Potash from Polyhalite by The reduction of polyhalite was carried out as a continuous operation in a laboratory rotary kiln, using a mixture of carbon monoxide and hydrogen derived from natural gas. A maximum temperature of approximately 830" C. was most desirable. Too high a temperature caused fusion of the polyhalite, and too low a temperature slowed down the reduction process. No volatilization of potassium compounds occurred during reduction at temperatures up to approximately 900"C. Polyhalite reduced in the rotary kiln by means of a mixture of CO 3H2 derived from natural gas, with a maximum
F. FRAAS AND EVERETT P. PARTRIDGE' Nonmetallic Minerals Experiment Station, U. S. Bureau of Mines, Rutgers University, New Brunswick, N. J.
A
S PART of a general investigation of the industrial possibilities of polyhalite (K2SO4.MgSO4.2CaBO4.2Hz0) i t was previously demonstrated (2, is) that reduction with hydrogen, extraction of the reduction product with water, and evaporation of the concentrated extract liquor would yield a mixture of potassium monosulfide and hydrosulfide. The high recovery of potassium compounds and the sharp separation from calcium and magnesium compounds indicated by the early experiments led to a more detailed study of the reduction step, as described in the present paper. Because natural gas is available in large quantities a t low cost in the region in Kew Mexico and Texas underlain by polyhalite deposits, particular attention has been paid to its utilization in the reduction operation, both directly and after conversion into a mixture of hydrogen and carbon monoxide. The earlier experiments with hydrogen alone have also been supplemented by reduction tests in which carbon monoxide and carbon, respectively. have been employed. In general it has been possible to obtain satisfactory results with each of these materials.
+
Reductions with carbon monoxide and with mixtures of carbon monoxide and hydrogen were carried out in porcelain boats inserted in a silica combustion tube, the temperature being maintained with little variation by heating in an automatically controlled electric furnace. A sample of polyhalite, heated to 920" C. for one hour in a current of carbon monoxide previously dried over concentrated sulfuric acid, failed to give any perceptible reaction. The only change was a fusion of the polyhalite. However, when a mixture of carbon monoxide and water vapor was used, reduction of the polyhalite resulted, although the reaction apparently was much slower than with hydrogen. Even after 2.25 hours a considerable amount of sulfate was still found to be present in the solid product, so that the sample was reground and reduced further in separate batches in which various mixtures of carbon monoxide, water vapor, and hydrogen were used. The results of these reductions are summarized in Table 11. Stage 1 is the first reduction of polyhalite with carbon monoxide and water vapor. Stage 2 is the second reduction of the product from stage 1 after regrinding. The significant feature of the carbon monoxidewater vapor reductions is an unusually high percentage removal of sulfur, owing to the large partial pressure of water vapor, and an apparently much slower rate of reaction. The rate of reduction with mixtures containing 75 per cent of hydrogen is comparable to that with pure hydrogen. Further reductions in porcelain boats were carried out using Pittsburgh natural gas, the composition of which is represented by lot A in Table 111. Two runs were made, one with the dry gas directly from the cylinder and the other with gas and water vapor. These experiments are summarized in Table IV. The reductions with natural gas proceeded just about as readily as those with hydrogen. The significant difference, however, was a deposition of carbon, which was especially noticeable in the reduction with dry gas. The carbon deposited not only on the silica tube but also in the reduction mass itself, giving the latter a pronounced jetblack color instead of the usual pink tint. With the 1 to 1 mixture of natural gas and water, no carbon deposited in the reduction mass and the amount on the silica tube was only about one-fourth that from the reduction with dry gas.
