Desulfurization of phosphogypsum

Phosphogypsum is mixed with fine coal, balled, and charged to a travelling grate where the charge is heated under reducing conditions to evolve sulfur and/or sulfur dioxide for conversion into sulfuric acid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to methods and apparatus for producing useful 
products from phosphogypsum, a by-product of fertilizer manufacturer, and 
particularly to the removal of sulfur, as sulfur dioxide and/or sulfur, 
for producing sulfuric acid. 
2. Background Art 
In the manufacture of phosphoric acid from phosphate rock such as is 
utilized in the manufacture of fertilizer, substantial quantities of 
gypsum are produced creating gypsum piles which are an eyesore as well as 
possibly presenting a hazard to the environment. Utilizing the waste 
gypsum to produce a useful product has been previously suggested, for 
example, see U.S. Pat. Nos. 3,729,551; 4,162,170; and 4,247,518. One 
suggested process is to convert the phosphogypsum into sulfuric acid and 
lime or cement, including the utilization of coal to reduce the calcium 
sulfate within a fluidized bed reactor. The prior art processes for 
converting phosphogypsum to sulfuric acid and lime or cement, including 
those employing fluidized bed reactors, have heretofore been uneconomical 
because of energy, labor and capital costs associated with the processes 
considered. 
SUMMARY OF THE INVENTION 
An object of the invention is to produce a process and apparatus which is 
economical for removing sulfur from phosphogypsum suitable for manufacture 
of sulfuric acid. 
In accordance with this and other objects, the invention is summarized in a 
process and apparatus for producing sulfur dioxide and/or sulfur from 
phosphogypsum wherein the phosphogypsum is mixed with fine coal and formed 
into pellets which are charged on a travelling grate where the charge is 
heated under reducing conditions to produce an effluent containing the 
sulfur dioxide and/or sulfur.

DESCRIPTION OF PREFERRED EMBODIMENT 
As illustrated in FIG. 1, an apparatus or plant for producing sulfuric acid 
from gypsum such as phosphogypsum in accordance with the invention 
includes facilities 10 for forming a mixture of gypsum and carbon material 
such as coal or coke, a balling mechanism 12 for forming the mixture into 
pellets or balls, and a travelling grate mechanism such as a circular 
travelling grate 14 for heating and reducing the gypsum in the pellets to 
produce an effluent containing sulfur dioxide and/or sulfur which is 
conducted by facilities 16 to a plant 18 for converting the sulfur dioxide 
into sulfuric acid. Additionally, the gypsum reducing plant includes 
phosphogypsum handling facilities 20, coal handling facilities 22, fuel or 
coal preparation facilities 24, pelletized lime handling facilities 26, 
and recycle handling facilities 28. The preferred apparatus and process 
are particularly designed for processing phosphogypsum, a waste product of 
phosphoric acid production from phosphate rock; however the apparatus and 
process can be used on other gypsum materials. 
Generally, phosphogypsum is a fine material with particles sized in the 
range from about 20 mesh to 500 mesh, and contains from about 60% to 95% 
CaSO.sub.4 in the form of fine crystals, the remainder being silica and 
other impurities. The following Table I is a size analysis of a typical 
Florida phosphogypsum and Table II is a chemical analysis of a typical 
Florida phosphogypsum. The phosphogypsum handling facilities 20 includes 
conveyors, slurry equipment, temporary storage facilities, and/or any 
other equipment handling and supplying the phosphogypsum to the mixing 
facilities 10. The phosphogypsum facilities 20 may include additional 
grinding or processing prior to mixing depending upon the properties of 
the phosphogypsum. 
