Chemistry of inorganic compounds – Phosphorus or compound thereof – Oxygen containing
Reexamination Certificate
1994-07-01
2001-05-22
Langel, Wayne (Department: 1754)
Chemistry of inorganic compounds
Phosphorus or compound thereof
Oxygen containing
C210S710000, C210S724000, C423S157400, C423S170000
Reexamination Certificate
active
06235257
ABSTRACT:
TECHNICAL FIELD
This invention relates to the generation, use, and treatment of wet-process phosphoric acid process waters, eliminating conventional acid pond water and cooling ponds and increasing product recovery, thereby minimizing environmental disturbance and/or product loss.
BACKGROUND OF THE INVENTION
Wet-process manufacturing of phosphoric acid is so standardized that its procedures are well known and are described as conventional in authoritative texts. An accepted reputable authority upon this subject is Pierre Becker,
PHOSPHATES AND PHOSPHORIC ACID—Raw Materials, Technology, and Economics of the Wet Process
, published by Marcel Dekker, Inc. (New York, 1983). Of particular interest is chapter
9
thereof, entitled “What to Do with Gypsum” (pp. 471-496). The present inventors consider Becker's identified treatise as the prime authority on conventional wet-process phosphoric acid practice, from which the present invention departs as described below.
Such conventional processing produces by-product gypsum stacks from which very acidic water drains into extensive cooling ponds, often measuring several hundred acres each. Conventional gyp[sum] pond water is extremely acid, usually having a pH of about 1.8-2.0. Also, it is composed of weak phosphoric acid, as in a range from 1.3-2.5% P
2
O
5
, fluosilicic acid and soluble fluorides ranging from 0.5-1.8%, sulfuric acid and soluble sulfates ranging from 0.5-1.6%, plus soluble contaminating metal salts and radioactive compositions Conventional acid gypsum pond water retains in solution—and so is likely to pollute ground water with—residual phosphoric acid and soluble fluorides, dissolved metal impurities and radioactive compounds, and it emits fluoride-containing gases. This is a serious drawback of conventional wet-process phosphoric acid manufacturing.
Attempts to limit resulting air, water, and soil contamination have had limited success. Randolph in U.S. Pat. No. 3,625,648 proposed a rather diffuse range of neutralization of acidic gypsum pond waters effective to reduce some contaminants by precipitation. The present inventors also undertook to reduce fluoride contamination, Palm as in U.S. Pat. No. 3,699,212 and Hartig as in U.S. Pat. No. 3,720,757, and jointly they subsequently developed a two-stage neutralization of recirculated acid waters, as in U.S. Pat. No. 4,320,012.
Other examples of ameliorative efforts include treating gypsum pond waters to remove metallic ions as in Mills U.S. Pat. No. 4,303,532; to recover calcium fluoride as in Hirko & Mills U.S. Pat. No. 4,171,342 or O'Neill U.S. Pat. No. 4,374,810; or to remove calcium fluoride so as to provide a reduced fluoride solution for use in wet ball-milling of phosphate rock as in O'Neill et al. in U.S. Pat. No. 4,472,368. Hirko et al. U.S. Pat. No. 4,171,342; Zibrida in U.S. Pat. Nos. 4,657,680 and 4,698,163; and Murray et al. in U.S. Pat. No. 5,112,499.
The industry and the inventors, despite such efforts, have not successfully eliminated the dependency of conventional wet-process phosphoric acid manufacturing upon acid pond water with its load of contaminants threatening pollution of adjacent air, soil, and water.
The present invention, while retaining benefits of the present inventors' identified early joint invention, is directed to modifying the wet-process manufacturing of phosphoric acid with the result of substantially eliminating pollution attributable to conventional gypsum pond waters and minimizing the loss of P
2
O
5
while doing so.
SUMMARY OF THE INVENTION
A primary object of this invention is to eliminate conventional acid pond water cooling ponds and their attendant pollution.
Another object of the invention is to supersede a conventional gypsum stack, often constructed unlined on raw land, with a gypsum stack improved by having a liner against ground water contamination.
A further object of this invention is to replace conventional contaminant-bearing acid gypsum pond water by new gypsum stack water whose contamination is limited to reduced F and P
2
O
5
content.
Yet another object of the invention is to minimize P
2
O
5
losses in wet-process phosphoric acid manufacturing.
A still further object is to limit water discharged into surface waters to decontaminated water of compatible pH.
In general, the objects of the present invention are attained by providing new not-so-acidic process waters in place of conventional wet-process acid gypsum pond water, which is discontinued and is superseded. Such new acidic process waters result from aqueous collection of waste gases and liquids from the conventional process.
Circulating clean water with a pH of about 6 to 7 through conventional wet-process flash coolers and evaporators/condensers, and mopping up of P
2
O
5
process leaks, spills, and wash liquids provides an acid process water (APW) useful in preliminary washing of process gypsum filter cake. The resulting APW is supplied via filter cake washing to the wet-process phosphoric acid reactor, thus recovering substantially all water-soluble P
2
O
5
normally lost in conventional processing. Some of its fluosilicic acid reacts with phosphate rock therein, producing phosphoric acid, precipitating some fluorides, and reducing the consumption of sulfuric acid. This acid process water is contained in process tanks, pipelines, and like equipment, well segregated from contact with external air, soil, and water.
Scrubbing vent gases from the process reactor and other process equipment captures gaseous fluorides. Partially neutralized scrubber water at a pH of about 4 to 5—typically 4.5—and clarified is thereby freed of most fluorides and of other contaminants insoluble at that pH. This water is useful in part in wet-rock grinding at the head-end of the wet-process, and in other substantial part is alkalized to a pH of about 10.5 to 12—typically 11—freeing it of contaminants insoluble at this pH (but soluble at lower pH) and clarified, then re-acidified to a nearly neutral pH between 6 and 7. The latter decontaminated pH-adjusted water (here called pH-AW) is non-scaling and is useful for such purposes as filter wash, scrubber water make-up, cooling tower water make-up, acid process water makeup, limestone slurrying, and lime slaking and slurrying.
Alternatively, pH-AW may be discharged to surface water after ascertaining that their respective pH's match reasonably closely.
Washed gypsum filter cake is slurried with part of the water drained from stacked gypsum (i.e., gypsum stack water or GSW), and whatever GSW is not so recycled is fed to the first of two neutralizing stages in which scrubber water is processed, as above.
Other aspects of the present invention, together with means and methods for attaining the various objects, will be apparent from the following description and accompanying diagrams of preferred embodiments, which are presented by way of example rather than limitation.
REFERENCES:
patent: 3625648 (1971-12-01), Randolph
patent: 4320012 (1982-03-01), Palm et al.
patent: 4402923 (1983-09-01), Lang
patent: 4698163 (1987-10-01), Zibrida
patent: 4777027 (1988-10-01), Davister et al.
patent: 5112499 (1992-05-01), Murray et al.
patent: 5316748 (1994-05-01), Palm et al.
patent: 5366640 (1994-11-01), Palm et al.
Pierre Becker,Phospates and Phoshoric Acid—Raw Materials, Technology, and Economics of the Wet Process, published by Marcel Dekker, Inc. (1983), pp. 471-496.
Hartig R. George
Palm Gordon F.
Langel Wayne
McClure Charles A.
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