Dolomitic Hydrated Limes

Lime is among a family of chemicals which are alkaline in nature and contain principally calcium, oxygen and, in some cases, magnesium. In this grouping are included quicklime, dolomitic lime, hydrated lime, dolomitic hydrated lime, limestone, and dolomite. The most commonly used additives are quicklime and hydrated lime, but the dolomitic counterparts of these chemicals (i.e., the high-magnesium forms) are also widely used in wastewater treatment and are generally similar in physical requirements. 

High-calcium and dolomitic hydrated limes for use in building are classified in terms of their CaO and MgO contents [20.19]. 

Reaction rates of high calcium limes are relatively high at all pH levels up to 12.4 and increase with finer particle size distributions, subject to the above comments regarding blinding by insoluble reaction products. It is for this reason that, when preparing milks of lime from quicklime, it is usually beneficial to slake under conditions which give a finely divided milk of lime (see chapter 22). The solution rates of dolomitic hydrated limes up to pH 9 also depend on particle size, but are appreciably lower than those of high calcium limes. 

Compound fertilisers (including calcium nitrate and calcium ammonium nitrate) sometimes contain 1 to 2 % of dolomitic hydrated lime. The lime confers similar benefits to pulverised dolomitic limestone (see section 10.3). 

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. 

Quicklime and hydrated lime are reasonably stable compounds but not nearly as stable as their limestone antecedents. Chemically, quicklime is stable at any temperature, but it is extremely vulnerable to moisture. Even moisture in the air produces a destabilizing effect by air-slaking it into a hydrate. As a result, an active high calcium quicklime is a strong desiccant (qv). Probably hydrate is more stable than quicklime. Certainly hydrated lime is less perishable chemically because water does not alter its chemical composition. However, its strong affinity for carbon dioxide causes recarbonation. Dolomitic quicklime is less sensitive to slaking than high calcium quicklime, and dead-burned forms are completely stable under moisture-saturated conditions. 

Miscellaneous. Both whiting and hydrated lime are used as diluents and carriers of pesticides, such as lime—sulfur sprays, Bordeaux, calcium arsenate, etc. The most widely used bleach and sterilizer, high test calcium hypochlorite, is made by interacting lime and chlorine (see Bleaching AGENTS). Calcium and magnesium salts, such as dicalcium phosphate, magnesium chloride, lithium salts, etc, are made directly from calcific and dolomitic lime and limestone. 

The trade association for U. S. and Canadian manufacturers of high calcium quicklime, dolomitic quicklime, and hydrated lime, collectively referred to as lime."... 

Calcium oxide is a white caustic crystalline alkali substance that goes by the common name lime. The term lime is used both generically for several calcium compounds and with adjectives to qualify different forms of lime. This entry equates lime, also called quicklime or burnt lime, with the compound calcium oxide. Hydrated lime, made by combining lime with water, is calcium hydroxide and is often referred to as slaked lime (Ca(OH)2). Dolomite limes contain magnesium as well as calcium. Limestone is the compound calcium carbonate. The term lime comes from the Old English word lim for a sticky substance and denotes lime s traditional use to produce mortar. Calx was the Latin word for lime and was used to name the element calcium. 

To alleviate operating difficulties in the pretreater caused by unwanted steam condensation, a nitrogen gas feed was substituted for the steam feed of this pretreater. The coal feed was Illinois No. 6, River King Mine, 20 X 0 mesh. Additives were hydrated lime and dolomite, and additive concentrations used in the coal feed mixtures were 5 and 2 wt %, respectively. The analyses and sizes of the additives were as follows ... 

The effect of the additive upon yield of hydrogen and methane in the pilot-plant unit is shown in Figure 9. At an average gasification temperature of 914 °C, addition of 5% hydrated lime in the coal feed increased the hydrogen yield approximately 30% from 6.25 to 8.1 scf/lb of moisture-and-ash-free coal feed. A similar increase of 17% was obtained when 5% dolomite was used in the coal feed at 945 °C average gasifica-... 

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. 