TABLE I 
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PHOSPHOGYPSUM TYPICAL SIZE ANALYSIS 
(BASED ON FLORIDA ROCK) 
Mesh Cumulative 
Size % 
______________________________________ 
+20 M 4.6 
+28 M 9.1 
+48 M 21.0 
+65 M 28.8 
+100 M 37.7 
+200 M 62.6 
+325 M 69.3 
-325 M 30.7 
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TABLE II 
______________________________________ 
PHOSPHOGYPSUM TYPICAL CHEMICAL ANALYSIS 
Weight 
% 
(Dry Basis) 
______________________________________ 
Total P.sub.2 O.sub.5 
0.78 
Insol. P.sub.2 O.sub.5 
0.44 
Water Soluble P.sub.2 O.sub.5 
0.34 
CaO 37.70 
F.sub.2 O.sub.3 0.10 
Al.sub.2 O.sub.3 0.03 
Na.sub.2 O 0.11 
K.sub.2 O 0.05 
SiO.sub.2 3.90 
F 0.12 
Cl 0.01 
MgO 0.00 
SO.sub.3 54.60 
Total C 0.14 
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The carbon material used to reduce the gypsum is preferably a high sulfur 
content coal. Optionally other carbonaceous or reducing materials such as 
coke, petroleum coke, elemental sulfur, pyrite or other sulfides, etc. may 
be used in place of coal. The fuel or coal supply facilities and fuel 
preparation facilities, illustrated by respective boxes 22 and 24, include 
storage facilities, conveyors, grinding mechanism, e.g. wet ball milling 
equipment, coking facilities, and/or other equipment suitable for the 
handling and preparation of the fine coal, coke or other carbon material 
supplied to the mixing facility 10 as well as coal or other fuel necessary 
for producing heat in the travelling grate 14. 
For proper balling, it is generally preferable that a portion of the 
mixture fed to the balling mechanism 12 be relatively fine; these fines 
may be coal, gypsum, lime or mixtures thereof, or some other ingredient. 
The coal may be ground to fines 10 to 15%-10 microns in size prior to 
mixing, for example by open circuit wet ball milling, or the coal may be 
coarse when fed to the mixing facilities 10, and the coarse coal and 
phosphogypsum mixed and ground together so that the combined mixture, 
after grinding, will be 10 to 15% of less than 10 microns in size. As 
another alternative, recycle lime used as a binding agent may form a 
portion or all of the small particle material. 
The proportioning and mixing facilities 10 includes suitable surge hoppers, 
weigh feeders and other equipment for producing a mixture of phosphogypsum 
and coal at a ratio to produce a ratio of gypsum to fixed carbon of about 
92-97% gypsum and about 7-3% fixed carbon, by weight. Recycle solids and 
fines from the proportioning and blending 10, the circular grate 14, and 
lime handling 26 are handled by recycle facilities 28 for reintroduction 
into the proportioning and blending 10. Recycle materials of fines and 
lime are proportioned as generated; lime, e.g. 1 to 10% lime by dry 
weight, particularly being included for aiding in the inhibition of 
corrosion during the process as well as for aiding in the balling 
procedure. 
The balling mechanism can be an open circuit balling pan arrangement or a 
closed circuit balling drum arrangement with sizing devices such as 
vibrating screens or roller separators. The balling mechanism is designed 
to produce balls or green pellets about 0.5 inches (13 mm) in diameter. 
One example of a suitable pelletizing pan apparatus is illustrated in U.S. 
Pat. No. 3,169,269. Water and/or other ingredients may be added to the 
mixture being balled to aid in the forming of green pellets. 
The travelling grate mechanism 14 includes sealed hoods and burners for 
heating the pellets under controlled reducing atmospheric conditions to 
evolve the fixed or combined sulfur as element sulfur and/or sulfur 
dioxide from the phosphogypsum. Preferably the travelling grate mechanism 
14 is a liquid sealed circular grate (Carousel type) similar to the 
circular travelling grates commercially available from Davy McKee 
Corporation, 6200 Oak Tree Boulevard, Cleveland, Ohio 44131 U.S.A., having 
sufficient size in order to economically handle large quantities of 
pellets. 
An example of a suitable circular travelling grate mechanism 14 is 
illustrated in FIGS. 2 and 3. The mechanism 14 includes facilities 40 for 
depositing a charge of green pellets upon a moving grate 42 which 
successively moves the charge through various zones, such as predrying 
zone 44, drying zone 46, firing zone 48, post-firing zone 50 and cooling 
zone 52, within a sealed hood to a facility 54 for discharging lime 
pellets from the travelling grate. In the predrying zone 44, air from 
blower 56, and which is heated in heat exchangers 58 and 60 by the product 
gas, is employed to remove at least a portion of the moisture from the 
green pellets. Blower 62 drives air upward through the hot charge on the 
grate 42 in the cooling zone 52 and thence to the drying zone 46 where the 
air completes the drying of the green pellets. The moist waste drying air 
is removed by blower 64 from a wind box 66 extending in the predrying and 
drying zones. A burner, such as a fixed bed gas producer 68 operating on 
high sulfur content coal to produce hot raw low BTU gas, supplies heated 
gas to the firing zone 48 sufficient to heat the pellet charge to a 
temperature within the range from 1800.degree. to 2200.degree. F. 
