Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Alkaline earth metal
Reexamination Certificate
2000-11-15
2003-05-13
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Alkaline earth metal
C423S157400
Reexamination Certificate
active
06562308
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the area of ore processing, particularly to the wet processing of phosphate rock.
BACKGROUND OF THE INVENTION
One of the primary uses of phosphoric acid is in the production of artificial fertilizers. Phosphoric acid is made on an industrial scale by extracting phosphate (expressed as phosphorus pentoxide (P
2
O
5
)) from phosphate rock by acidulating the phosphate rock to form a solution of phosphoric acid. In the past, easily mined high-grade phosphate rock deposits provided phosphate without the need for complicated purification processes. With the depletion of such high-grade deposits, lower grade deposits must be employed. However, the presence of a larger amount of impurities, such as metals including iron, in the lower grade rock has necessitated the increased usage of processes for the purification (beneficiation) of the phosphate rock. Such beneficiation processes add to the cost of producing the phosphate and negatively affect the overall P
2
O
5
recovery.
Purification of the phosphate rock is generally desirable in industry, such as in the fertilizer industry, since the absence of impurities means that there is more phosphate present per unit weight of the final acid product. Less of the product is then required leading to a decrease in the cost of transporting it. Furthermore, a higher-grade product leads to a decrease in the amount of handling and reduces the amount of sludge produced during processing. Finally, higher-grade phosphate rock reduces scaling concerns faced by processors.
Fertilizers, for example, may be produced from phosphate solutions by concentrating the phosphoric acid solution obtained from the acidulation of phosphate rock followed by reaction of the solution with anhydrous ammonia to form monoammonium phosphate (MAP) as a wet solid. MAP is then granulated and dried to yield dried granules of fertilizer. Concentration of the phosphoric acid solution may be accomplished by such methods as vacuum evaporation and submerged-combustion direct heating. Superphosphate fertilizers such as normal superphosphate (NSP) and triple superphosphate (TSP) may also be produced from phosphoric acid solutions. For a review of fertilizer production, see
Fertilizer Manual
, T. P. Hignett ed., (International Fertilizer Development Center, Muscle Shoals, Ala., USA (1985)) pp. 187-202.
The wet processing of phosphate rock generally involves the reaction of ground phosphate rock with an acid such as sulphuric acid or mixtures of different acids. The reactant solution used in such a process is often based on a recycled acid solution already containing phosphoric acid to which more sulphuric or other acid is added. Phosphate dissolves in the acid solution and is present in solution in the form of phosphoric acid. The solution can be isolated from the residue by a variety of methods including filtration, centrifugation and froth flotation. The following reference provides a discussion of the wet process technique: Becker, P., “Phosphates and Phosphoric Acid”,
Fert. Sc. and Tech. Series
, (Marcel Dekker, Inc., N.Y. (1983)) pp. 369-403.
Wet processing has disadvantages. The residue may be slime rather than a crystalline solid making the isolation of the solution more difficult. This can be ameliorated by digesting the phosphate rock for a longer period of time, as in the Prayon process (Slack, A. V., “Phosphoric Acid”, Vol. 1
, Fert. Sc. and Tech. Series
, (Marcel Dekker, Inc., N.Y. (1968) pp. 253-258) thus promoting the growth of larger insoluble solid residue particles, generally gypsum (calcium sulphate) formed from the calcium in the phosphate rock and the sulphate from sulphuric acid. While larger residue particles may be formed in this manner, the longer digestion time results in more of the impurities solubilizing, thus contaminating the product phosphoric acid. The temperature and time of reaction in the Prayon process are responsible for the unwanted solubilization of impurities. Therefore, there is a need for a process that favours the dissolution (and therefore recovery) of phosphate while reducing the amount of impurities recovered with the phosphate and favouring the formation of easily separable residue.
U.S. Pat. No.4,039,624 issued on Aug. 2, 1977 to Hill, discloses a process for producing phosphoric acid from high iron and aluminum content phosphate rocks using nitric acid. This process employs relatively coarse particles of phosphate rock (~0.5 mm) and the leach time is very long (~1 hours). Furthermore, this process requires the presence of high levels of iron and aluminum and requires iron in a non-hydrated form (e.g. hematite). There still remains a need for a process that can be generally applied to phosphate rock containing different impurities and that requires less time to effect good separation of the phosphate from the impurities.
U.S. Pat. No. 3,919,395 issued on Nov. 11, 1975 to Hauge discloses a process for extraction of phosphorus compounds from low and high grade phosphate ore using dilute mineral acids whose calcium salts are water soluble. This process employs coarsely ground ore (larger than 100 mesh (150 microns) and requires a neutralization step using ammonia or lime.
In another process, high iron containing phosphate rock is leached with nitric or hydrochloric acid to form a solution of phosphoric acid and a concentrate containing the iron (Forssberg, Eric and Adolfsson, Goran, “Dephosphorization of High Phosphorus Iron Ores by Acid Leaching ”,
Erzmetall
. 34(6): 316-322 (1981)). This process is focussed on the recovery of iron rather than phosphorus. The process uses relatively coarse particle sizes (in ranges from 75 to 6700 microns) and leaching occurs over a long period of time (~24 hours). The paper states that sulphuric acid is unsuitable in the process because the formation of calcium sulphate in the concentrate lowers the Fe-content from 61 to 56%. This means that a significant amount of iron is being leached into the acid solution along with phosphate.
U.S. Pat. No. 4,828,811 issued on May 9, 1989 to Derdall et al discloses a process and apparatus for producing phosphoric acid from phosphate ore wherein a slurry of phosphate ore in phosphoric acid is processed in a multi-zone reactor in which coarse solids and “fine” solids are processed separately. This patent refers to “fine” solids which are typically about—65 mesh (>150 microns).
The effect of particle size on the dissolution of phosphate rock by mixtures of sulphuric acid and phosphoric acid has been studied (Gilbert, Richard L. and Moreno, Edgar C. “Dissolution of Phosphate Rock by Mixtures of Sulfuric and Phosphoric Acids”, I&EC
Process Design and Development
, 4(4): 368-371 (Oct., 1965)). While this study generally shows that reducing phosphate rock to smaller particle sizes favours the dissolution of phosphate, there is no teaching of favourable separation of impurities present in the phosphate rock. There is no indication in this reference that reducing the particle size not only increases the solubilization of phosphate but also selectively increases the solubilization of phosphate in relation to an impurity.
SUMMARY OF THE INVENTION
There is provided a process for recovering phosphate from phosphate rock comprising:
(a) leaching finely divided particles of phosphate rock with a protic acid at a temperature and for a time that favours dissolution, into a leachate, of phosphate in relation to an impurity; and,
(b) isolating the leachate.
There is also provided a process for separating phosphate from an impurity in phosphate rock comprising leaching finely divided particles of phosphate rock with a protic acid at a temperature and for a time that favours retention of the impurity in a solid residue in relation to retention of the phosphate in the solid residue.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Phosphate rock includes all naturally occurring mineral deposits containing phosphate as a component. Phosphate deposits can encompass variations and differing compositions within the same source and can hav
Carstens Leslie L.
Wynnyk Nick P.
Agrium Inc.
Langel Wayne A.
Woodcock & Washburn LLP
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