Production of phosphates

Various stages of the process of producing phosphates are subject to a vacuum produced by a liquid ring pump in various materials of construction using relatively high concentrations of sulfuric acid as the compressant liquid.

BACKGROUND OF THE INVENTION 
This invention is concerned with the production of phosphates. The basic 
process involves subjecting phosphate bearing rock to a sulfuric acid 
treatment to produce phosphoric acid, commonly called P.sub.2 O.sub.5 in 
the trade, and a precipitation of sulphates, particularly calcium 
sulphate. The phosphoric acid is then purified by subjecting it to 
filtration, evaporation, crystallization and other processes. 
The vacuums used in these processes have been produced by steam ejectors, 
the steam for which is derived from the sulfuric acid plant which 
generates steam as a by-product. Although it is well known that the 
efficiency of steam ejectors, as compared with other vacuum producers is 
poor, the carry-over of the corrosive materials of the process to the 
vacuum source has discouraged the use of other vacuum producers. 
When the sulfuric acid plants are new, the steam generated by them is 
sufficient to run the ejectors, but as the plants age and their 
efficiencies decrease this is no longer the case, particularly at times of 
peak production. To meet the steam demand of the ejectors, it has been the 
practice to utilize either an extra sulfuric acid plant or a donkey 
boiler. The capital investment, operating and maintenance costs of the 
additional sulfuric acid plant or donkey boiler would be considerable. 
A vacuum pump capable of producing the vacuums required in the processes is 
a liquid ring pump such as that described in U.S. Pat. No. 3,154,240 
issued Oct. 27, 1964, to Irving Callender Jennings. However, those pumps, 
even though much more efficient than steam ejectors, may not be used since 
they increase the fresh water requirements of the plant, increase the 
waste water volume and the waste water treatment necessary to handle this 
increase. Further, these pumps are subject to corrosion when constructed 
of cast iron. 
Cast iron pumps of this kind utilized in the production of phosphates would 
be chemically attacked by carryover from the vacuum filter, the 
crystallizer and phosphoric evaporators. This enters the pumps to form 
chemically aggressive compounds with the water of the ring. Additionally, 
the gases being handled include silicon tetrafloride which also reacts 
with the water of the ring to produce aggressive compounds. Similarly, the 
third stage evaporators of conventional phosphate plants produce hydrogen 
fluoride which again is corrosive to cast iron when reacted with the water 
of the ring. 
Additionally, these third stage evaporators run at higher vacuums than a 
liquid ring can efficiently produce using the warm sealing water available 
at most phosphoric acid plants, because of the vaporization of water from 
the ring. 
According to this invention, there is provided a phosphate producing plant 
which includes liquid ring pumps of which those parts contacted by the 
liquid seal or ring liquid of the pumps is of cast iron, stainless steel 
or other material and the pumps are operated with a sulfuric acid ring 
liquid. There is also provided a method of producing phosphates which 
includes subjecting the various purification stages for the phosphoric 
acid to vacuums produced by liquid ring pumps which have a sulfuric acid 
ring liquid. Preferably, the method includes the recirculation of the 
sulfuric acid of the ring liquid back to the process system to be utilized 
in the treatment of the phosphate bearing rock. Alternatively, the ring 
liquid may be cooled and recycled to the vacuum pump with periodic 
additions of fresh acid to keep the proper concentration. 
The corrosion rate of cast iron is low in the presence of concentrated 
sulfuric acid. The acid further minimizes the corrosion by absorbing water 
vapor from the incoming gas, to preclude formation of aggressive chemical 
compounds in the liquid ring. Acid concentration can be regulated to avoid 
dilution and subsequent corrosion of the cast iron pump. 
While for cost considerations a cast iron pump is desirable, pumps of other 
materials--for example stainless steel--may be used in this process with 
sulfuric acid seal. The use of stainless steel lowers the corrosion rate 
still further and allows the use of warmer and less concentrated sulfuric 
acid compressant. 
An added advantage of sulfuric acid over conventional water compressant is 
its lower vapor pressure. It is known that the volumetric efficiency of 
liquid ring pumps decreases with increasing vapor pressure of the 
compressant liquid--especially at higher vacuum levels. For a given 
temperature, the vapor pressure of sulfuric acid is very much lower than 
that of water. This enhances the high vacuum capability of the liquid ring 
pump. 