(980.degree. to 1200.degree. C.). Quantities of fresh air from the blower 
56 and recycled product gas from blower 70 are also supplied to the firing 
zone 48; the atmosphere in the charge being maintained at least 
non-oxidizing and preferably reducing. The product gas is removed from the 
firing zone 48 via wind box 72 and is then passed through an incinerator 
74 where combustible gas products are burned with fresh air from blower 56 
and hot low BTU gas from burner 68. In post-firing zone 50, a portion of 
the product gas from blower 70 is recycled to pass upward through the 
charge and then downward into the product receiving wind box 72 to remove 
the greatest portion of product gas from the charge. The output product 
gas from the incinerator 74 is passed through a waste heat boiler 76 and 
the heat exchangers 60 and 58 where heat from the process is recovered. 
Heating the pellets in the firing zone 48 within a neutral or reducing 
atmosphere causes one or both of the following endothermic reduction 
reactions between the carbon and gypsum. 
EQU CaSO.sub.4 +C.fwdarw.CaO+CO+SO.sub.2 
EQU CaSO.sub.4 +3C.fwdarw.CaO+3CO+S 
These reactions may generally occur with intermediate production of calcium 
sulfide and its reaction with calcium sulfate. Carbon dioxide may be 
produced in place of at least some of the carbon monoxide, particularly at 
lower temperatures and under more oxidizing conditions. Additionally, 
oxygen in the draft passing through the charge may exothermally react with 
carbon and carbon monoxide. 
EQU 2C+O.sub.2 .fwdarw.2CO 
EQU CO+1/2O.sub.2 .fwdarw.CO.sub.2 +(heat) 
This helps maintain a reducing environment during the reduction as well as 
supplying additional heat to maintain the reduction reaction. Carbon 
monoxide and elemental sulfur produced in the firing zone 48 will 
subsequently be oxidized in the incinerator 74 to carbon dioxide and 
sulfur dioxide. The total reaction process may be expressed in the 
following equation: 
EQU 2CaSO.sub.4 +2C+O.sub.2 .fwdarw.2CaO+2SO.sub.2 +2CO.sub.2 
In FIG. 4, curves 80, 82 and 84 show percentages of sulfur removed for 
varying percentages of coke (containing about 80% fixed carbon) in the 
pellets at respective temperatures of 1800.degree. F. (980.degree. C.), 
2000.degree. F. (1100.degree. C.) and 2200.degree. F. (1200.degree. C.). 
As shown by these curves, the most efficient removal of sulfur is 
performed at temperatures above 2000.degree. F. (1100.degree. C.) with a 
coke content of about 6 parts (about 5 parts fixed carbon) by weight 
relative to a phosphogypsum content of about 94 parts (about 87 parts 
gypsum) by weight. Excess carbon above the stoichiometric quantity may be 
included in the pellets to produce combustible material for the 
incinerator 74. 
The following Tables III, IV, V and VI represent raw materials, material 
balance, overall heat balance and product gas analysis derived from 
calculations concerning the process. 
TABLE III 
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RAW MATERIALS 
Phosphogypsum 
% Coal % Recycle % 
______________________________________ 
CaSO.sub.4.2H.sub.2 O 
92 Fixed 60 CaO 76.7 
Carbon 
CaCO.sub.3 1 Volatile 30 CaSO.sub.4 
7.8 
Matter 
CaF,SiO.sub.2,P.sub.2 O.sub.3 
7 Ash 10 CaF,SiO.sub.2, 
16.6 
Sulfur 5 P.sub.2 O.sub.3 
______________________________________ 
Pellet Composi- 
tion (Dry Basis) 
% 
______________________________________ 
Phosphogypsum 
87.03 
Coal 6.97 (Above stoichiometric, excess for 
incineration.) 
Recycle 6.00 
______________________________________ 
TABLE IV 
______________________________________ 
MATERIAL BALANCE 
Basis: 1 Minute of flowsheet rate, 2000 lbs 
phosphogypsum, dry basis. 
Stoichiometric C 
Excess Coal 
lb/min lb/min 
______________________________________ 
Input 
Green Pellets 2872 lbs 2872 lbs 
Low BTU Gas to Hood 
911.2 911.2 
Low BTU Gas 759.3 445.3 
to Incinerator 
Air to Cooling 3920 3920 
Air to Predrying 
12604 12604 
Air for Combustion 
1385 1385 
in Hood 
Air for Combustion 
1154 1435 
in Incinerator 
23605.5 23572.5 
Output 
Spent pellets 947.6 lbs 947.6 lbs 
Exhaust Product Gas 
5174.6 5141.6 
Waste Drying Gas 
17483.3 17483.3 
23605.5 23572.5 
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TABLE V 
______________________________________ 
OVERALL HEAT BALANCE 
Basis: 1 Minute of flowsheet rate, 2000 lbs 
phosphogypsum, dry basis. 
