Production of Hydrated Lime

The term hydrated lime is widely used to describe a powdered calcium hydroxide product made by reacting quicklime with a controlled excess of water. The product is essentially dry and generally contains less than 1 % of unreacted water. 

The process is called hydration and should be differentiated from slaking which involves the production of a dispersion of calcium hydroxide in water (i.e. a milk of lime or a lime putty, see chapter 22). However, the expression slaked lime is used as a generic term for hydrated lime, milk of lime and lime putty. 

An estimated 10 to 15 % of the quicklime produced in developed countries is converted into hydrated lime and the percentage may be higher in countries which do not have a large steel industry. Because hydrating plants are relatively complex and can be fed with surplus grades of quicklime, there are relatively few of them and they are normally located at a lime works. 

While the chemical reactions involved in the formation of hydrated lime are simple, the physical chemistry (i.e. the kinetics, crystallisation and agglomeration) is complex. Some of the information in the literature is apparently contradictory, presumably owing to unquantified variations in process conditions and the quality of the reactants. The following sections aim to present a coherent account of the topic. References are given for those wishing to delve more deeply into the subject. 

Because of the above complexities, before deciding to invest in a hydrating plant, the hydration characteristics of the available grades of quicklime should be evaluated. The quality of the water supply should also be assessed, bearing in mind any seasonal variations. 

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Although most lime is sold as quicklime, production of hydrated lime is also substantial. This product is made by the lime manufacturer in the form of a fluffy, dry, white powder, and its use obviates the necessity of slaking. Small lime consumers caimot economically justify the additional processing step that hydration entails. 

There are many applications where magnesium is regarded as an undesirable impurity. These include the production of hydrated lime (at atmospheric pressure), aerated concrete, sandlime bricks, and precipitated calcium carbonate, for which MgCOa levels should preferably be less than 2 % and, ideally, less than 1 % in the limestone. 

Although impurities in water affect the rates of reaction between quicklime and water when producing milks of lime (sections 22.2 and 22.8.2), relatively little has been reported on their effect on the production of hydrated lime. This could be because ... 

Improved absorbents have been developed — see section 20.6, which describes the production of hydrated limes with high specific surface areas. [29.6] mentions a calcium lignosulfonate-modified hydrated lime for the high temperature injection process (presumably a high surface area hydrate). 

Most carbide acetylene processes are wet processes from which hydrated lime, Ca(OH)2, is a by-product. The hydrated lime slurry is allowed to settle in a pond or tank after which the supernatant lime-water can be decanted and reused in the generator. Federal, state, and local legislation restrict the methods of storage and disposal of carbide lime hydrate and it has become increasingly important to find consumers for the by-product. The thickened hydrated lime is marketed for industrial wastewater treatment, neutrali2ation of spent pickling acids, as a soil conditioner in road constmction, and in the production of sand-lime bricks. 

Ethylene was reacted with chlorine water, or with a mixture of hydrated lime and chlorine. In the latter case the Ca(OCl)2 formed decomposes to yield HOC1. The aqueous opening of the intermediate chloronium ion leads to the formation of the product. Ethylene chlorohydrin then was cyclized to ethylene oxide by addition of calcium hydroxide. 

The effect of additives on the yield of carbon monoxide is shown in Figure 10. Addition of hydrated lime and dolomite at the 5% concentrations brought respective increases of 23 and 26% in yield of carbon monoxide. Figure 11 shows similar increase in product gas yield (CO -f-H2 + CH4) for the same additions. Adding 2% dolomite failed to bring any significant increase in yields of methane, hydrogen, or carbon monoxide. 

Slaked lime is used in both the production and refining of sugar. Sugarcane is cut, shredded and treated with water to produce raw juice with a pH of 4 to 5. The juice is treated with lime to raise the pH and heated to approaching 75 °C to destroy invertase and other enzymes [30.5]. Typically some 2 to 5 kg of hydrated lime is used per tonne of sugar produced. 

Loss on ignition is an analytical term for the loss on weight produced by heating a sample to a specified temperature. In the case of quicklime and limestone products, it generally refers to heating to 1000 50°C in air to decompose all carbonates and hydroxides to the oxides and to burn-off organic matter. In the case of hydrated lime, it may refer to 575 25°C, at which temperature calcium hydroxide decomposes and any organic matter is burnt-off. 

