Abstract |
Wet-process acid is produced by treating fluorapatite (Ca10(PO4)6F2) or phosphate rock, with sulfuric acid. Phosphate rock must be finely ground to react properly with sulfuric acid, and standard control equipment is normally used to prevent objectionable dust emissions. The emissions of most concern are fluoride compounds liberated from the rock by the sulfuric acid. These consist of hydrogen fluoride, silicon tetrafluoride, and some products of reaction and decomposition of the latter. Most phosphate rock contains 3.5 to 4 percent fluorine, and half of this may be volatilized in the processing. This represents a large potential source of pollution. Because of the principal atmospheric contaminants generated in the process are gaseous fluorides, vapor scrubbing is universally employed to control emissions. Specific devices used for control include venturi scrubbers, impingement scrubbers, and various kinds of spray towers. The results of MCA-PHS stack tests on ten wet-process phosphoric acid plants in various parts of the country are tabulated. The major source of gaseous fluoride emissions in wet-process phosphoric acid plant is the digester. Only trace quantities of particulate fluorides are normally present in exit gases from digesters and filters, and these can be removed effectively by scrubbing. (Author) |