A Method for Recovering Fluorine as Sodium Silicofluoride SYDNEY ATKlN and ENRICO PELITTI Chemical Construction Corp., New York, ALAN P. VILA and JOHN HEGEDUS American Cyanamid Co., New York,
N. Y.
N. Y.
This continuous process gives:
d Reduction of fumes d Reduction of effluent neutralizing costs
d Recovery of a valuable by-product -EDITOR'S
NOTE
The predominant use of wet-process phosphoric acid is for the manufacture of phosphatic fertilizers. This use as a proportion of total phosphorus distribution has changed little in the past five years-about 82%. Two processes are used for manufacture of phosphoric acid, furnace method and wet-process method. In 1959, some 1,140,700 short tons of phosphoric acid as P205were made by the wet-process method. This represents about 61% of the total phosphoric acid produced that year. And is a 47% increase over that produced in 1955. The pre-eminence of the wet process is expected to continue, judging by the relatively greater expansion projected for this method. The following four articles are from the Symposium on Production of Wet-Process Phosphoric Acid and Its Uses in Fertilizer Processes, which was given at the 138th Meeting of the American Chemical Society in New York, September 1960. Three other articles from this symposium dealing with the uses of wet-process phosphoric acid in fertilizer processes will appear in the September/October issue of the Journal of Agricultural and Food Chemistry.
Wet-Process Phosphoric Acid in Mixed Fertilizers 6y D . 0. Walstad Sludge-Free Wet-Process Phosphoric Acid for Use as Liquid Fertilizer by E. J . Fox and W .A. Jackson Liquid Fertilizers from Wet-Process Phosphoric Acid -Suspension of Impurities by A . Y Slack and M . C. Nason
PHOSPHOR~O
ACID, an important chemical in our national economy, is produced from mined phosphate rock. I n the United States, there are extensive deposits of phosphate rock in Florida, Tennessee, and some Rocky Mountain states. This article relates the work done with Florida pebble phosphate rock only. There are two general methods used for the manufacture of phosphoric acid. In the first, known as the wet-process, phosphate rock is treated with a sufficient amount of sulfuric acid to convert completely the P205 content into orthophosphoric acid. An insoluble residue, largely calcium sulfate (gypsum) is filtered off. The acid produced is saturated with gypsum and contains other impurities introduced with the rock. Essentially the following reaction occurs :
Ca@'04)2
+ :3HsS04 + 6 H p 0 -, + 3CaSO4.2HZO
2H3P04
(1)
The second is the furnace method in which phosphate rock is fused in the presence of reducing materials. Elemental phosphorus is produced and burned with air to form P2O5. This is absorbed in water to give an acid of high uniform purity. I n general, and under prevailing economic conditions, it is substantially cheaper to make phosphoric acid by the wet-process than by the thermal reduction process. The wet-process, however, has some disadvantages, such as impurities which are retained in the phosphoric acid solution. The nature and amount of impurities are largely dependent on the composition of the rock used as raw mateVOL. 53, NO. 9
*
SEPTEMBER 1961
705
rial. One of the most important impurities, both because of the quantity in which it occurs, and of the effects it has on process, equipment, and materials, is fluorine. I t is usually present as calcium fluoride, and in quantities between 2 and 6% of the rock. Fluorine content of Florida phosphate rock is generally between 3.5 and 4.0%. In the wet-process, 55 to 60% of the fluorine in the rock is retained in the phosphoric acid produced. A typical composition of Florida pebble phosphate rock, and the wet-process phosphoric acid produced from it is shown below.