In the Olin process, chlorination of a slurry of hydrated lime in sodium hypochlorite leads to the crystallization of the triple salt Ca(OCl)2 NaOCl NaCl I2H2O. This product is filtered and the filter cake is mixed with a chlorinated lime slurry. The molar concentration of the NaOCl in the triple salt is equivalent to that of CaClj in the chlorinated lime. Heating this mixture leads to the precipitation of neutral calcium hypochlorite dihydrate crystals, which are filtered and dried [6]. 

In a process used from 1937 to 1984, a solution of 10-15% hypochlorous acid, which contains very little chloride, is mixed with a slurry of hydrated lime at pH 10.0-10.5 to form a 15-20% solution of calcium hypochlorite. Sodium hypochlorite may be added to convert calcium chloride to calcium hypochlorite. The solution is spray dried, and the resulting solid is granulated and vacuum dried. Figure 24.7 depicts a current process, which uses salt-free 45-50% hypochlorous acid to make a slurry of hydrated calcium hypochlorite that is spray dried to make calcium hypochlorite with up to 82% available chlorine. The product may also be agglomerated. ... 

Some carbonates are important industrial chemicals. Calcium carbonate occurs naturally in several forms, including limestone, and is used in the production of quicklime, calcium oxide CaO, slaked (or hydrated) lime, calcium hydroxide Ca(OH)2 and cement. 

The term lime also has a broad coimotation and frequently is used in referring to limestone. According to precise definition, lime can only be a burned form quicklime, hydrated lime, or hydraiflic lime. These products are oxides or hydroxides of calcium and magnesium, except hydraiflic types in which the CaO and MgO are chemically combined with impurities. The oxide is converted to a hydroxide by slaking, an exothermic reaction in which the water combines chemically with the lime. These reversible reactions for both high calcium and dolomitic types are Quicklime... 

Texture. All limestones are crystalline, but there is tremendous variance in the size, uniformity, and arrangement of their crystal lattices. The crystals of the minerals calcite, magnesite, and dolomite are rhombohedral those of aragonite are orthorhombic. The crystals of chalk and of most quick and hydrated limes are so minute that these products appear amorphous, but high powered microscopy proves them to be cryptocrystalline. Hydrated lime is invariably a white, fluffy powder of micrometer and submicrometer particle size. Commercial quicklime is used in lump, pebble, ground, and pulverized forms. 

Lime. The total toimage of quicklime and hydrated lime sold and used in 1992 was 16.2 x 10 t. Captive lime accounted for 11% of this total, nearly the same percentage as captive limestone. Captive lime production decreased drastically since the 1970s, falling from 6.7 x 10 t in 1973 to 1.82 X 10 in 1992. Whereas commercial lime production was at an all time high in 1992, total lime (commercial lime plus captive lime) was 18% below that... 

Manufacture. A high purity, completely hydrated lime of high reactivity is employed in the manufacture of calcium hypochlorite. It should contain low levels of impurities such as siUca, MgO, CaC03, CaSO, AI2O3, Fe203, and trace metals such as Co, Ni, Cu, and Mn, since some of these can cause process difficulties and can also affect the quaUty and stabiUty of the final product. 

Bleaching Powder. This material, known siace 1798, is made by chlorination of slightly moist hydrated lime, calcium hydroxide [1305-62-0] Ca(OH)2- It has the empirical formula Ca(OCl)2 CaCl2 Ca(OH)2 2H20. Its compositioa, loag a subject of coatroversy, was estabHshed by phase studies, microscopy, and x-ray diffraction techniques (241). The initial chlorination products are monobasic calcium chloride [14031-38-4] and dibasic calcium hypochlorite [12394-14-8] ... 

Corrosion inhibitors are used to reduce the corrosion of surface equipment, surface casing, and the drill string by drilling and well treatment fluids. Many different corrosion inhibitors have been used. These include amine salts such as ammonium sulfite -bisulfite blends, zinc carbonate, zinc chromate, hydrated lime, fatty amine salts of alkylphosphates, cationic polar amines, ethoxylated amines, and tertiary cyclic amines. Commercial products are usually proprietary blends of chemicals. 

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