Operation of the 4-inch diameter Synthane pilot-plant gasifier at 40 atm pressure and average temperature of up to 945°C with dolomite and hydrated lime additives at 5% concentration has increased product gas (CO -f H2 + CH4) yield significantly and has increased allowable operating temperatures. 

Specific gravity. High-calcium hydrated lime has a specific gravity of 2.24 g/cm [19.1]. The values for partially and fully hydrated dolomitic limes are about 2.7 and 2.5 g/cm, respectively [19.2]. 

The requirements for highly hydrated dolomitic lime, (also called di-hydrated or double hydrated lime) are specified in [20.15, 20.16]. It is required to contain less than 8 % of free lime, to ensure that it is sound. 

Lime specifications. Only slaked lime should be used in hot-mix asphalt. Both powdered hydrated lime and milk of lime meeting the requirements of Types CL 70, 80, or 90 [26.18] would be fully satisfactory, as would Type S dolomitic limes [26.19]. 

Limes for building purposes are specified in CEN Standard EN 459 [26.18]. They are divided into the 8 groups listed in Table 26.7. Calcium limes (CL, which may be in the form of quicklime, powdered hydrated lime, or putty) are sub-divided into three - CL 90, CL 80 and CL 70 — on the basis of their chemical analysis. Dolomitic limes (DL) are sub-divided into DL 85 and DL 80, also on the basis of their chemical analysis. Hydraulic limes (HL) are described in section 26.9. 

One non-standard desulfurisation technique, used on a rotary kiln by a producer of calcined dolomite, is to include finely divided dolomite in the feedstone. The fines calcine, become airborne as a result of the action of the kiln internal fittings (i.e., trefoils and lifters), and remove a significant amount of the sulfur dioxide. It is not known whether this technique would be as successful with high calcium limestone, which calcines at higher temperatures than dolomite. Other techniques will no doubt be evaluated, such as the injection of hydrated lime into the back-end of the kiln. The cost-effectiveness of such techniques in relation to alternatives, and their effects on kiln operation, would need to be assessed. 

Completely hydrated dolomitic limes see Type S hydrated lime. 

Hydrated dolomitic limes are powdered slaked limes consisting of calcium hydroxide, magnesium hydroxide and magnesium oxide (see semi-hydrated lime and Type S hydrated lime). 

Hydrated limes are hydrated air limes, calcium limes, or dolomitic limes, resulting from the controlled slaking of quicklimes to produce a dry powder. 

T pe S hydrated lime (also called special hydrated lime) is defined in ASTM specification C-207. It is produced by heating lime (generally dolomitic) in an autoclave at ca. 180 °C. It may contain up to 8% of unhydrated oxide. It is required to meet specified plasticity, water retention and particle size requirements. 

Lime, hydrated, dolomitic Finishing lime CaO MgO 2H2O CaO 0.423 2.363 Source of CaO... 

The sulfur dioxide penetrates the pores and reacts with the calcium oxide to form solid calcuim sulfate that can be removed with the ash. Dolomite (CaCOs MgC03) and hydrated lime [Ca(OH)2] are also used as sorbents. Sulfur scrabbers based on a variety of chemical reactions have become more common since 1990. Such systems produce by-products with some commercial value, such as elemental sulfur, sulfuric acid, and gypsum. Scrubbers have added benefits of removing some NOx, mercury, arsenic, and other pollutarrts that either crrrrerrtly or in the futrrre may fall rrrrder formal regulation. However, scrubbers add about 25% to the capital and operating costs of a power statiorr, leading most power stations to switch to low-sulfirr coals rather than build scrubbers. 

A high level of MgCOs can create a thermal demand on the roaster, resulting in the need for auxiliary fuel (e.g. coal). Furthermore, the dolomite content in the roaster product will be a net acid user in the downstream atmospheric leach step. On the other hand, the MgO inside the fluidized bed roaster can capture some of the released SO2 values, and lower the amount of neutralizing agent (e.g. hydrated lime) required for the off-gas scrubbing. 

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