaches are produced by reacting lime with chlorine. These bleaches are widely used as swimming pool chemicals and in paper production.
Calcium Magnesium Acetate
Dolomitic lime is reacted with acetic acid to produce calcium magnesium acetate (CMA), a de-icing chemical. CMA is not corrosive and is very compatible with the environment. It is much less damaging to road surfaces and automotive parts than salt. It is effective in melting ice at very low temperatures, and is not toxic to vegetation.
Miscellaneous Chemical Uses
Lime is employed in the manufacture of many other inorganic and organic chemicals and pharmaceuticals. Most of the calcium organic and inorganic salts are produced by reacting the various acids with lime (the base). Among the most important inorganic products that use lime are calcium phosphates (mono, di, and tri), fluoride, bromide, ferrocyanide, and nitrate. Lime is used in the manufacture of organics such as calcium acetate, stearate, oleate, tartrate, lactate, citrate, benzoate, and gluconate.
Lime is also used as a neutralizing agent in the manufacture of chrome chemicals (bichromate); for purification of salt brines; to aid in the concentration of glucose and dextrin; to make metallic calcium; as soda-lime, an adsorbent and gas purifier; and for countless other minor or isolated purposes, such as for CO2 absorption, as a desiccant, etc.
Lime is used in the production of ethylene or propylene glycol via the chlorohydrin process. Ethylene gas, obtained readily from petroleum refineries, is chlorinated to form ethylene dichloride, which in turn is reacted with lime to produce ethylene glycol.
Hydrated lime is essential to the production of sugar from both sugar cane and sugar beets. It is also used to purify sugar from other sources, such as maple or sorghum, although these are produced in much smaller quantities.
Sugar cane and sugar beets are harvested and processed with water to form raw juice, which has low pH and contains dissolved impurities. Hydrated lime is added to the juice to raise the pH and to react with the impurities to form insoluble calcium organic compounds that can be removed. Excess lime is removed by carbonation or by the addition of phosphoric acid. This process may be repeated several times depending on the purity of final product required.
High-purity refractory “dolomite” (frequently called Doloma) and lower purity fettling grade “dead-burned dolomite” (usually referred to as DBD) are both manufactured by calcining dolomitic limestone. The production method is similar to the manufacture of ordinary lime, except that the burning time is longer and temperatures considerably higher (in the range of 1600 – 1850º C). High purity doloma is fired in rotary kilns to the upper end of the temperature range and without the addition of impurities. High purity doloma is used to manufacture refractory bricks employed in cement and lime rotary kiln linings, and in steel ladles and refining vessels. The lower-purity DBD is fired in rotary kilns to the lower end of the temperature range, and iron oxides are added during calcinations to stabilize the resulting hard-burned quicklime against decomposition from moisture. DBD is used for the manufacture of monolithic patching and repair materials for steel furnaces.
Large quantities of light-burned dolomitic lime are used in the production of synthetic refractory grade magnesia (MgO). The quicklime is slaked in magnesium chloride brine, precipitating magnesium hydroxide. The mag-hydroxide is calcined and fired into dense, high-purity magnesium oxide. Refractory magnesia is used in the production of linings for cement and lime kilns, in addition to steel ladles and furnaces.
Stabilized zirconium oxide (ZrO2) is produced by adding about 3% lime during the fusion or sintering process. ZrO2 is widely used for analysis crucibles and thermocouple tubes, in addition to crucibles and furnaces used for the production of high-temperature aero-space alloys.
One form of silica brick, a specialized refractory used for lining coke ovens and glass furnaces, is made by thoroughly mixing ground silica (usually quartzite) with the addition of ½ to 3 percent milk-of-lime as a sintering (firing) aid. This mixture is formed into various shapes and then fired in kilns.
Lime serves a myriad of uses in the food industry. Some examples include:
When cream is separated from whole milk during the production of butter, lime water is often added to the cream to reduce acidity prior to pasteurization. The skimmed milk is next acidified to separate casein. The casein is mixed with lime and a small amount of sodium fluoride to produce calcium caseinate, a form of glue. Fermentation of the remaining skimmed milk (whey) and the addition of lime forms calcium lactate, which is marketed as a medicinal or acidified to produce lactic acid.
Glue and Gelatin
Waste materials from rendering plants are treated with lime in slurry form. This process swells the collagen, thereby facilitating subsequent hydrolysis. Afterwards, the treated stock is washed to remove lime, albumin, and mucin. The washed stock is dried, and the final product is sold as glue or gelatin.
In the preparation of a common type of baking powder, monocalcium phosphate, lime is required as an ingredient. This baking powder is made by reacting pure phosphoric acid with a high calcium lime.
Fruit and Vegetables
In the controlled atmospheric storage of fruit and vegetables, bags of hydrated lime are placed on racks in the storage room to absorb CO2 that exudes from ripening fresh produce. The co-location of hydrated lime in the storage room maintains a higher ratio of oxygen to CO2, permitting vegetables and fruit to be stored fresh for much longer periods. When placed in close proximity to the produce, carbon dioxide penetrates easily through the multi-wall paper bags into the lime. For apples, lime consumption averages about 1 to 1.5 lb./bushel. Pears, plums, and tomatoes, in addition to apples, have been stored this way in Canada and the Northeastern U.S., as well as in Oregon and Washington. In California, this technique is commonly employed in storing lettuce.
All quality tortillas are treated with lime. Corn is first soaked in milk-of-lime before its conversion to cornmeal. Lime is also used in the corn chip business. Grape leaves from wineries are treated with lime to precipitate calcium tartrate, which is sold as such or converted to tartaric acid. Several recipes for making watermelon pickles require the melon rind be soaked in milk-of-lime.