WATER
PHOSPHORIC ACID F R O M PH PLANT
I
$-I
I
FLUOSILICATE SETTLING
CLARIFYING
I t SLUDGE T O
MIXER
DIG EST ION
WASTE
1
Florida Components PZOS
Pebble Phosphate Rock, %
Wet-Process Phosphoric
31.60 45.50 3.70 0.90
32.00 0.30 2.10 0.60 1.00 1.50
CaO
F Fez03 AlzOa
Si02
9.60
t
FILTRATE TO
-\
PHOSPHORIC ACID PLANT
PHOSPHORIC AClO To FURTHER PROCESSING
Acid, yo
1.40
TANK,
FUEL+
DRYER
SODlUM FLUOSILTCATE
Chemic0 process for fluorine recovery from phosphoric acid
SETTLED ACID - 5 f c
NA;! COS FEEDER 37.5 G.F?M*
In the example shown, a plant producing 100 tons of PzO6 per day will have about 6.5 tons of fluorine in the acid. Presumably, the fluorine present i n the rock as calcium fluoride reacts with the mixed acids to form hydrogen fluoride as represented by Equations 2 and 3.
+ 2H&0' 4 Ca(HZP04)2 + 2HF (2) CaFz + HzSOa + H20 CaS04.2HzO + 2HF ( 3 ) CaF2
-L
+
The hydrofluoric acid thus produced reacts immediately with silica always present in the phosphate rock to produce hydrofluosilicic acid. Si02
+ 6HF
4
+ 2H20
H2SiF:6
(4)
As silica is present in considerable excess in Florida rock, reaction 4 goes to completion. Practically all processes used today for large scale commercial production of wet-process acid on a continuous basis produce an acid strength between 25 and 32% of PzOS. Concentration of the acid to a higher strength is required in most plants before it can be used to make triple superphosphate or diammonium phosphate, or for solid or liquid fertilizer mixtures. There are two basic methods for concentrating the acid: vacuum and high temperature. In the high temperature method, fuel can be burned under the surface of the acid solutionsubmerged combustion-r air previously heated is introduced under the liquid levelkdrum concentrator-or hot air is mixed with the acid in a spray tower. In vacuum concentrators, steam is used to heat the acid under high
706
~
Pilot plant, Brewster, Florida
vacuum in falling film evaporators, or in conventional heat exchangers, with water vapor and noncondensable gases flashed off in a vapor head and removed continuously through barometric condensers and steam vacuum ejectors. Irrespective of the concentration method used, a large percentage of the fluorine present in the acid as fluosilicic acid is volatilized according to the following reaction : HzSiFs
-.f
SiFa
+ 2HF
(5)
If a barometric condenser follows the concentrator, the cooling water absorbs the HF and reacts with the silicon tetrafluoride to form again hydrofluosilicic acid : 3SiFa
+ 2H20
+
2HzSiFa
+ Si02
(6)
This same reaction takes place, if a wet scrubber is used on the exhaust gases. The residual fluorine content of the acid in a typical vacuum process, con-
MDUSTRIAL AND ENGINEERING CHEMISTRY
centrating phosphoric acid from 32 to 54% of PzO5, is given below.
% PtOa
%'oF
32.00
2.10
40.00 50.00 54.00
1.90 1.20 1.00
I t is, therefore, apparent that fluorine will be present either in the exhaust gases or in the waste water from the condenser. In view of current restrictions on the pollution of air or river streams, such presence can represent a serious problem for the producing plant. Exhaust gases must be scrubbed with expensive scrubbing equipment, and waste streams from the scrubbers or the barometric condensers must be neutralized with lime before they can be disposed of to nearby
SODIUM S I L I C O F L U O R I D E rivers or brooks. As much as 1 ton of neutralizing lime might be used for every 10 tons of PzOs produced. Recently, a process has been developed to recover fluorine from the condenser water discharge of vacuum concentrators as a 15% solution of HzSiFo. Fluorine can also be recovered, in the same form, from scrubbing equipment. Apart from the cost of the additional equipment required, however, which is very high, the product obtained is of no value unless a market for it can be developed within a very limited distance of the producing plant because of transportation costs. If fluorine could be removed from the acid prior to concentration, a great deal of the problems described would be minimized, or disappear altogether. The precipitation of fluorine as sodium silicofluoride with anhydrous sodium oarbonate affords a simple and convenient procedure for the removal of most of the fluorine in wet-process phosphoric acid. The following reaction occurs: HZSiF6
+ NazC03
-+
NaeSiFs
+ COz + HsO
(7)
Sodium silicofluoride is used in the manufacture of vitreous enamels and opalescent glass, coagulant for latex, insecticide, and in the fluoridation of water. I t represents, therefore a valuable by-product. for which credit can be taken, to offset the cost of installing and operating the equipment required for precipitation of the fluorine. A continuous process has been developed for producing high grade sodium silicofluoride from wet-process phosphoric acid (32y0 PZOE) in a pilot plant operated by The Chemical Construction Corp. a t the American Cyanamid Co. plant in Brewster, Fla. A product with a purity greater than 98y0 was produced, with 80% of the fluorine in the acid removed as sodium silicofluoride.