Batch Tests with Various Reducing Agents Polyhalite, of the composition of lot A of Table I, reduced in a loosely covered crucible with wood charcoal in 50 per cent excess a t a temperature of 950" to 1070" C. for 2 hours, yielded a material containing only 0.71 per cent sulfur in the form of sulfate. After extraction of 500 grams of the reduction product with 500 grams of water at the boiling point for 70 minutes, washing with an additional 500 grams of water, and drying, the residue showed a content of 2.46 per cent potassium. ~~
TABLEI. COMPOSITION OF RAWPOLYHALITE Lot A B
--
Compn., Per CentMg Ca 6 C1 11.68 3.69 14.26 20.97 0.13 11.98 3.90 14.09 21.43 0.07
K
Screen Mesh -1O-I-30 -4+10
When a quantity of the reduced material was moistened in one portion with water, it spontaneously heated to interspersed incandescent particles, which phenomenon was transient but passed over the whole mass. The reaction occurring on adding the material to water was more violent than with the hydrogen-reduced material. During the extraction a t the boiling point a rather distinct odor of ammonia could be detected which was evidently a decomposition product of nitrides or cyanamides. 1
Present address, Hall Laboratories, Inc., Pittsburgh, Pa.
2214
Reduction Process ~~~~
temperature of 830" C., lost sulfur corresponding approximately to the reduction of the magnesium sulfate to the oxide. The calcium sulfate was reduced to the sulfide, except that an excess of water vapor caused further removal of sulfur, yielding some calcium oxide. The potassium sulfate was reduced to potassium sulfide under all conditions employed. On the basis of assumed efficiencies for heat utilization in a converter unit for natural gas and in a reduction unit using this converted gas, it is estimated that in practice approximately 16,000 cubic feet of methane would be required for the reduction of one ton of polyhalite.
Continuous Reduction with Converted Gas peratures below 855' C. in porcelain boats with the same apparatus as in the previous carbon monoxide reductions. The results of these experiments are listed in Table V. As represented by the standard extraction ( 2 ) , the degree of reduction in each case was rather complete, although for some unaccountable reason the reductions with hydrogen apparently were not as good as the one with natural gas. The data show that the reaction can be carried out a t comparatively lorn temperatures. The natural gas reduction a t 800" C. gave no deposition of carbon whatever a t any place in the apparatus or in the reduction mass itself. The product from these low-temperature reductions had an altogether different appearance from that reduced a t 905 O C., especially in the case of the larger particles close to the maximum size that could pass a 0.25-inch grizzly; these particles retained their original outline and were in no way bound together, so that they could be poured directly out of the boat.
Reductions at Lower Temperatures Polyhalite reduced in stationary containers a t temperatures above 900" C. was found to cake into masses of a hardness comparable to chalk. Since this behavior did not seem promising for industrial operation, the effect of slightly lower temperatures was investigated. By two successive coolings from a temperature of 935" C., polyhalite of the composition of lot A in Table I was found to have a solidifying point a t 855' C. Reductions were accordingly carried out a t tem-
Heat of Reaction Tables 11, IV, and V show that chemically it is immaterial
whether the reducing gas is a 1 to 1 mixture of natural gas and water vaDor or a 1 to 3 mixture of , carbon rnonokde and hydrogen (the water-gas reaction product of natural B~ATS TABLE 11. REDUCTION OF POLYHALITE IN COMBUSTION WITH CARBON MONOXIDE gas) since there is not much difference in the rates of reaction and both give S Gas Weight Reducing Time a t Compn. of Productb Degree, of Run Stage Vol. Sample Gas 905' C. Total S S o d - - Mg Removal Reduction fairly complete r e d u c t i o n . PractiLiters Grams Min.a %S %S % % % cally, however, there is a significant 31.5 72.9 1 1 75 50.5 Cod 135 23.27 9.24 5.99 difference between the two because of 43.3 98.9 2 2 :1 8.0 cod 65 22.75 0.437 7.09 35.6 98.9 3 2 8.7 CO + 3Hae 10 27.20 0.447 7.43 the energy changes involved. 38.7 99.7 4 2 CO + 3H2e 60 28.15 0 , 1 4 5 8.08 16 5.7 The heat balance of the reaction in6 Additional time of 45 minutes to bring furnace up t o temperature. b ComDosition of Dolvhalite eiven in Table I. lot A . volving the reduction of polyhalite C Rate'of gas 5ow-th; same is in run 4. with hydrogen cannot be accurately d Saturated with water a t 75-80' C. e Saturated with water a t room temperature. calculated a t the reaction temperature because specific heat data, especially the variation a t elevated temperatures, TABLE 111. COMPOSITION OF NATURAL GAS are not available for potassium sulfate and sulfide, magnesium Compn., % sulfate and sulfide, and calcium sulfide. Calculations based Lot CHI C2H4 Nz on broad assumptions show that the heat of reaction probably A 83.0 15.8 1.1 is small, the degree of exothermicity being increased as more B= 94.0 6.1 0.0 of the alkaline earth sulfides are converted to oxides, and de0 Gas converted f o r early rotary kiln reductions. creased when carbon monoxide is substituted for hydrogen. Since natural gas usually consists primarily of methane, O F POLYHlLITE IN COMBUSTION BOATS the conversion to a mixture of carbon monoxide and hydrogen TABLE Iv. REDUCTION WITH NATURAL Gas. may be represented by the reaction: Run
Time at -Compn. 905' C. Total S
Mi%.