Additionally, environmental problems associated with a water seal are 
averted by using sulfuric acid. The spent water from liquid ring pumps is 
contaminated with carry-over from the phosphate process and so it must be 
impounded or treated. On the other hand, effluent sulfuric acid may be 
circulated to the production line. Whatever water, chemical and solid 
contamination it picks up in the vacuum pump should not prevent the use of 
the acid for treating phosphate bearing rock. Thus, there is negligible 
added burden to waste treatment or impounding facilities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
In FIG. 1 there are illustrated conventional parts of a phosphate 
production line including a vacuum filter 10 from which the filtrate is 
delivered to a receiver 12, filtrate or product being removed from the 
receiver at line 14. Line 16 extends between the receiver and a demister 
18 and the demister is connected by line 20 to the suction inlet 22 of a 
liquid ring pump 24, the discharge of the pump being connected to a 
separator 26. The pump has sulfuric acid liquid delivered through line 25 
and from the lower part of the separator 26 the sulfuric acid is delivered 
by pump 28 to the production line to be utilized in the processing of the 
phosphate bearing rock. From the upper end of the separator 26, a line 30 
extends to atmosphere or to other treatment stages that line 30 including, 
if necessary, a demister 32 and a return line 34 for delivering a 
disentrained sulfuric acid to the separator 26. 
FIG. 2 of the drawings illustrates typical arrangements for first, second 
and third stage phosphoric acid evaporators. 
Phosphoric acid is delivered as a solution along line 40 to evaporator 
chamber 42 from the lower portion of which phosphoric acid slurry is 
removed as a concentrate at line 44. Vapor is passed through line 46 to a 
barometric condensor 48 to which water or other condensing liquid such as 
sulphuric acid is delivered along line 50 and from which condensate is 
delivered to seal pit 52. The condensor 48 is connected to the inlet of a 
liquid ring pump 54, the outlet of which is delivered to separator 56. As 
with the filter illustrated in FIG. 1 from the separator gas is delivered 
via an appropriate line to atmosphere, that line including a demister, if 
necessary, and sulfuric acid is pumped to be recycled in the rock treating 
process. 
The embodiment of FIG. 3 shows a crystallizer or flash cooler section of a 
phosphate production line in which phosphoric acid solution is delivered 
along line 60 to the crystallizer or flash cooler 62 from which phosphoric 
acid slurry is removed at 64. As in the embodiment illustrated in FIG. 2, 
the gas from the crystallizer is delivered to a barometric condensor 66 
provided with a water or other condensing liquid such as sulfuric acid 
line 68 and condensate is delivered to a seal pit 70. The condenser is 
connected to the inlet of a liquid ring pump 72 which, as in the 
embodiments of FIGS. 1 and 2, delivers to a separator 74 from which 
noncondensibles are discharged through a demister and from which sulfuric 
acid is returned to the process via a pump. 
The utilization of sulfuric acid as a condensing liquid may be feasible in 
certain situations and this will appreciably reduce the overall water 
requirements from the system resulting in the advantages discussed supra. 
It will be recognized that with the conventional steam ejector the use of 
acid is not possible since, of course, the propellant of the ejector 
issues to atmosphere and would include an unacceptably polluting carryover 
from the condenser. By the utilization of the liquid ring pump as in this 
invention, this problem would not occur. 
FIG. 4 illustrates an alternate technique for the handling of sulfuric acid 
compressant. Acid is recirculated through a cooler 57 to the vacuum pump 
58. Acid concentration is maintained within acceptable limits by addition 
of fresh, concentrated acid on a continuous or batch basis. An acid 
reservoir 59 may be used to augment the liquid reserve of separator 60. 
An alternative technique for maintaining the concentration of sulfuric acid 
re-circulated to the pump is indicated in chain line in FIG. 4. In this 
arrangement a supply 80 of concentrated sulfuric acid is provided and a 
line 82 leads from that supply to the connection between the separator and 
the cooler. Line 82 includes a valve V which is controlled by an acid 
concentration monitor C arranged to sample the concentration of acid in 
the line connecting the separator and pump. 
It will be understood that the valve could, if desired, be manually 
operable. The line issuing from the separator to the cooler will include a 
liquid pump to provide the necessary pressure at the seal liquid inlet to 
the vacuum pump. To remove from that line the excess acid there is 
provided a pressure controlled bleed off valve 84, the outlet of which 
leads, most desirably, back to the process. The valve is controlled by a 
controller 86 which modulates valve 84 to maintain a seal liquid inlet 
pressure to vacuum pump 58. 
The liquid ring pump of each of the embodiments herein may take the form of 
any of the well-known kinds, it may be a center ported pump or it may be a 
side ported pump, or it may be a combination of those pumps. Parts of the 
pump contacting the ring liquid may be of cast iron, stainless steel or 
other material. The ring liquid is to be sulfuric acid in concentrations 
chosen for minimum corrosion rate of the pump. By the adoption of this 
arrangement, an inexpensive plant is provided with efficient pumps not 
subject to the corrosive action of the materials of the process and which 
minimizes adverse environmental efforts. Further, this eliminates the 
shortcomings of the steam ejectors discussed supra.