Instructional Manual For Clarification Of Startup In Source Categories Affected By New Source Performance Standards - U.s. Environmental Protection Agency - 1979 Page 97
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146 WET PROCESS - PHOSPHORIC ACID PLANTS - SUBPART T
§60.200 - 60.204
Introduction
For the purposes of this regulation, wet process phosphoric acid plants
are defined as any facilities manufacturing phosphoric acid by reacting phos-
phate rock and acid.
Affected facilities include any combination of reactors,
filters, evaporators, and hotwells.
Emissions of total fluorides are limited
to 10 g/metric ton (0.02 lb/ton) of equivalent P20S feed.
Equivalent P2 0 S
feed is defined as the quantity of phosphorous, expressed as phosphorous
pentoxide, fed to the process.
Sources constructed, reconstructed, or modi-
fied after October 22, 1974, are subject to the regulation.
Process Description
The sulfuric acid or wet process produces phosphoric acid by the reaction
of phosphate rock with sulfuric acid which also results in the precipitation of
calcium sulfate (gypsum).
Figure 18 is a simplified flow diagram of the pro-
cess.
Phosphate rock is ground in a special ball mill with dilute phosphoric
acid (recycled) and the resultant slurry mixture is then passed into multiple
reactor and digesting tanks for reaction with sulfuric acid.
Cooling is re-
quired to remove the heat of reaction.
Water vapor and gaseous impurities are
carried to an absorber where fluosilicic acid is recovered.
Acid digestion of
the slurry requires 4 to
8
hours at temperatures of about 750C (167 0 F).
Violent
agitation and close temperature control are required for the production of
uniform, easily washed and filtered gypsum (CaS04 • 2H20) crystals.
Without
this close control, the anhydrite would form, become subsequently hydrated, and
result in plugging of pipes.
Slurry from the digester tanks passes into horizontal, rotating, tilting-
pan-type vacuum filters where phosphoric acid (30 to 35 percent P20S) is removed
from the filter cake.
This acid filtrate is then concentrated to 54 percent
P20S by evaporation.
Pre-Startup Operations
Mechanical checkout is begun as soon as specific units or groups of inter-
related units are reported as mechanically complete by the construction con-
tractor.
These checks include:
•
alignment of all motor driven equipment
•
hydrostatic testing of all liquid handling equipment
85
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