% S
of Product804 .Mg
%S
%
Degree of Sulfur Reduction Removal
%
CH4
%
+ HzO (gas)
= CO
+ 3Hz
(1)
The heat of formation from the elements for methane and liquid water are given by Storch (15) while that for carbon monoxide was calculated from data of Lewis and Randall (IO) giving B value of AH2g1 = 26,150. For the heat of vaporization of water a t 298" K., the value AH = 10,450
120 22 89 .. 57 08 0.083 66 .. 77 41 99 99 .. 81 22 2 2 2l b= 120 0.348 1 .. 9 Composition of raw polyhalite given i n Table I, lot A; composition gas given in Table 111, lot A . b Gas direct from cylinder. C Gas saturated with water a t 80-85' C. Q
1
225
VOL.28. NO. 2
INDUSTRIAL AKD ENGINEERING CHEMISTRY
226
would be prohibitive from the standpoint of the carbon deposition t h t would ensue. If a mixture of natural gas and water vapor were used directly, a considerahle amount of heat would have to he supplied to the reaction, whereas with the use of a 1 to 3 mixture of carbon monoxide and hydrogen it would be necessary only to take care of heat losses from the apparatus. From an engineering standpoint it seems preferable h t to convert natural gas into a mixture of hydrogen and carbon monoxide, using apparatus specifically adapted forthis operation which involves little corrosion and the transfer of a considerable amount of heat. In the second stage, where the converted gas reacts to reduce the polyhalite, it is not necessary to supply heat, and in the construction of the apparatus a t this point more emphasis may he placed on faotors such as the resistance to corrosion.
Continuous Countercurrent Reduction The first attempts a t studying the rate of reduction of polyhalite and the degree of oxidation of the exit gases in a continuous operation were made with a laboratory apparntus simulating a shaft kiln. Difficulty in maintaining the movement of solids led to the development of a rotary kiln which functioned quite satisfactorily.
FIQURE 1. DIAGHAM OF APPARATUSFOR TION OF
CoNTINDoU0
%Doc-
POLYAALITE
I. Pre~mreremiator on RBR oulinder 2.
3.
4. 5.
6. 7. S.
9.
IC. 11. 12. 13. 14. '
15. 16.
17.
1s. 19.
20. 21.
Flowmeter Loostion of 888 meter wheo natursi pas voirirnea are indicated Mercury temperature regulator Eirictriediy heated Bask oonteining wster Thermometer LIlgein. Catelyat oontainei for oonveiter wwisting of a binck iron pipe ( L = 81 om., i. d. = 3.6 om.), later replwed by w a r t s tube (L 121.9 am., i. d. = 2.34 om.) CunYeiter iurnaee Cwdensw Wet g*s meter Sampling outlet Location of carbon dioxide and water absorber used oirly i n r u m 11 and 12 Glass window Gss-tight benriog surfaoa Rubber tubs Nitrogen-flushing Bpertuie Disobaqe coliecDor Looation of therrnoeoupie Furnsoe ( L = 76.2 cm.) Kiln oonsisting of 18-8 eiirome-niokal steel pipe (L = 106.7 cm.. i.d. -3.51cm.,o.d. =4.22crn.)pLaeedinfuroauaofS.OB.cm.bore for rum 1 to 10, inoiusive. For rum 11 and 12, lining of short lengths of aluodum tubing (i. d. = 3.61 ~ m . .9. d. 4.43 cm.) collared to fit i n 18-8 chrome-nickel steei pipe ( L 10B.7 em.. i. d. 5.25 om., a. d. = 6.03 om.) which WB(I inaerted in fomsoe of 7.02-om. bore About 0.1-mm. clearance 26. Compreaaed sir Jar sieator Poiyhelite feeder 27. Eieotoi for wmte gas Palvhslite delivew tube 28. Weate gas sampler Aperture for sampling inaide kiln
-
-
22. 23.
