Chemistry of inorganic compounds – Halogen or compound thereof – Hydrogen halide
Reexamination Certificate
1999-04-05
2002-03-12
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Halogen or compound thereof
Hydrogen halide
C423S490000, C423S158000
Reexamination Certificate
active
06355221
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for removing soluble fluoride from a waste solution containing such soluble fluoride. In one embodiment, the invention relates to a process for recovery of synthetic fluorspar-grade calcium fluoride from a fluoride-containing aqueous effluent using calcium sulfate to form the calcium fluoride. The present invention is particularly useful in removal of fluoride from an effluent originating from an alkylation process utilizing hydrogen fluoride (HF; hydrofluoric acid) as the catalyst. The calcium sulfate used in the inventive process may originate as a byproduct from a process for converting calcium fluoride into hydrogen fluoride. The fluorspar-grade calcium fluoride recovered from the inventive process may be used as feedstock in the hydrogen fluoride production process, which converts calcium fluoride into hydrogen fluoride. Thus, the present invention further relates to a continuous, closed-loop process recycling both fluoride and calcium.
BACKGROUND OF THE INVENTION
Environmental laws and efficient operation of wastewater treatment plants require that fluoride be removed and/or recovered from wastewater prior to discharge into the environment. Increasingly, it is being recognized that economic concerns demand recycle of materials such as fluoride for which environmental concerns demand removal from waste streams. A number of processes have been used for removal and/or recovery of fluoride from wastewater. For example, U.S. Pat. No. 4,734,200 discloses a process for treating acidic process waste water containing SiF
6
−2
and phosphorus-type contaminants, in which the waste water is treated with a strong base ion exchange resin to remove SiF
6
−2
.
Similarly, U.S. Pat. No. 4,965,061 discloses a process for producing hydrofluoric acid from waste water containing SiF
6
−2
, in which the SiF
6
−2
is reacted with (NH
4
)
2
SO
4
at a high pH to form (NH
4
)
2
SiF
6
, from which ammonia, Si(OH)
4
and HF are eventually produced.
U.S. Pat. No. 5,215,632 discloses a method of removing fluoride and sulfate from an aqueous stream by adding calcium chloride and phosphate to form CaSO
4
and a compound of calcium, fluoride and phosphate. The CaSO
4
is removed as a precipitate, and carbonate is added to cause the precipitation of calcium fluoride.
A system for removal and recycle of fluoride is known as the HARDTAC process and is available from Modular Environmental Technologies, Inc. According to the company's website, (www.modenvtech.com), the HARDTAC process uses calcium chloride or lime as a source of calcium for reaction with fluoride to form calcium fluoride, and may be applied to treatment of a neutralized hydrogen fluoride acid blowdown from a refinery.
A process used and licensed by Phillips for neutralization of hydrogen fluoride and removal of fluoride uses sodium hydroxide to neutralize refinery waste hydrogen fluoride, and soluble calcium chloride as the source of calcium for formation of CaF
2
. The process separates solid CaF
2
for disposal as solid waste and discharges an effluent containing sodium chloride. Another known process uses KOH to neutralize HF, and uses lime (Ca(OH)
2
) as the source of calcium for reaction with fluoride to form CaF
2
and regenerates KOH for recycling to the hydrogen fluoride neutralization step.
Hydrogen fluoride for use in refinery alkylation processes is usually made from high-grade fluorspar, a CaF
2
mineral, in a process wherein the CaF
2
is treated with concentrated sulfuric acid or oleum (H
2
SO
4
+SO
3
). The products of this reaction are HF and CaSO
4
. The by-product CaSO
4
is generally put in a landfill, since it has previously had little or no economic value.
The hydrogen fluoride produced for use in refinery alkylation processes, as well as for other uses in the chemical processing industry, must be free of chloride contamination, due at least in part to stress corrosion cracking of steel which results from or is exacerbated by the presence of chloride. Hydrogen fluoride produced from a recovery process which utilizes calcium chloride is not suitable for such uses due to the unavoidable presence of chloride contamination in the CaF
2
. In the process using KOH to neutralize HF, the emphasis mainly has been on recovery and recycle of KOH, and the CaF
2
is generally recovered as a gel, dewatered, solidified and discarded in a landfill.
Lower grade hydrogen fluoride, such as that obtained from recycling calcium fluoride obtained by using calcium chloride to precipitate fluoride and thus containing elevated levels of chloride, or which has a broad range of particle sizes and/or very small particle sizes, or which contains some excess organics, base and water, may be suitable for use in aluminum processing, but is not suitable for use in refinery alkylation processes, and for use in other areas of the chemical industry.
SUMMARY OF THE INVENTION
One aspect of the present invention is a process for removing soluble fluoride from a waste solution containing the soluble fluoride, the process including the steps of:
(A) mixing the waste solution with an aqueous slurry containing seed calcium fluoride particles to form a first aqueous composition;
(B) mixing the first aqueous composition with a calcium containing reagent to form a second aqueous composition;
(C) advancing the second aqueous composition through a tubular reactor for an effective period of time and at a velocity sufficient to permit the soluble fluoride and the calcium containing reagent to react and attach to the seed particles of calcium fluoride, the seed particles of calcium fluoride thereby increasing in size to become enhanced particles; and
(D) removing a portion of the particulate of the enhanced particles from said second aqueous composition.
Another aspect of the present invention is a process for removing soluble fluoride from a waste solution containing the soluble fluoride and manufacturing hydrogen fluoride from the fluoride so removed, the process including the steps of:
(A) mixing the waste solution with an aqueous slurry containing seed calcium fluoride particles to form a first aqueous composition;
(B) mixing the first aqueous composition with a calcium containing reagent to form a second aqueous composition;
(C) advancing the second aqueous composition through a tubular reactor for an effective period of time and at a velocity sufficient to permit the soluble fluoride and the calcium containing reagent to react and attach to the seed particles of calcium fluoride, the seed particles of calcium fluoride thereby increasing in size to become enhanced particles; and
(D) removing from the second aqueous composition substantially only enhanced particles having an average particle size of at least about 20 &mgr;m;
(E) treating the enhanced particles removed in step (D) with sulfuric acid to form hydrogen fluoride and calcium sulfate.
In one embodiment, the present invention includes a process for removing soluble fluoride from a waste solution containing the soluble fluoride, the process including the steps of:
(A) mixing the waste solution with an aqueous slurry containing seed calcium fluoride particles to form a first aqueous composition;
(B) mixing the first aqueous composition with calcium sulfate to form a second aqueous composition;
(C) advancing the second aqueous composition through a tubular reactor for an effective period of time and at a velocity sufficient to permit the soluble fluoride and the calcium containing reagent to react and attach to the seed particles of calcium fluoride, the seed particles of calcium fluoride thereby increasing in size to become enhanced particles; and
(D) removing a portion of the enhanced particles from the second aqueous composition, wherein the portion contains substantially only particles having a size greater than about 20 &mgr;m; and
(E) removing water from the second aqueous composition, wherein the water removed contains less than about 50 ppm of soluble fluoride.
Thus, the present invention provides a continuous
BP Corporation North America Inc.
Kim Patrick
Nguyen Ngoc-Yen
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