These basic steps are used in producing NazSiF6 by the continuous process:
e e
Settling of solids in H,PO,
Warming of acid prior to precipitation e Precipitation of Na,SiF, with NazCOs e Addition of Aerosol O.T., d e foaming - ag - ent e Settling of slurry 0 Filtering and washing e Drying a n d bagging
The plant intermediate storage as a 32% q2Ob solution at a temperature of 57' C. was allowed to settle for 12 hours. Most of the suspended solids in the wet-process acid settled out fairly rapidly, and can be removed by a clarifier mechanism. The amount of solid residue in the acid as a function of time is indicated below.
.Hours Settled 0 1 3 5 7 12 16
Grams Solid/Liter of Settled Acid 3.00 dark residue 0.96 dark residue 0.79 dark residue 0.73 dark residue 0.70 dark residue 0.40 light residue 0.30 light residue
The clear supernatant liquor was pumped into an acid feed tank and heated to 65' C. Heating the acid prevents deposition of gypsum crystals during the precipitation of the sodium silicofluoride. Anhydrous sodium carbonate and the clear warm acid were fed simultaneously to the mixing tank. A sodium carbonate to fluorine weight ratio of 1.25 to 1.50 was most effective for removal of the fluorine from the acid.
Time, Minutes 3 5 10
20 30 45
Clear Liquor, Inches 4.0 8.0 10.0 12.5 12.5 12.5
1.25 1.50 1.50 1.50
Foaming was effectively controlled by adding 1.5 ml. of a 2.0y0Aerosol O.T. solution per gallon of acid. The Aerosol O.T. solution was added dropwise The slurry from the mixing tank flowed into a settling tank. The overflow from the settling tank was analyzed periodically for fluorine concentration. Complete settling of the sodium silicofluoride was obtained in 20 minutes. The table above indicates settling progress as a function of time. Other data obtained in the settling step and subsequent filtration were: Solids in feed slurry to settling tank, % Slurry at 60' C., sp. gr. Solids in settled slurry, % Settled slurry, sp. gr. Moisture in filter cake, yo Bulk density of dry cake, lb./cu. ft.
2.80 1.370
30.0 1.650 22.00 52.0
Countercurrent washing of the filter cake using a reslurry wash with dilute acid, followed by two water displacement washes was most effective in washing out most of the PzO5 left in the cake to produce a high-grade product. For one displacement wash, 0.25 gallon per pound of dry cake was used. The filter rate of the sodium silicofluoride was established as 140 pounds per square foot per hour using a polyethylene PO-801 filter cloth and a 17inch Hg. vacuum. An Eimco pan filter, 1.3 square feet filter area, was used in the tests. The filter cake was dried and analyzed. A typical analysis of the product, given in per cent, is as follows: Purity
99.00 0.25 0.00 0.00
PnOs Ca
so1
Moisture Water-insoluble FezOa A1201
Continuous precipitotion of Na&iFs in plant H 3 P 0 4 with NaZCO3
Settled Cake, Inches 0.5 1.0
0.10 0.035 0.004 0.003
The sodium remaining in solution after the precipitation of the sodium silicofluoride has very little effect on the quality or the strength of the final fertilizer product. RECEIVED for review September 28, 1960 ACCEPTED February 8, 1961 Division of Fertilizer and Soil Chemistry, 138th Meeting, ACS, New York, September 1960. VOL. 53. NO. 9
SEPTEMBER 1961
707