24. 26.
--
wits used. The hest capacities for methane, water vapor, and hydrogen are also given by Storch (25),while that for carbon monoxide was obtained from Lewis and Randall (9). Utilizing these values and assuming that the heat of formation tor carbon monoxide a t 298" K. does not differ much from t.hat at 291° K., the iolloming equation for the hest of resction was obtained: AH = 45,980 i- 14.45T - 0.24 X 10-8Tz - 0.266 X IO-*Ta
In the range of temperatures at which the reduction reactions take place or at 827" C., we have the value ArfLtu8= 54,030 calories, which is decidedly endothermic. The use of methane alone in the reduction of polyhalite
Rotary Kiln A description of the rotary kiln is given in Figure 1. The driving mechanism including the supporting rolls for the kiln and the polyhalite feed device were the same as that described hy Clarke and eo-urorkers (I) for the calcination of polyhalite. As shown in Figure 1, two kilns were employed; the inner surface in one ease was 18-8 chrome-nickel steel and in the other alundum. Most of the work cited in thk paper was completed before i t was found that the eoutaminstion introduced from Olxe chromenickel st,eeIwas detrimental to the proper extraction of the potassium. Consequently two runs (11 and 12) in the kiln with the alundum lining were added to verify the reaction rates. A single temperature measurement was made a t the center of the furnace, between the outside of the kiln and the interior of the furnace wall. No temperature measurements were made along the interior of the kiln during actual operation because of the small diameter of t.he kiln and the corrosive nature ol the gases. However, au approximate temperature distribution for the stationary kiln, as shown in Figure 2, was obtained with a movable couple. The temperature superimposed on each curve is that indicated by the controlling thermocouple on the outside of the kiln. This couple was checked against an optical pyrometer, and the accuracy is probably 1 3 " C. The gases for the reductionsin the rotary kiln were obtained by converting natural gas into a mixture of hydrogen and carbon monoxide. In the conversion, natural gas saturated with water a t 80" to 90'C. was passed a t 950" C. over a catalyst consisting of metallic nickel deposited on broken alundum in the ratio of 0.9 kg. of nickel nitrate hexahydrate to 2 liters of broken alundum as described by Hawk and coworkers (6). The container for the catalyst is shown in Figure 1. Sampling and Analysis Samples of converted gas representative of the entire run were obtained by slow, continuous removal from tho system between the gas meter and the kiln by displacemept of saturated water from a liter bottle through 8 siphon. To sample the outlet gases, a device (shown in Figure 1) consisting of a 200-eo. spherical glass bulb to rrhicb was attached a spout 40 em. long snd 0.2 em. outside diameter was used. The smut. the Sore of whinh WR,S ad__ justed according to the rate ~f~sampling desired, was provided with a hook-shaped seal at its end. A tube of small diameter open at both ends and large enough to slide over the spout, nl~~
FEBRUARY, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE v. REDUCTIONOF POLYKALITE IN COMBUSTION BOATS AT Low TEMPERATURES~ Kin Standard Extn. Reduced Kin K Temp. Time Polyhalite residueb extd. CMin. % % 0.862 95.3 120 18.65 Hz 835 1.145 94.3 120 20.25 Hz * 800 20.70 1.003 95.1 775 60 Hz 0 120 20.70 1.175 94.2 HZ c 750 Naturalgasd 800 120 21.70 0.308 98.5 a Composition of raw polyhalite given i n Table I, lot A . b Percentage based on weight of unextracted solid. 0 Saturated with water a t room tem erature. Composition of natural gaa given d Saturated with water a t 80-85' C? in Table 111, lot A . Reducing Gas
227
ents in the outlet gas, it remained in the sample bulb after the other gases were pumped out. This sulfur was determined as barium: sulfate after dissolving out with carbon disulfide and oxidizing with sodium peroxide in a Pennock and Martin crucible. For the solids, potassium and magnesium were determined gravimetrically as the chloroplatinate and pyrophosphate, respectively, and sulfate as barium sulfate. Carbon dioxide, after being evolved in the presence of chromic acid and hydrogen peroxide, was gravimetrically determined by absorption in ascarite. Several bulbs containing a solution of chromic acid in concentrated sulfuric acid were placed in the absorption train. Calcium was found by the volumetric permanganate method. In the total sulfur determinations the sulfides were oxidized with hydrogen peroxide in ammoniacal solutions.
