Chemistry of inorganic compounds – Halogen or compound thereof – Binary fluorine containing compound
Reexamination Certificate
1999-09-10
2001-05-01
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Halogen or compound thereof
Binary fluorine containing compound
C423S483000, C423S484000, C423S157300, C423S158000
Reexamination Certificate
active
06224844
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the manufacture of calcium fluoride and more particularly to its production by the reaction of fluosilicic acid (i.e., FSA or aqueous H
2
SiF
6
) with phosphate rock containing fluorapatite.
BACKGROUND
Fluorine is an essential element for producing fluorochemicals and fluoropolymers and is currently derived, primarily, from fluorspar, a mineral which is a crystalline form of calcium fluoride. However, world-wide reserves of fluorspar are being depleted and new economical sources of fluorine are desired.
An important reserve of fluorine is phosphate rock which contains a variety of apatites including fluorapatite, i.e., CaF
2
.3Ca
3
(PO
4
)
2
, a mineral which is used for the manufacture of phosphoric acid. This mineral constitutes a fluorine reserve which is about four times greater than the proven reserves of fluorspar. During the manufacture of phosphoric acid most of the fluorine is removed as fluosilicic acid (FSA). There is some demand for FSA for fluoridating drinking water and for the manufacture of cryolite and aluminum fluoride. However, because this demand is small, most of the FSA produced during phosphoric acid manufacture is wasted, e.g., it is sent to a waste-water pond with the fluorine ending up in the surrounding environment and can cause an environmental pollution problem.
Over the years numerous processes, several of which are described below, have been developed to recover the fluorine from phosphate minerals. The United States Bureau of Mines (Chem. Abst., 75:23270, 1971) has shown how waste fluosilicic acid can be converted to an acid-grade fluorspar (CaF
2
). A first step involves treating the FSA with ammonia to precipitate silica, which is removed by filtration to form NH
4
F. In a second step ammonium fluoride is treated with lime to form CaF
2
.
U.S. Pat. No. 5,531,975 describes a process for reacting phosphate rock and FSA to produce a slurry comprising phosphoric acid, calcium fluoride, silicon dioxide and undigested phosphate rock. An excess stoichiometric amount of calcium to fluoride is initially present in the slurry. The product slurry is pumped into a vacuum filter or centrifuge where the phosphoric acid and colloidal calcium fluoride are separated from the undigested phosphate rock and silic. In Example 1 of the '975 patent, the weight ratio of F:Si in the product (initial filtrate) is shown to be about 30:1.
Surprisingly, a process has been discovered which yields a much more efficient process for producing calcium fluoride from FSA, with much higher recovery of fluorine from the FSA and calcium fluoride containing much lower levels of silica than previously.
SUMMARY OF THE INVENTION
This invention provides a process for producing calcium fluoride comprising:
(a) mixing aqueous H
3
PO
4
with aqueous H
2
SiF
6
to form a mixture such that the concentration of H
3
PO
4
is at least about 3 moles/liter,
(b) adding phosphate rock to the mixture at a rate such that the pH of the mixture is maintained at less than about 1.0 and at a temperature sufficient to form a second mixture containing calcium fluoride, and
(c) separating calcium fluoride from the second mixture.
DETAILED DESCRIPTION
The phosphate rock useful in the current process is any naturally occurring phosphate rock typically composed primarily of tricalcium phosphate (Ca
3
(PO
4
)
2
), calcium carbonate (CaCO
3
) and calcium fluoride (CaF
2
). This phosphate rock may also be used for the manufacture of phosphoric acid.
The concentration and source of aqueous H
2
SiF
6
used in this process is not critical. The aqueous fluosilicic acid (FSA) produced by a phosphate plant may be used, which is typically 20-30% by weight H
2
SiF
6
.
The concentration and source of aqueous phosphoric acid used in step (a) described above is not critical and may be from about 50 to 100 weight %, but is preferably from about 80 to 90 weight %, but more preferably about 85 weight %.
The acids may be mixed in any manner known to a person of ordinary skill in the art to form a mixture of the acids. For example, they may be combined in a stirred reactor such that the molar phosphoric acid concentration is at least about 3 moles/liter, preferably 4 moles/liter.
The phosphate rock must be added to the mixture of the acids at a temperature sufficient to form calcium fluoride. This temperature may be achieved by heating the combined-acid mixture to a temperature sufficient to initiate the decomposition of FSA and to form calcium fluoride when phosphate rock is added. By “a temperature sufficient to form a second mixture containing calcium fluoride” is meant a temperature which is sufficient to initiate the decomposition of FSA when the phosphate rock is added to the mixture of acids and to cause calcium fluoride to form. Typically, this temperature may be at least about 50° C., preferably at least about 80° C., and more preferably at least about 90° C.
Phosphate rock should be added to the mixture of the acids at a rate such that the pH of the mixture is maintained at less than about 1.0, preferably at less than about 0.7, and more preferably at less than about 0.5. The quantity of phosphate rock is not critical and may be added until all the FSA is decomposed. Typically, addition of the phosphate rock is continued until the Ca:F weight ratio is at least 1.2:1.
After addition of phosphate rock to the mixture of the acids to form a second mixture containing calcium fluoride, the calcium fluoride may be separated from any undigested phosphate rock and silica by any method known to those skilled in the art. For example, the calcium fluoride may be recovered by any solid-liquid separation technique such as filtration, decantation or centrifugation. Filtration may be done using the second mixture containing the calcium fluoride before it is cooled, or the second mixture may be cooled prior to the separation. The separation step can be operated either in batch or continuous modes. Separation may include washing of the calcium fluoride using standard techniques (e.g., with water following initial filtration, centrifugation or decantation).
Separation may be enhanced by heating the second mixture to near reflux for at least about 0.5 hours, preferably for about 1 hour. This heating is done in part to insure agglomeration of the insoluble silicon compounds present in the second mixture. The second mixture may then be sent to a centrifuge or filter to separate the colloidal (less than 1&mgr;) calcium fluoride and phosphoric acid from the undigested phosphate rock and silica. The components of the second mixture can also be separated by gravity settling.
The separated calcium fluoride may contain about 20-50 wt. % phosphate, depending on the molarity of the phosphoric acid originally used. After the silica and undigested phosphate rock have been removed, calcium fluoride remains suspended in aqueous phosphoric acid. Additional rock may be added to the suspension in an amount such that the ratio of Ca:F is at least about 1.3:1. This addition may be done to remove any soluble silicon compounds remaining in the calcium fluoride suspension. In a further step sulfuric acid may be mixed with this calcium fluoride suspension to produce hydrogen fluoride. If the temperature of the mixing of sulfuric acid with the calcium fluoride suspension is about 120° C., then nearly anhydrous hydrogen fluoride is produced.
In another embodiment, the calcium fluoride product may be separated from the phosphoric acid after the removal of any undigested phosphate rock and silica, but before the addition of sulfuric acid. The calcium fluoride may be separated from the phosphoric acid using a centrifuge or any other technique known to one skilled in the art. Alternatively, mixing the suspension with an organic solvent that is miscible with phosphoric acid, such as methanol, ethanol, or isopropanol, reduces the calcium fluoride solubility and decreases the specific gravity of the solution, facilitating isolation of the calcium fluoride by centrifugation or gravity settling.
Suprisin
Michel John Byrne
VerNooy Paul Douglas
E. I. Du Pont de Nemours and Company
Griffin Steven P.
Ildebrando Christina
Shipley James E.
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