High-calcium lime

Hydraulic limes (84) may be used for mortar, stucco, or the scratch coat for plaster. They harden slowly under water, whereas high calcium limes, after slaking with water, harden in air to form the carbonate but not under water at ordinary temperatures. However, at elevated temperatures achieved with steam curing, lime—silica sand mixtures do react to produce durable products such as sand—lime bricks. 

With regard to chemical use -diluted ferric chloride solutions (10% to 20%) usually give better results in the conditioning of the sludge. A high calcium lime is preferable or sludge filtration work. One should avoid excessive use of chemicals. The quantities of chemicals used for conditioning can be frequently reduced by careful control of the... 

If young leaves are pale and growing tips die, the problem may be calcium deficiency. Add high-calcium lime, dolomitic lime if magnesium is also low, wood ashes, or gypsum to the soil. 

EN 12485 Chemicals used for treatment of water intended for human consumption — calcium carbonate, high-calcium lime and half-burnt dolomite — Test methods (in preparation). 

Nevertheless, it is believed that a significant quantity of high-calcium lime-stone/calcium carbonate is used for this purpose. A two-stage neutralisation process may offer the best overall economics. In the first stage, calcium carbonate is used to neutralise the greater part of the acid under acidic conditions. In the second stage, an alkali is used to neutralise the residual acid and to raise the pH to the required level. For more details of the chemistry involved, see section 28.3. 

Above the dissociation temperature of calciiun hydroxide (ca 540 °C), and below the dissociation temperature of calcite (ca 800 °C), equation (15.1) (see section 15.2.1) can proceed to the left with evolution of heat. Porous high-calcium limes can absorb over 50 % of the theoretical amount of CO2, whereas dense limes may only absorb 3 to 5 % of CO2 [15.8]. 

Average long-term values, for high calcium limes, excluding use for heating or drying fuel. 

Some very light-burned dolomitic limes are hydrated at atmospheric pressure in a similar way to high-calcium limes. They use maturing bunkers to provide 12 to 24 hours of residence at 80 to 90 °C in the presence of excess water. 

Tables 27.3 and 27.4 summarise typical specifications for high-calcium lime [27.11] and calcined dolomite [27.12] used as fluxing agents in the BOS process. Both products should be ...

High-calcium limes, which include lump and pulverised quicklime, hydrated lime and milk of lime, are categorised in [28.3] into three Types (Table 28.1), depending on the levels of the major and minor components, namely ... 

Table 28.1. High-calcium lime for use in water treatment grading (proposed limits)...

Ground" refers to pulverised high-calcium quicklime, lump" refers to granular high-calcium quicklime, hydrate" refers to hydrated high-calcium lime and milk" refers to milks of lime % values are by mass. 

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. 

High calcium lime is generally specified, although some users require dolomitic lime [30.1]. A low level of impurities is desirable to minimise the amount of sludge produced. Some impurities such as arsenic can pass into the final product and limits may be imposed (e.g., less than 2mg/kg of arsenic). 

ASTM C911 [31.5] (formerly ASTM C 258) specifies the quality of quicklime required by the process. The particle size requirements are described in section 31.4.1. From the chemical viewpoint, a high calcium lime is required with a MgO content of less than 2 % as it volatilises and can produce troublesome deposits. The level of minor impurities should be limited (Si02 < 2 %, Fe203 + AI2O3 < 1 %) and phosphorous should be less than 0.02 %. The loss on ignition (combined CO2 and H2O) should not exceed 4 %. 

Relatively little lime is used in glass manufacture as limestone is generally more cost-effective (see section 12.2). However, dolomitic lime and occasionally high calcium lime are used in finely ground forms under specific circumstances. 

High-calcium lime is quick- or slaked lime produced from high-calcium limestone. 

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