Direct production of coarse particle inorganic fluorides

Relatively large particles of calcium carbonate or magnesium oxide are reacted with a solution of ammonium fluoride or with aqueous hydrofluoric acid to form particles of calcium or magnesium fluoride, similar in size to the starting material by pseudomorphic alteration.

BACKGROUND OF THE INVENTION 
In the past calcium fluoride has been produced by various dry processes. 
For instance, U.S. Pat. No. 2,573,704, N. Gilbert et al., teaches making 
calcium fluoride by reacting calcium carbonate with gaseous hydrogen 
fluoride. In this process hot waste gases containing hydrogen fluoride are 
passed through a bed of 1/4-2 inch oolitic and pisolitic limestone. The 
hot gases react with the outer surface of the limestone, producing calcium 
fluoride which decrepitates as a powder. The calcium fluoride rich portion 
is separated by screening, and the coarser material composed mainly of 
limestone is recycled. 
Another dry process is described in Ross's U.S. Pat. No. 3,357,788 wherein 
calcium carbonate is reacted with ammonium bifluoride or with ammonium 
fluoride. The calcium fluoride produced has an average particle size of 
about 8 microns. 
Both of these dry process patents refer to wet processes as the undesirable 
approach to produce calcium fluoride. The Ross patent states that large 
volumes of solutions must be held at their boiling temperature for several 
hours in order to develop calcium fluoride crystals from extremely fine 
particles to desirable sizes such as about one micron average diameter. 
The Gilbert patent states that various types of wet treatments of partially 
reacted limestone for removal of calcium fluoride have been suggested, but 
that these methods are generally too expensive and complicated for 
commercial production. 
Other patents of interest include U.S. Pat. Nos. 3,551,332; 3,529,929, and 
less pertinent 2,213,907, 2,781,244, 2,781,245, and 2,839,369. 
The search conducted was in Classes 23, 423 and 210. 
The processes in the known art produce a finely-divided powder or small 
particles in a large volume of liquid which requires filtration and 
drying. The drying step is necessarily difficult, and energy and 
time-consuming because the calcium fluoride is finely divided and thus 
difficult to dry. 
With the present emphasis on energy and mineral conservation throughout the 
world, there is real value in the improvements provided by the present 
invention. Not only does this invention offer a method of removing 
fluorides from solutions that might possibly cause disposal problems, but 
also produces a potentially valuable product that can easily be dried with 
essentially no dust or handling problems. 
SUMMARY OF THE INVENTION 
It is an object of this invention to produce a coarse particle calcium 
fluoride product utilizing an economical process. 
It is a further object to provide an economical process for producing 
calcium fluoride from limestone which avoids the usual finely-divided 
powder as the product with the attendant drying and handling problems. 
The invention involves starting with a relatively coarse particle of 
limestone and retaining that same coarse particle in the calcium fluoride 
product formed. The particle size and particle shape of the limestone is 
essentially retained in the product formed. None of the commercial 
processes utilized in the prior patents or known art has accomplished 
this. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Granular limestone, preferably oolitic limestone, is reacted with aqueous 
hydrofluoric acid. Alternatively, an aqueous solution of ammonium fluoride 
is reacted with the coarse grain limestone to produce the CaF.sub.2 
product as follows: 
EQU 2 HF + CaCO.sub.3 .fwdarw. CaF.sub.2 + CO.sub.2 + H.sub.2 O 
EQU 2 nh.sub.4 f + caCO.sub.3 .fwdarw. CaF.sub.2 + 2 NH.sub.3 + CO.sub.2 + 
H.sub.2 O 
granular magnesium oxide can also be reacted with aqueous solutions of 
hydrofluoric acid or ammonium fluoride to produce granular magnesium 
fluoride.

