Liquid purification or separation – Processes – Preventing – decreasing – or delaying precipitation,...
Reexamination Certificate
2002-02-21
2004-07-06
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Preventing, decreasing, or delaying precipitation,...
C210S638000, C210S639000, C210S724000, C210S726000, C210S906000, C423S321100, C423S331000, C423S332000
Reexamination Certificate
active
06758976
ABSTRACT:
BACKGROUND OF THE INVENTION
Production of phosphoric acid by what is commonly know as the “wet process” involves the reaction of finely ground phosphate rock with sulfuric acid. As a result of the various reactions, a slurry is produced containing phosphoric acid, calcium sulfate and various impurities derived from the phosphate rock. This slurry is normally filtered to separate the phosphoric acid product from the byproduct calcium sulfate. The phosphoric acid thus obtained is then used in the production of various phosphate fertilizers that are used in agriculture. Water is normally used to wash the calcium sulfate filter cake and thereby increase the recovery of the phosphoric acid product. Most of this wash water is fed back into the phosphoric acid production process as make-up water. However, a portion of this water remains trapped in the calcium sulfate filter cake and is discharged from the filter with the filter cake. This trapped water contains small amounts of phosphoric acid and small amounts of all other impurities that were present in the phosphoric acid product. Additional water is normally used to wash the calcium sulfate filter cake off of the filter and transport it, by pumping as a slurry, to a storage or disposal area.
At the storage or disposal area the calcium sulfate will settle and the excess water will be liberated. This liberated water will normally be collected in a system of channels and ponds and recycled to the phosphoric acid production plant for reuse (i.e., washing the calcium sulfate filter cake). These channels and ponds also serve as a collection means for other water that is used in and around the phosphoric acid plant, such as for cleaning or washing, fresh water fume scrubbers, and as a collection means for phosphoric acid spills or leaks within the plant. Also, since these channels and ponds are located outside, they collect rain water.
Since all of the water contained within these channels and ponds contains small amounts of phosphoric acid and other impurities normally present in the phosphoric acid, it is considered contaminated. Thus, before this water can be released to the environment, it must be treated or purified to remove the phosphoric acid and other impurities. In some cases, in an efficiently operated phosphoric acid plant, in the absence of severe weather conditions, a balance will exist between water input to the pond system and water evaporation such that virtually all of this contaminated water can be recycled and used within the plant. In this case, treatment and discharge of the contaminated water, commonly known as pond water, is not necessary.
However, there are circumstances under which treatment and discharge of the contaminated pond water is necessary. One such circumstance could be an extended period of abnormally heavy rainfall. Another such circumstance would be when the phosphoric acid plant has ceased operation ether for an extended period of time or permanently.
Many factors influence the specific components and their concentrations in this contaminated pond water. Thus, there is no typical composition for the pond water other than the fact that it will contain some phosphate. However, some of the chemical components that could be found in pond water, and an example of their range of concentrations, are as follows:
CHEMICAL COMPONENT
RANGE OF CONCENTRATION
P
1700-12,000
ppm
SO
4
4300-9600
ppm
F
200-15,000
ppm
Si
100-4100
ppm
(ammoniacal) N
40-1500
ppm
Na
1200-2500
ppm
Mg
160-510
ppm
Ca
450-3500
ppm
K
80-370
ppm
Fe
5-350
ppm
Al
10-430
ppm
Cl
10-300
ppm
One method of treating or purifying this pond water well known in the art is double liming. This method consists of adding a calcium compound (such as CaCO
3
, Ca(OH)
2
or CaO) to the pond water, in two stages, such that the phosphate and other impurities form solid precipitates that settle and are separated from the thus purified water. This method is described in Francis T. Nielsson, ed.,
Manual of Fertilizer Processing
, Marcel Dekker, Inc. (1987), pp. 480 to 482; G. A. Mooney, et al.,
Removal of Fluoride and Phosphorus from Phosphoric Acid Wastes with Two Stage Line Treatment
, Proceedings of the 33rd Industrial Waste Conference, Purdue Univ. (1978); G. A. Mooney et al.,
Laboratory and Pilot Treatment of phosphoric Acid Wastewaters
, presented at the Joint Meeting of Central Florida and Peninsular, Florida A.I.Ch.E. (1977); and U.S. Pat. Nos. 5,112,499; 4,698,163; 4,320,012; 4,171,342; 3,725,265 and 3,551,332. However, there are several problems associated with this method. One problem is the large volume of sludge produced. Sludge (i.e., a mixture of the precipitated impurities, un-reacted calcium compound and water) is produced in both the first and second stages of this process. These sludge materials are normally deposited in settling ponds that require large land areas. While it is possible to reclaim and recycle some of the sludge from the first stage of this process, the sludge from the second stage tends to be very voluminous, extremely difficult to de-water and of little economic value. Thus, large impoundment ponds are required to permanently store this sludge. Another problem with this method of pond water treatment is that, because of the large quantity of water tied up with the sludge, only about 50% to 60% of the pond water fed to the process can be discharged. Thus, the process equipment must be significantly larger than would otherwise be needed. A third problem with this process is that virtually all of the economically valuable phosphate contained in the pond water is converted to a form that renders it unsuitable for use as a fertilizer without significant re-processing at added cost. Finally, a fourth problem with this treatment process is that the purified pond water often just barely meets the criteria for discharge and cannot be used as a substitute for the fresh water that would normally be required in a phosphoric acid plant, such as for steam production.
Another general method of water purification is reverse osmosis. This process is based on the application of external pressure on an aqueous salt solution in contact with a semi permeable membrane, such that the applied pressure exceeds the osmotic pressure of the water component of the solution in contact with the membrane. Thus, some of the water is forced through the membrane in the reverse direction, while the other components in the solution (i.e., soluble salts) do not pass through the membrane. This results in a stream of purified water, known as permeate, and a stream of increased salt content, known as the reject or concentrate. Reverse osmosis is well known in the art and is described in
Encyclopedia of Separation Technology
, Volume 2, pp. 1398-1430, edited by Douglas M. Ruthven, John Wiley & Sons, Inc., 1997; Reverse Osmosis/Ultrafiltration Principles by S. Sourirajan and T. Matssuura, National Research Council of Canada, Ottawa, Canada, 1985; Reverse Osmosis Technology, B. Parekh, ed., Marcel Dekker, Inc., New York, 1988; Membrane Processes by R. Rautenbach and R. Albrecht, John Wiley & Sons, Inc., New York, 1989 and other publications. Reverse osmosis is also described in a variety of U.S. Patents, for example, U.S. Pat. Nos. 4,110,219; 4,574,049; 4,876,002; 5,006,234; 5,133,958 and 6,190,558.
Several attempts have been made to use reverse osmosis for the purification of contaminated phosphoric acid plant pond water. However, these attempts have generally failed due to the fact that the pond water is a saturated solution. Thus, as soon as any water is removed from the pond water the solution becomes supersaturated and salts precipitate that quickly clog the membranes used in reverse osmosis and prevent additional pure water from flowing through them.
However, if reverse osmosis could be made to function in the treatment of contaminated phosphoric acid plant pond water, many economic and environmental benefits would result. One benefit is that the phosphate values contained in the pond water would be recovered in an economically useful form (i.e., as a concentrat
Astley Vaughn V.
Jardine Kenneth J.
Michalski Dennis
Hruskoci Peter A.
IMC Global Operations Inc.
Sidley Austin Brown & Wood LLP
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