Question

During the manufacturing of superphophates, fluorine and silicon impurities combines with water to form:

• A. silicic acid
• B. fluorosilicic acid
• C. silicon tetrafluoride
• D. form their oxides

Answer 1

Answer: (B)

fluorosilicic acid

The reactions occurring during the production of superphosphate are complex and are usually summarised as follows :

1. $C{a}_5(P{O}_4{)}_{3}F+5H_{2}S{O}_4\to 5CaS{O}_4+3H_{3}P{O}_4+HF$

2. $C{a}_5(P{O}_4{)}_{3}F+7H_{3}P{O}_4+5H_{2}O\to 5Ca(H_{2}P{O}_4{)}_2{H}_{2}O+HF$

These reactions can be combined to give the overall equation :

$2C{a}_5(P{O}_4{)}_{3}F+7H_{2}S{O}_4+3H_{2}O\to 7CaS{O}_4+3Ca(H_{2}P{O}_4{)}_2{H}_{2}O+2HF$

There are other reactions occurring at the same time. For example, virtually all the HF reacts with other silica minerals associated with the fluorapatite (silicates, quartz) to form silicon tetrafluoride. These gaseous emissions are recovered as hydroflusilicic acid $\left(H_{2}Si{F}_6\right)$ in the scrubbing system. Carbonates in the rock also react with sulfuric acid.