Preliminary Reductions lowed the breaking of the hook-shaped seal by means of a forward motion. After evacuating the sample bulb to a fraction of a millimeter pressure, the spout with its outer concentric tube was inserted into the kiln through an opening at the end where the by-product gases were removed. Then by a forward motion of the outer tube the hook-shaped seal was broken. After sufficient time had elapsed to fill the bulb, the spout was withdrawn from the kiln and its tip quickly sealed in a Bunsen flame. The reduced solid was sampled by collecting at the end of the run the material on the last 15 om. (5.9 inches) of the discharge end of the kiln. The Orsat apparatus was used for the determination of the gas constituents with the exception of water and hydrogen sulfide which were determined as previously described (3). Values for oxygen and unsaturated hydrocarbons are omitted from the data since the quantities determined were smaller than the experimental error. When elemental sulfur was one of the constituTABLE VI.
REDUCTION OF POLYHALITE IN ROTARY KILN 880
2 832
3 780
4 802
5 832
6 832
7 882
8 832
9 832
110 832
12" 832
1.75 2.4
1.75 2.4
1.75 2.4
1.75 2.4
1.75 4.6
1.75 4.6
1.75 4.6
1.75 2.4
1.56 2.4
.. , ..
...
58 0.08 420
49 0.11 421
38 0.09 401
43 39 0.10 0.04 400 215
33 0.05 220
36 0.06 218
22.45 28.20 2.99 3.54 20.13 19.09
23.45 29.30 1.30 3.92 19.80 19.94
16.34 23.05 20.73 0.01 14.44 13.89
18.00 23.30 19.05 0.15 15.68 15.30
23.10 29.15 2.42 3.96 19.90 19.64
19.08 24.80 13.38 1.91 16.85 16.22
19.79 24.05 12.75 1.82 17.40 16.83
24.03 30.60 0.33 4.46 20.35 20.43
24.40 31.00 0.23 4.83 20.30 20.75
74.3 1.2 0.0 21.5 1.8 1.1
76.0 1.9 0.0 17.2 0.8 4.0
66.7 1.1 0.0 26.6 1.6 3.9
57.0 0.1 0.6 30.9 3.9 7.5
75.4 1.1 0.0 23.2 0.3 0.0
70.7 1.2 0.0 27.5 0.5 0.0
72.0 1.5 0.0 26.1 0.4 0.0
73.3 1.7 0.0 24.6 0.4 0.0
73.3 2.0 0.0 24.1 0.6
15.6 4.6 58.7 0.3 0.3 4.1 13.7 2.7 0.0 1.89
3.2 2.4 66.2 0.0 0.0 0.3 17.5 9.6 0.8 1.72
53.2 2.1 12.2 0.0 1.2 21.7 6.6 2.9 0.0 1.89
23.6 2.3 37.2 0.4 0.0 13.9 11.6 10.7 0.1 1.94
21.3 2.1 58.0 0.9
19.6 3.4 54.7 0.9 0.0 8.0 13.0 0.0 0.0 4.27
15.7 5.4 55.7 0.8 0.0 7.5 14.9 0 0 0.0 4.35
19.2 2.3 55.9 1.2 0.0 6.8 12.9 1.3 0.2 1.92
19.5 2.0 66.4 1.4 0.0 7.9 13.8 0.0
1.63 0.14 0.06 0.02 1.64 0.44 1.13 27.4 45 73
1.16 0.13 0.05 0.02 1.17 0.10 1.03 30.1 14 92
1.39 0.14 0.05
1.23 0.15 0.05 0.00 1.13 0.62 1.16 26.3 -3 45
2.97 0.25 0.13 0.04 3.06 1.08 1.97 30.3 55 65
2.36 2.17 0.32 0.33 0.11 0.10 0.03 0.04 2.43 2.26 0.87 0.69 2.56 2.61 31.5 29.9 -5 -13 64 70
1.61 0.15 0.06 0.03 1.69 0.58 1.15 27.8 47 Q6
1.61 0.15 0.06 0.03 1.70 0.60 1.19 27.5 43 65
1
Run:
Max. temp
C.
