Limestone and Lime

Description and general properties. Lime or calcia are common names for calcium oxide 

The hydratation of quickhme is highly exothermic and it releases circa 1.19 MJ per kilogram of lime. If not enough water is added, the heat released can increase the temperature of the water until it reaches its boiling point. Once the reaction is complete, the product obtained is calcium hydroxide, Ca(OH)j [1305-62-0], also called hydrated lime or slaked lime. The solution saturated with calcium hydroxide is called milk of lime and has a pH of 12.25. Hydraulic lime is an impure form of hme that will harden under water. Lime has been used for thousands of years for construction. Archeological discoveries in Turkey indicate lime was used as a mortar as far back as 7000 years ago. Ancient Egyptian civilization used lime to make plaster and mortar. 

Industrial preparation. Most hme worldwide is obtained from quarries of carbonated rocks such as limestone, marble, chalk, and dolomite, or even from oyster shells. The suitable raw materials are usuaUy selected because of their low silica and iron contents. After the rock is blasted away, the material is then crushed and sized before being calcined into vertical shaft furnaces (Europe) or rotary kilns (USA) at 1010 to 1345 C. During calcination, the carbon dioxide is driven off and leaves calcium oxide or quicklime according to the following reaction  

Theoretically, 100 kg of pure calcium carbonate yields 56 kg of quicklime. 

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Thermal Properties. Because all limestone is converted to an oxide before fusion or melting occurs, the only melting point appHcable is that of quicklime. These values are 2570°C for CaO and 2800°C for MgO. Boiling point values for CaO are 2850°C and for MgO 3600°C. The mean specific heats for limestones and limes gradually ascend as temperatures increase from 0 to 1000°C. The ranges are as follows high calcium limestone, 0.19—0.26 dolomitic quicklime, 0.19—0.294 dolomitic limestone, 0.206—0.264 magnesium oxide, 0.199—0.303 and calcium oxide, 0.175—0.286. 

Pit and Quarry Maga fine, Cleveland, Ohio, numerous articles on production of limestone and lime. 

Lime has been manufactured for more than 2000 years and was the product of one of the first chemical processes used in the U.S. by the early settlers (the manufacture of rum being another). The many uses of lime are so varied that limestone and lime production are greater than any other natural substance. It is a low-cost commodity in the U.S. because there are limestone deposits in many parts of the country. Lime plants are always close to the limestone source in order to minimize freight costs. 

Before going further, let us clarify the various common names of limestone and lime. The following is a summaiy 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. 

Flue gas treatment systems can also be classified in terms of the reagent used in the process. Limestone and lime are the two most commonly used reagents for flue gas desulfurization. This is mainly because of the low cost of these reagents, especially limestone. Low reagent costs and simplicity of design make flue gas treatment systems using lime or limestone the most economical of the systems currently available. 

The reactivity of limestones with respect to the reaction with sulfur dioxide varies markedly. For example, for a given fluidised bed combustor, the Ca S stoichiometric ratio required to achieve a 90 % reduction in sulfur emission at atmospheric pressure, varies from 2 to 5. The reasons for such a variation are not understood, but are likely to include decrepitation, catalytic effects of minor components such as iron, and the structure of the limestone and lime [12.12]. Laboratory test methods have been developed for predicting the performance of sorbents [12.13,12.14]. 

These equations are good for 0.01 < (Re)t < 16000 and 0.67 < cj) < 1. The equation fitted the experimental terminal velocities of limestone and lime particles to better than 20%... 

On the basis of the Scholtz Station test results, Rush and Edwards (1977) concluded that the overall performance of a properly designed and operated double alkali system should be superior to that of direct limestone and lime systems because... 

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