Patent Application
20060093548
The present invention relates to recycling the excess heat of reaction when producing aluminum fluoride from hydrogen fluoride gas and aluminum oxide preferably as Al2O3.3H2O (gibbsite or alumina trihydrate or aluminum tri hydroxide).
The Hall-Heroult process was first used commercially around 1900. In this process, aluminum is extracted by electrolyzing aluminum oxide (also known as “alumina”) dissolved in a molten salt bath based on cryolite, Na3AlF6. The molten cryolite is operated at a temperature generally with the range of 950° C.-1000° C.
In U.S. Pat. No. 5,279,715 (LaCamera et al.) tried to lower the cryolite operating temperature to between 685° C.-900° C., by utilizing a bath comprising 36 wt % NaF and 64 wt % AlF3. With widespread use of aluminum, materials such as AlF3, used to make aluminum are in demand.
British Patent Specification No. 1,026,131 (Pegler) teaches producing aluminum fluoride (AlF3) of high purity in a two stage process in which hydrated alumina (containing H2O) is treated at a temperature below 550° C. with an excess of anhydrous (without H2O) hydrogen fluoride followed by treating the product at a temperature of at least 650° C. with excess anhydrous hydrogen fluoride over that required to react stoichiometrically with any unreacted Al2O3.
In one process to make AlF3, fluorspar (CaF2) is reacted with sulfuric acid (H2SO4) in a kiln, where HF gas product passes into a reactor to contact alumina trihydrate (Al2O3.3H2O). A description of this process is found in U.S. Pat. No. 6,517,790 B1 (Eisele). This reaction releases heat. In one instance, to cool the reactor, water is passed through Inconel tubes within the reactor. These tubes could develop cracks or the like, through which water or steam could pass to stop the reaction in the reactor. The tubes could also warp when the temperature reaches about 550° C. The water would quickly change phase to vapor with an attendant and large volume expansion, where the pressure gradient could rupture the tubes. Temperature control was also difficult because a small amount of water would produce very large temperature changes.
What is needed is an improved means to cool the countercurrent reactor. It is a main object of this invention to provide improved cooling of the reactor.
The above problems are solved and needs are met by providing a method of producing aluminum fluoride in a reactor vessel by countercurrent contact of HF gas with Al2O3.3H2O comprising the steps: reacting fluorspar and a strong containing sulfuric acid in a heated kiln to produce HF gas; feeding the HF gas to a reactor having at least two stages, where Al2O3.3H2O is also feed into the reactor to countercurrent contact the HF gas to produce heat and AlF3; passing pressurized cool air into an external cooling jacket surrounding the outside of at least one of the stages of the reactor to remove excess heat of reaction and provide heated air at a temperature of about 210° C. (410° F.) to about 410° C. (770° F.); circulating the heated air into a combustion furnace to react with natural gas providing combusted exhaust gas; and passing the combusted exhaust gas into a jacket surrounding the heated kiln of step (1), to heat the kiln.
Preferably the pressurized cool air flows into the cooling jacket at the rate of about 400 cu ft/min (11.3 cu meters/min) to about 700 cu ft/min (19.8 cu meters/min) and the heated air discharge is from about 260° C. (500° F.) to about 360° C. (680° F.) so that the cooling jacket temperature is substantially less than the usual 550° C. (1020° F.) temperature to which Inconel tubes had been subjected in one prior art embodiment of the invention. This provides improved cooling of the reactor.
Preferably the reactor contains three stages with the cooling jacket being disposed about the outer circumference of the second (middle) stage of the reactor. The kiln in previous processes had been heated by combusting natural gas. By use of heated air, from about 300,000 BTU/hr to about 375/000 BTU/hr energy from natural gas is displaced, providing efficient, less costly kiln heating, and allowing recycle of the excess heat of reaction from the countercurrent reactor.
One advantage of the new process is the recycling of waste heat from one part of the process and using it another. The other advantage is eliminating the internal cooling tubes that used water and were prone to failure and caused excessive down time. And finally, feedback temperature control is much easier with the air as opposed to the water.
A full understanding of the invention can be appreciated from the following detailed description of the invention when read with reference to the accompanying drawings wherein:
The reaction for hydrogen fluoride gas and alumina trihydrate solids is given below:
with literature values of released heat of reaction reported to be between 126 to 133 kJ/mol Alf3. This reaction is exothermic and so generates substantial heat which requires cooling control. One prior art system to produce aluminum fluoride is shown in
Preferably the feedstock Al2O3.3H2O 16 is heated up to about 55° C. (131° F.) to about 65° C. (149° F.) to ensure quick reaction in the reactor 9 and the HF gas is supplied from the kiln 22 to the bottom of the reactor at from about 75° C. (167° F.) to about 85° C. (185° F.). The hot air 46 exits the reactor at between about 210° F. (410° F.) to about 410° C. (770° F).
Air 42 is used as the cooling medium on the outside cooling jacket 44. The air flow to the cooling jacket can be about 500 to 600 cubic feet per minute based on a cooling jacket per surface area of 60 square feet and a cylindrical reaction vessel of from 6.5 to 7.0 feet in diameter. The discharge temperature of the heated air 46 is preferably from 260° C. (500° F.) to 360° C. (680° F.) with automatic feedback temperature control. The hot air that is discharged from the jacket can then be reused in another part of the aluminum fluoride process, namely to supply heat to the kiln 22 that produces the hydrogen fluoride gas 18.
The first part of the aluminum fluoride process involves producing hydrogen fluoride gas. The hydrogen fluoride gas is produced in a kiln by the reaction between sulfuric acid or sulfuric acid plus fuming sulfuric acid and fluorspar (calcium fluoride). This is an endothermic reaction and requires heat. The heat is usually supplied by burning natural gas in a furnace and circulating the hot combustion gases around the outside of the kiln in a heating jacket. In this invention, the heated/hot air 46 from the outside cooling jacket is brought back to a combustion air blower 50, that then passes combustion air to burn with natural gas 52, in a furnance 54 that supplies heat to the kiln 22. The hot air from bed 12 of the fluidized bed reactor, when mixed with natural gas, displaces about 300,000 to 375,000 BTUs per hour that were previously supplied solely by natural gas for heating the kiln.
The results of this invention have shown that the published values in the literature for the heat of reaction of aluminum try-hydrate reacting with hydrogen fluoride gas are incorrect. The use of the air jacket has shown that the actual heat of reaction produces approximately −10,000 kJ/mol of AlF3. In thermodynamics, the negative sign indicates that heat is given off.
Further elaborating on
While this invention and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various modifications and changes can be made in the process without departing from the spirit and scope of the invention.
