Phosphate acidulation utilizing HF acid

Details Phosphate acidulation utilizing HF acid Phosphate acidulation utilizing HF acid

2000-06-09

2004-01-13

Langel, Wayne A. (Department: 1754)

Chemistry of inorganic compounds

Phosphorus or compound thereof

Oxygen containing

C423S157300, C423S490000

Reexamination Certificate

active

06676914

ABSTRACT:

BACKGROUND
According to Dittmar (U.S. Pat. No. 1,018,746) phosphoric acid is obtained in a high degree of purity by mixing a phosphate of an alkali metal or of an alkaline earth metal with hydrochloric acid or hydrofluoric acid in excess “as the chlorid or the fluorid of the metal is insoluble in hydrochloric acid or hydrofluoric acid respectively, whereupon the metal is precipitated as an insoluble chlorid or fluorid, the phosphoric acid being liberated and remaining in solution so that it can be separated by filtration.” In the case of phosphates or superphosphates of alkaline earths, for example calcium or magnesium phosphate, excess of hydrofluoric acid is used.
Hechenbleikner (U.S. Pat. No. 1,313,379) treats phosphate rock with a mixture of dilute hydrofluosilicic and hydrofluoric acids.
Cross (U.S. Pat. No. 2,493,915) treats phosphate rock with a minimum amount of moisture present with sulfuric acid, hydrogen chloride or even hydrogen fluoride or any other strong acid to form phosphoric acid together with the calcium salt of the acid used in treatment. This phosphoric acid is then extracted with sulfuric ether, amyl alcohol or other volatile solvent. Any organic solvent which will dissolve the phosphoric acid but will not dissolve the calcium salt may be used. Accordingly, his claims are directed to reacting calcium phosphate material in a tower by introducing into the tower and passing upwardly through the calcium phosphate material therein a gaseous reagent selected from the group consisting of hydrogen chloride and hydrogen fluoride.
Lapple (U.S. Pat. No. 3,323,864) points out that a “hydrofluoric acid-based process is generally impractical for commercial operation because the rate of rock attack with an acid of practical strength is too slow and because of the high cost of this acid.”
Lynn (U.S. Pat. No. 3,792,153) notes, with regard to the digestion of phosphate rock to form phosphoric acid, that current “processes digest the mineral with sulfuric acid during which process HF may be liberated, creating a severe pollution problem for the phosphate industry. By-product gypsum is formed in quantities much too large for the available market, and the digestion is relatively slow because of the formation of the insoluble gypsum.”
Russian Patent No. 880,974 refers to decomposition of phosphate rock with perchloric and hydrofluoric acid and then with nitric acid.
Claim 2 of Belgian Patent No. 750,498 relates to heating a mixture of calcium phosphate mineral with a water-soluble fluoride and acid, wherein the acid can be, inter alia, hydrofluoric acid.
SUMMARY OF THE INVENTION
Aqueous hydrofluoric acid is mixed with phosphate rock in a relatively small reactor or in one or more small vessels for a retention time of at least 10 seconds, usually between 10 and 30 minutes. After complete reaction, a slurry of calcium fluoride (CaF
2
), phosphoric acid and some excess HF is separated, using normal separation techniques, such as a basic table filter. The first wash from the filter, containing from 10 to 15% P
2
O
5
and 2 to 10% HF, is recycled back to the reaction section to regulate the amount of solids in the slurry to the filter and to recover the P
2
O
5
that is washed from the filter cake. The return stream, which contains some dissolved Ca
++
ions, must be added to the reactor slurry after the reaction between the phosphate and the HF has taken place to prevent the formation of very small crystals of CaF
2
.
Filter grade acid of from 10 to 33% (usually from about 25 to 28%) P
2
O
5
is concentrated from 40% to >60% P
2
O
5
in a standard vacuum evaporator. The product concentration step is similar to that used in conventional wet-process phosphoric acid production. However, in the subject process essentially no solids are precipitated during this step, thus reducing a major waste material-handling problem that is currently faced by the industry.
Clarification techniques and/or activated carbon absorption may be used to produce a clear-product acid for use as technical or food-grade acid.
The CaF
2
filter cake (is in the form of pseudomorphs having greater than 95% CaF
2
) recovered from the filter is mixed with sulfuric acid and thermally treated in a rotary kiln in which HF fumes are liberated from the solids. The fumes generated during this acidification/thermal treatment are scrubbed from the exit gas stream using standard absorber technology. A by-product of the rotary (regeneration) kiln is calcium sulfate which is stacked in contained piles. The CaF
2
/H
2
SO
4
reaction, thermal treatment, and HF recovery steps of the process rely on proven existing technology and process equipment commonly used in the HF production industry.
The scrubbed/recovered HF, less than 50% and normally less than 37% HF, is subsequently concentrated in an HF concentrator and recycled to the reactor; excess HF is marketed. Gas vented from the HF concentrator is returned to the HF recovery system.
Scrubbing HF vapors from the reactor, from the filter and from the H
3
PO
4
concentrators serves to both control and recover the HF emissions from these pieces of apparatus. Recovery of the HF and return to the reaction system are advantageous from both an environmental and economic point of view. Clean vapors from the HF process scrubber(s) and from the HF recovery absorber(s) are vented to the atmosphere.
An advantage of this invention is that it requires significantly fewer pieces of process equipment that are generally smaller in size than the equipment used in the conventional phosphoric acid-producing methods. A further advantage is a reduction in reactor retention time from the 6 to 12 hours necessary to effect good rock dissolution and to grow desired-size gypsum dihydrate (CaSO
4
.2H
2
O) crystals (essential for good filtration and high P
2
O
5
recovery) to only about 10 minutes to effect good conversion of the phosphate rock to CaF
2
. [Crystals of CaF
2
take the form of the phosphate rock (pseudomorphs) in the feed, and dissolution and recrystallization are not required.]
A still further advantage is the reduction in agitation requirements for reaction; the conventional process requires agitation to effect complete dissolution of phosphate rock. The degree of agitation is extremely high and results in high-energy usage and in erosion of the equipment located inside the reactor. The agitator drives generally require up to or greater than 100 HP for each compartment (up to 13); the subject process requires only one low energy agitator per reaction vessel which operates at a more gentle level of agitation, just enough to keep the solid particles suspended without causing particle attrition. (If a pipe reactor is used, no agitation is required.)
Another advantage is in the required management of heat, which is essential to the proper operation of the conventional process, in which heat is normally removed using a large flash cooler that operates at about 4 inches of Hg absolute pressure. The vacuum is generated by the use of a steam ejector followed by a barometric condenser or by a vacuum pump. The subject process requires a substantially lower level of removal of the heat of reaction. This is the result of the formation of pseudomorphs rather than the growing of gypsum crystals. It is anticipated that no heat removal will be required.
An additional advantage is in process control and automation. In the conventional process, the free sulfate level in the reactor is probably the most important process variable and is also one of the most difficult to measure and control on a steady/predictable basis. The automation of the measurement and/or control of the free sulfate level has been attempted many times over the years, but the problem has remained essentially unsolved. In addition, when an adjustment is made to the system, it takes several hours to see the final results. This is due in part to the long retention time in the reaction section and to slow changes that occur with the growth of CaSO
4
crystals. Total automation of the conventional system is very diff

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