Kiln settin;: Slope degrees Rate 'of rotation, r. p. m Time of retention,b min.: Solids Gases a t 800' C. Duration of run, min. Compn. of reduced solids, per cent:
ca n
K, calcd. Gas compn., per cent: Inlet, gas: Ha CHI CiHs
co coz N 2
Outlet gas: H2
His
Hz0 CH4 CiH4
co
coz Nn
sz
Rate of flow of oolvha1ite.O - . n_r a m d. m i n Rate of flow of gaaes,d liters/min., S. T . P.: f l ) Drv inlet sa8
rhalite
The first reduction in the rotary kiln constituted a reduction of (-6+10) mesh polyhalite that was carefully hand picked to free it from all obvious crystals of sodium chloride. On passing this material through the kiln heated to a temperature of approximately 840" C., considerable balling up occurred; lumps were formed almost 2 cm. in diameter without any signs of reduction. On raising the temperature to 900" C . , only a slight reaction was visible where the lumps were mechanically abraded by the walls of the kiln. A qualitative test showed considerable chloride t o be present. The polyhalite for the subsequent reductions was then washed practically free of sodium chloride. Entirely different behavior was observed in these experiments. No two particles were found to cling together, and the material re-
0.00
1.36 1.26 1.13 26.8 20 7
Reduction of iolids; per cent: Calcd. from gas 106 104 9 Calcd. from solid 91.4 96.5 18.4 a With alundum-lined kiln; all other runs with 18-8 chrome-nickelsteel kiln. b For 53.4 cm. length of midportion of kiln. 0 Composition of polyhalite given i n Table I, lot B . d Calculated as follows: 1) From actual measurement of gas saturated with water. 2) From theoretical content of water of hydration i n polyhalite. .(3) From gas temperature in meter. Water lost to reduced solid not accounted for. (4) From COZi n reduction product. A (5) = (1). W0'
0.0
7.6 10.1 0.0 0.0
3.29
71 92 59 0.08 0.05 ... 363 397 543
0.0
0.0
1.99
25.71 30.30 2.21 3.98 21.92 21.86
...
,..
,.. , , ,
,.. , , ,
I . .
... ... ... ... ... , . .
. . 64
...
511
26.74 31.36 0.52 3.73 22.81 22.74
...
... ...
...
... ...
...
... ... .. ... ... ...
...
...
2.27
2.33
... ... ...
...
...
...
1.8'
... ... 2.50
1.36
1.39
...
...
...
24
...
... ...
44
...
61 60 97 93 ... 55.0 58.6 99.1 99.4 94.4 98.8 C) (1) B (6) = 7 100 (2) (3) (4)]. (7) From sulfate content. A = Burn of percentage constituents in inlet gas equivalent t o HZ = H 2 CO 4CH4 -I-,7CzH6;. A' f o r gas out. B = sum of percentage constituents i n inlet gas equivalent to Ha and HzO, = A H?O COz,+ H z S ; , B' f o r p s out. C = approximate correction for air oxidation equivalent to NZ = 0.532 [(%Nz out) (%Nz in)]. Approximate value = 4 times natural gas flow.
44 32.0
100 93.3 (A'
1
+ [ +
a
,
+
+ +
+
-
+
-
.
.
INDUSTRIAL AND ENGINEEIUKG CIfEM1SlIIY
228
nun
No.
ME
I
1:SOJ 2.05
.. 5
..
8
~. .. ..
6 7
9 10 lid 126
13 14
15 16 17 18
.. 3'06 3.07
..
