It is important to know the free, or available, CaO in burnt or hydrated limes, especially for the laboratory determination of lime consumption in cyanide tests. The so called sugar method is a convenient one and is widely used. It is based on the solubility of the CaO present in sugar solution. The carbonates and other oxides are unaffected.
Procedure
Add 1.0 gram of the lime, ground to minus 100 mesh, to a 250-cc measuring flask, then add 20 grams of cane sugar and 100 cc water. Shake the flask vigorously for several minutes, then dilute to the 250-cc mark. Let stand at least 2 hr. shaking occasionally, then let settle until the solution is clear. Pipette 25 cc, and titrate, using either sulphuric or oxalic acid as under “Protective Alkalinity,” using phenolphthalein as the indicator.
The amount of lime (CaO) found by titration multiplied by 10 equals the lime in 1 gram of the sample.
Determination of Protective Alkalinity
Protective alkalinity is usually defined as the “alkaline hydrates and half the monocarbonates.” The effect is to protect the cyanide from decomposition by acid constituents of the ore and by carbon dioxide in the atmosphere. Either oxalic acid or a mineral acid may be used as a standard. If oxalic acid be used, the reaction is
CaO + H2C2O4.2H20 = CaC2O4 + 3H2O
Thus, 126 grams H2C2O4.2H2O saturates 56 grams CaO, or 5.62 grams H2C2O4.2H2O saturates 2.5 grams CaO. Therefore, if a solution containing 5.62 grams H2C2O4.2H2O per liter is made up, 1 cc of the solution (— 0.00562 gram oxalic acid) will saturate 0.0025 gram CaO. If 25 cc of mill solution be taken for titration, each cubic centimeter of acid required will equal 0.0025 gram CaO or 0.01 per cent CaO. For example, a 25-cc sample of solution is titrated, and it is found that 3.6 cc standard acid is used. The protective alkalinity of the solution then is equivalent to 0.036 per cent CaO.
An oxalic acid solution containing 5.62 grams H2C2O4.2H2O per liter has a normality of 0.0892. A mineral acid such as H2SO4 or HNO3 of the same normality will serve equally well. The acid used should be standardized against a standard alkali.
A normal solution is one of which one liter contains a quantity of the substance, expressed in grams, equivalent to one gram hydrogen. When the solution is to be made of a salt that contains water of crystallization, the weight of such water must be taken into consideration. As in the case of oxalic acid.
H2C2O4 + 2H2O
The total atomic weight, including the water, is 126. Therefore, H2 = 126, and H = 63 = grams of the salt to be added to 1 liter of water to make a normal solution. A decinormal solution contains one-tenth of this amount, or 6.3 grams per liter.
Phenolphthalein (1 gram phenolphthalein in 50 cc alcohol, dilute to 100 cc with water) is used as the indicator, as it gives a value for monocarbonates corresponding with the definition of protective alkalinity already given.
The determination (Clennell’s method) is made on the same portion of solution used for the free cyanide test as outlined. After the required amount of silver nitrate has been added for the determination of free cyanide, a drop or two of the phenolphthalein indicator is added. If the solution be alkaline, a pink color results. The standard acid is then run in until the pink color disappears, and the burette reading is noted.