MMBtu/ 
min 
______________________________________ 
Heat In 
Low BTU Gas - 
latent 2.378 
Firing Zone sensible .268 
Low BTU Gas - 
latent 1.982 Stoichiometric 
carbon .936 
Incenerator sensible .223 replaced by ex- 
4.851 cess coal 
in pellet 
Heat Out 
H.sub.2 O, latent and 1.131 
sensible gas 
Waste drying gases .572 
Endothermal reaction 1.277 
Exhaust product gases .164 
Steam (waste heat 1.494 
boiler) 
Spent pellets .041 
Losses .172 
4.851 
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TABLE VI 
______________________________________ 
PRODUCT GAS ANALYSIS 
Low BTU Residual 
Gas Com- Coal Phosphogypsum 
bustion Combustion 
Decomposition 
Total 
Moles Moles Moles Moles % 
______________________________________ 
CO.sub.2 
18.52 5.53 5.13 29.18 18.6 
H.sub.2 O 
11.43 11.43 7.3 
N.sub.2 
84.36 20.80 105.16 
67.1 
SO.sub.2 
.51 .25 10.25 11.01 7.0 
156.78 
100.0 
______________________________________ 
The calculations in these tables are based upon assumptions that (1) 96 
percent yield of sulfur from phosphogypsum as SO.sub.2, (2 ) use of 
stoichiometric carbon or surplus coal in the green pellets with excess 
fuel units consumed in the incinerator with stoichiometric oxygen from 
air, (3) maintenance of oxidizing zones for drying and cooling, and 
neutral to reducing zones for firing the post-firing, and (4) 
establishment of waste heat boiler, heat exchangers and draft 
recirculation systems within the temperture-draft composition allowances. 
The material balance presented in Table IV is based upon 2000 pounds of dry 
phosphogypsum. In the first column, the calculations are based upon 
inclusion of stoichiometric quantities of carbon as part of the pellet 
blend, and in the second column the calculations are based upon the use of 
excess coal wherein the amounts above stoichiometric carbon were 
considered to be gasified or evolved as condensibles for subsequent 
oxidation in the incinerator. 
From Table V, it is noted that 4.851 million BTU per ton of phosphogypsum 
are required for sustaining the process. Additionally, however, 
stoichiometric carbon from coal is required for the desulfurizaion 
reaction in the amount of 0.834 million BTU per ton. The heat balance 
shows 1.345 million BTU (assuming about 10% loss) generated as by-product 
steam. 
Table VI shows a product gas analysis with 7.0 SO.sub.2 without excess 
oxygen. If required, this could be increased to about 8 to 10 percent 
through use of a sulfur burner as fuel for the incinerator instead of 
using low BTU gas or by incorporation of sulfur or sulfides as additives 
to the phosphogypsum. 
The effluent gas removed by blower 76 through heat exchanger 58 is passed 
to the sulfuric acid plant 18. The plant 18 can be any suitable plant such 
as that available from Davy McKee, Lakeland, Fla. 33803 which employs the 
Davy Double Absorption catalytic process to convert sulfur dioxide into 
sulfuric acid. 
The facilities 26, include hoppers, conveyors, storage means, etc., for 
handling the lime pellet by-product from the travelling grate 14. This 
lime by-product may be sold or used in pollution control or as a 
flocculating agent for slimes associated with phosphoric acid production, 
for flue gas desulfurization, for stabilizing solid wastes from 
beneficiation plants, or other use. 
The present invention results in a more economical and useful method and 
apparatus for converting existing phosphogypsum piles into useful 
by-products (sulfuric acid, lime and steam) than has been possible in the 
prior art. The cost of the present method and apparatus is competitive 
with present sulfur burning plants for generating sulfur dioxide to be 
converted into sulfuric acid. 
Since many modifications, variations and changes in detail may be made in 
the above described embodiment without departing from the scope and spirit 
of the invention, it is intended that all matter in the foregoing 
description and shown in the accompanying drawings be interpreted as 
illustrative and not in a limiting sense.