Question

During the manufacturing of single superphosphate, excess silicon combines with acid to form:

• A. silicates
• B. silicic acid
• C. silanes
• D. hydride of silicon

Answer 1

Answer: (B)

silicic 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.