Gas separation: processes – Liquid contacting – And recycle or reuse of contact liquid for further contact
Reexamination Certificate
1999-06-14
2001-01-30
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
And recycle or reuse of contact liquid for further contact
C095S199000, C095S223000, C095S229000, C095S233000, C096S234000, C096S247000, C096S265000, C423S24000R, C423S488000
Reexamination Certificate
active
06179901
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to obtaining hydrofluoric acid as a byproduct of a process of deriving adjusted phosphorous compounds from naturally occurring phosphate rock.
2. Background Information
Mined phosphate rock is commonly defluorinated by heating the rock to an elevated temperature in a kiln or fluidized bed reactor. For simplicity of description, an exemplar kiln process will be described herein, but those skilled in the art will recognize that other defluorination processes can be used and that those processes would also deliver a fluorine-rich flue gas. In one version of the defluorination process a kiln is used that is about two hundred fifty feet long with an internal diameter of about six feet. This generally horizontal tubular kiln slopes downward from an infeed end with a slope of about three feet for each twenty feet of length. Crushed and chemically adjusted phosphate rock is fed into the kiln at the upper, low temperature, end and moves under the influence of gravity to the lower end where the temperature is commonly on the order of twenty six hundred degrees Fahrenheit. At these high temperatures the bound fluorine in the rock is liberated and the hot fluorine-rich gas flows back up the kiln and is exhausted at the upper end thereof at a temperature of about thirteen hundred degrees Fahrenheit. The exhausted gas is accompanied by a significant amount of phosphate-rich dust.
It is known in the prior art to capture and dispose of fluorine compounds contained in the hot exhausted gas mixture by conveying the gas mixture through a spray of water, collecting the liquid effluent and neutralizing the effluent liquid with limestone so as to tie the fluorine up as calcium fluoride. It is also known in the prior art to collect the effluent liquid, which is commonly less than 6% HF by weight, and recover a fraction of the fluoride values therefrom. Only about half of the available HF is recovered in these processes. The rest must be neutralized and the precipitate from the neutralization process dumped. Moreover, because the hot exhausted gas mixture contains a large amount of entrained phosphate rock dust, the product from prior art recovery processes is contaminated with phosphorous.
BRIEF SUMMARY OF THE INVENTION
A defluorination apparatus of the invention provides a means for removing phosphate rock dust from a hot gas mixture exhausted from a phosphate rock defluorination kiln; a heat exchanger for cooling the hot gas mixture to a low enough temperature that it can be brought into contact with acid-resistant polymeric materials without destroying them; a water spray scrubber for further cooling the gas and absorbing the fluoride values as HF-rich liquor, and a pumping arrangement for pumping the HF liquor to a holding tank. In a preferred arrangement the heat exchanger is a gas tube steam boiler and a plurality of series-connected scrubbers is used to extract the fluorine values from the kiln exhaust so that the residual gas mixture can be exhausted into the atmosphere without exceeding permissible fluorine value levels. In the preferred series connected set of scrubbers, water is fed into the scrubber most distal from the boiler to extract HF from the gas fed thereinto, and an HF-laden process liquor is fed out of the most distal scrubber into the second most distal scrubber, where it is sprayed into the gas stream. This counter-current flow arrangement continues until one reaches the scrubber closest to the boiler and extracts the most concentrated process liquor from a sump thereof.
The invention provides a method of recovering fluorine values from a dusty hot gas mixture exhausted from a phosphate defluorination kiln. A preferred method comprises the steps of conveying the hot gas mixture upwardly from a dust collection chamber to eliminate phosphate dust; cooling the hot gas mixture in a gas tube steam boiler to a temperature of approximately three hundred degrees Fahrenheit; passing the cooled gas mixture through a water spray to form a mist comprising droplets of a hydrofluoric acid solution; aggregating the HF-rich droplets to form an HF liquor having a concentration in the range of twelve to seventeen percent HF by weight; and pumping the HF liquor into a holding tank.
One of the features of the invention is the recovery of HF-rich liquor from a phosphate rock defluorination kiln, where the concentration of HF in the liquor can be more than three times as high as is found in prior art processes. These higher HF values are retained in the scrubbers by disposing a porous polymeric pad, having a preferred thickness of about one and one half inches, at the top of each scrubber.
Another of the features of the invention is the recovery of relatively phosphorous-free HF values from a phosphate rock defluorination kiln.
An additional feature of the invention is the recovery of waste heat from a phosphate rock defluorination kiln by means of a gas tube boiler used to cool the exhaust gas mixture to temperatures low enough that the mixture can be handled in polymer-coated vessels.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and who wishes to learn how to practice the invention, it will be recognized that the foregoing recital is not intended to list all of the features and advantages, Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages may be provided by some embodiments.
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Kiewit David
Smith Duane S.
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