The following examples illustrate the invention. 
EXAMPLE 1 
Eighty two grams of 28 .times. 65 mesh limestone was added to 500 grams of 
an aqueous solution of 6.8% hydrogen fluoride. (6.8% HF Acid). The mixture 
was reacted at 130.degree. F for 60 minutes with constant stirring. 
The CaF.sub.2 product was dried at 200.degree. F and an analysis showed it 
to contain 40.8%F (84% CaF.sub.2) and 2.0% CO.sub.2. The 28 .times. 65 
mesh size is essentially retained in the CaF.sub.2 product. There are 
little or no fines in the resultant product and, for this reason, the 
product is easily dried. 
EXAMPLE 2 
Forty four grams of 20 .times. 200 mesh limestone was added to 500 grams of 
an aqueous solution containing 4% HF. The mixture was reacted at 
100.degree.-140.degree. F for 60 minutes. The product was calcined for 1 
hour at 1000.degree. F to drive off volatiles. The calcined product 
contained 43.6% fluorine (90% CaF.sub.2). This product was essentially the 
same size as the starting limestone. 
EXAMPLE 3 
Seventy-five grams of 20 .times. 200 mesh calcium carbonate was reacted 
with 500 grams of an aqueous solution containing 6.8% HF. The mixture was 
reacted at 100.degree.-140.degree. f for 60 minutes. The product was 
calcined at 1000.degree. F for 1 hour and assayed 46.4% F (95% CaF.sub.2). 
The product had essentially the same size distribution as the starting 
limestone. 
EXAMPLE 4 
Ninety three grams of 20 .times. 200 mesh calcium carbonate was reacted 
with 500 grams of aqueous 10.9% HF at 100.degree.-140.degree. F for 60 
minutes. The calcined product contained 46% F (94% CaF.sub.2) and was 
essentially the same size as the starting limestone. 
EXAMPLE 5 
One-hundred grams of 20 .times. 65 mesh limestone was added to an aqueous 
solution of 20% HF. The sample was heated to 150.degree. F for 1 hour. The 
liquid was decanted and fresh solution containing 20% HF added and mixture 
reheated to 150.degree. F for another hour. The product was dried and 
calcined at 1000.degree. F for an hour. It assayed 47.9% F and 50.4% 
calcium (98.3% CaF.sub.2 based on F). The product was about the same size 
as the starting limestone. 
EXAMPLE 6 
Fifty grams of 20 .times. 65 mesh limestone was reacted with 208 grams of 
an aqueous solution containing 9.1% ammonium fluoride for 1 hour at about 
150.degree. F. The liquid was decanted and an additional 208 grams of an 
aqueous 9.1% NH.sub.4 F solution added and heated to 150.degree. F to 
boiling for 1 hour. The product was dried and calcined at 1000.degree. F 
for 1 hour and assayed 50.7% Ca and 48% F. The size of the product was 
similar to the starting limestone. 
EXAMPLE 7 
Fifty grams of 35 .times. 100 mesh 67% MgO* was added to 700 grams of an 
aqueous solution containing 5.4% HF and reacted at 160.degree. F for 2 
hours. The liquid was decanted, 600 grams of the 5.4% HF solution added 
and reacted for an additional 2 hours at 160.degree. F. The product was 
dried and calcined at 1000.degree. F for 1 hour. It assayed 60.0% F and 
36.9% Mg. The size distribution of the product was similar to the starting 
material. 
EXAMPLE 8 
Fifty grams of 35 .times. 100 mesh 67% MgO* was added to 250 grams of an 
aqueous solution of 15.2% HF. This mixture was reacted at 160.degree. F 
for 2 hours. The liquid was decanted, 250 more grams of the 15.2% HF 
solution added, and mixture reacted for 2 more hours at 160.degree. F. The 
product was dried and calcined at 1000.degree. F for 1 hour. Analysis 
indicated 60.0% F and 37% Mg. The size distribution of the product as 
essentially the same as the starting MgO. 
EXAMPLE 9 
Fifty grams of 67% MgO* was reacted with 346 grams of an aqueous solution 
containing 9.1% ammonium fluoride for an hour at about 150.degree. F. The 
liquid was decanted and an additional 346 grams of aqueous 9.1% ammonium 
fluoride added. This mixture was reheated to about 150.degree. F to 
boiling for 1 hour. The product was dried and calcined at 1000.degree. F 
for an hour. Chemical analysis indicated 37.4% Mg and 57% F. The size 
distribution of the product was similar to the starting MgO. 
FNT *The magnesium oxide was granular magnesium 67 supplied by Basic, Inc. This 
product is 67.5% MgO, 1.1% CaO, 0.03% Fe.sub.2 O.sub.3, 0.2% Al.sub.2 
O.sub.3. 
Although the strict chemical reactions involved in this discussion are 
simple, the exact mechanism by which these pseudomorphic changes take 
place within a particle or other physical form are not as well understood. 
It is known that large particles of calcium carbonate can produce almost 
the same size and shape calcium fluoride particles by using the procedures 
of the examples. A seashell composed of calcium carbonate when reacted 
with hydrofluoric acid will result in a duplicate seashell composed of 
calcium fluoride (a pseudomorph). Likewise, using the processes of this 
invention, various physical forms including relatively large particles of 
calcium carbonate when reacted with HF will result in duplicate large 
particles of calcium fluoride, a pseudomorph or false form. In the 
seashell or in the large particle, the calcium ions remain, and the 
carbonate anions are replaced with fluoride anions. The solid reactant is 
preferably one having an average particle size of from about 100 to 1000 
microns. 
The advantages of the present process include avoiding the production of 
finely-divided forms that are very difficult to dry, with resultant dust 
and pollution problems. The particle sizes desired are 16 .times. 100 mesh 
and preferably 20 .times. 65 mesh. The temperature range maintained to 
complete the reaction is from about 90.degree. F to boiling and preferably 
from about 110.degree. to 150.degree. F. The reaction time may be from 1/2 
to 5 hours. The drying temperature can be varied and is not critical but 
is generally about 180.degree. to 225.degree. F. To remove the remaining 
water and other volatiles, the material is calcined at 1000.degree. F for 
1 hour. 
The particle size of the oolitic limestone used as the preferred starting 
material is about 0.02 inch, which is about one hundred times larger than 
the calcium carbonate used in the Ross patent (U.S. Pat. No. 3,357,788). 
Typical screen analysis of the limestone starting material (reactant) and 
the CaF.sub.2 product follow. 
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CaF.sub.2 SCREEN ANALYSIS 
Reactant Limestone 
CaF.sub.2 Product 
Tyler Percentages Percentages 
Mesh On Passing On Passing 
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20 1.9 98.1 0.8 99.2 
28 12.0 88.0 7.4 92.6 
35 25.4 74.6 14.5 85.5 
48 48.4 51.6 33.1 66.9 
65 68.3 31.7 48.0 52.0 
100 87.3 12.7 65.3 34.7 
150 95.4 4.6 78.0 22.0 
200 98.2 1.8 87.3 12.7 
Pan 100.0 -- 100.0 -- 
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