141. Determination of Quantity of Sulfuric Acid Necessary for Solution of a Mineral Phosphate.—The theoretical quantity of sulfuric acid required for the proper treatment of any phosphate may be calculated from its chemical analysis and by the formulas and reactions already given. For the experimental determination the method of Rümpler may be followed.[123]
Twenty grams of the fine phosphate are placed in a liter flask with a greater quantity of accurately measured sulfuric acid than is necessary for complete solution. The acid should have a specific gravity of 1.455 or 45° B. The mixture is allowed to stand for two hours at 50°. It is then cooled, the flask filled with water to the mark, well shaken, and the contents filtered. Fifty cubic centimeters of the filtrate are treated with tenth normal soda-lye until basic phosphate begins to separate. The excess of acid used is then calculated. Example: Twenty grams of phosphate containing 28.3 per cent of phosphoric acid, 10.0 per cent of calcium carbonate, 5.5 per cent of calcium fluorid, and 2.4 per cent of calcium chlorid were treated as above with sixteen cubic centimeters of sulfuric acid containing 10.24 grams of sulfur trioxid. In titrating fifty cubic centimeters of the filtrate obtained as described above, 10.4 cubic centimeters of tenth normal soda-lye were used, equivalent to 0.0416 gram of sulfur trioxid. Then 10.24 × 50 ÷ 1000 = 0.5120 = total sulfur trioxid in fifty cubic centimeters of the filtrate, and 0.5120 - 0.0416 = 0.4704 gram, the amount of sulfur trioxid consumed in the decomposition.
Therefore the sulfur trioxid required for decomposition is 47.04 per cent of the weight of the phosphate employed. One hundred parts of the phosphate would therefore require 47.04 parts of sulfur trioxid = to 73.6 parts of sulfuric acid of 1.710 specific gravity or 92.1 parts of 1.530 specific gravity.
A more convenient method than the one mentioned above consists in treating a small quantity of the phosphate, from one-half to one kilogram, in the laboratory, or fifty kilograms in a lead box, just as would be practiced on a large scale. A few tests with these small quantities, followed by drying and grinding will reveal to the skilled operator the approximate quantity and strength of sulfuric acid to be used in each case. The quantities of sulfuric acid as determined by calculation from analyses and by actual laboratory tests agree fairly well in most instances. There is, however, sometimes a marked disagreement. The general rule of practice is to use always an amount of sulfuric acid sufficient to produce and maintain water-soluble phosphoric acid in the fertilizer, but the sulfuric acid must not be used in such quantity as to interfere with the subsequent drying, grinding, and marketing of the acid phosphate.
For convenience the following table may be used for calculating the quantity of oil of vitriol needed for each unit of weight of material noted:
One Part by Weight of Each Substance Below Requires:
| Sulfuric Acid by Same Unit of Weight. | |||||
|---|---|---|---|---|---|
| At 48° B. | At 50° B. | At 52° B. | At 54° B. | At 55° B. | |
| Tricalcium phosphate | 1.590 | 1.517 | 1.446 | 1.382 | 1.352 |
| Iron phosphate | 1.630 | 1.558 | 1.485 | 1.420 | 1.390 |
| Aluminum phosphate | 2.025 | 1.930 | 1.839 | 1.756 | 1.721 |
| Calcium carbonate | 1.640 | 1.565 | 1.495 | 1.428 | 1.411 |
| Calcium fluorid | 2.006 | 2.010 | 1.916 | 1.830 | 1.794 |
| Magnesium carbonate | 1.940 | 1.860 | 1.775 | 1.690 | 1.660 |
Example.—Suppose for example a phosphate of the following composition is to be treated with sulfuric acid; viz.,[124]
| Moisture and organic | 4.00 | per cent. |
| Calcium phosphate | 55.00 | “ |
| Calcium carbonate | 3.00 | “ |
| Iron and aluminum phosphate | ||
| nearly all alumina | 6.50 | “ |
| Magnesium carbonate | 0.75 | “ |
| Calcium fluorid | 2.25 | “ |
| Insoluble | 28.00 | “ |
Using sulfuric acid of 50° B., the following quantities will be required for each 100 kilograms.