Solid Reduced
Ca
5.7s 4.76
2.57 2.53 1.85 2.03 2.55 2.16
4.94
Z.23
6 00 6.09 5,49
2.60 2.59
5,6l 5.85 4.08 4.49
6.42
6 67
..
13:i4
S:44 6 68
16:86
..
VOL. 28, NO. 2
4.67 2.80 2.92
7.71
5:i5
13:in
..
8.79 9.14 7.19 7.27 9.09 7.73 7.50 9.34 9.67 7.27 9.45
... ...
0.41 6.46
6:ii
9.78 7.10 13.50
6 90 12 97
13.44
10 55 4 15 13 45
0.01 10.63 6.74
O.!X
9.33
000
5.94 0.76 4.17 B 98 0 10 0 07 2 60 0 68 0 16 0 02 0 03 0:n1 0.41 0.00
0.81 0.89 0.00 0.03 0.90 0.43 0.41
1.01
1.10
0.83 0 91 0.85 1 54
0 01 0 06
1:05
0.05
0.61 0.16 1.26 0.78 0.76 0.81 0.38 0.94 0.99
-2.89
0.28 0.19 -0.01 -0.44 0.01 -0 22 1.49 -0.04
880 832 780 802 832 832
..
s82 R32 832
880 832
0134 0.45 0.FO
832 833
a32
0.3$ 0.01 0.08
sp:!
Rate of Reduction Table V I summarizes a series of reductions in tlie rotary kiln. The data include values for the rates of flow, time of retention, and composition of gases and solids. The length of kiln and specific volume of gases, from which the time of retention was calculated, were arbitrarily chosm. The specific volumes of the gases a.re on the basis of a temperature of 800" C., since the ternlieratare varies dong the kiln, and the effect of the ternparatiire variation between the different
which may have been due to an uncontrolled variation in the rates of f l o r of the gases or solids. Experiments 5 and
1.94
3.29
4.27 4.35
1.02
1.99
1.98 2.27 2.33 J J
832
sa3
2.59 0 00
....
1.64 1.17 1.X 1.18 3.08 2.48 2.28 1.69 1.70 0.51 1.8'
2.5' 1.2:
-~__ -
5s
49 38 39 48 33 36
Poiytialite Polyl'Blite
Polyhaljte Polyhalite Poly1"lite
Polvllalite
71
1.2
180 160
1.2s 1.Z(
180
1.2' 1.2"
K32
~~
duced without disintegrating, althougli rupture cracks were visible on some of the particles. The particles were discharged with the same shape and size as they had on entering the kiln. Some adhesion of particles to the surface of the 18-8 alloy, probably due t o compound formation hetween the alkali sulfide and t.he sulfides of iron and chromium (5, 1% was observed but this uever became greater than one layer thick. With tlie alundum-lined kiln, run8 11 and 12, no adhesion occurred in the zone of reduction either t,o tlie walls of tlie kiln or bet,xveen particles. At, the discharge end of the kiln where the temperature was decreasing from the limit of %. risible red heat, some adhesion did occur, bot this vould not he present in the proposed industrial &-up since the product would be discharged at the rnarimum temperature of the reduction. In an indostrial kiln an inert lining of bauxite or carbon or a basic lining of rnagne.;ite n~ouldpresurnably solve the problem of adhesion to the walls.
1.89 1.72 1.89
IS0 180
180
~
.-
~
6 , in comparison with 2, sliow the effect of exceeding the capacity of tlie kiln, the time of retention of the solids having been reduced by increasing the rate of rotation of the kiln. Although an excess of gas was supplied in experiment 5, the solids were only 93.3 per cent. reduced. On increasing the rate of solid flow in experiment 6, the degree of reduction was decreased, but the oxidation of gases ma8 in no way increased. The oxidation of t.he gases was evidently detemiined by their time of retent,ion. Comparison of experiments 6 and 7 sl~owsthe change in the rate of reaction froln 832" to 882" C. That there is only a slight increase in both the oxidation of the @xes and tlie reduelion of the solids may be attributed to the fact that in this temperature interval, nlricli includes the solidification temperature of fused polylialite, an abrupt alterattbionin the structure of the unreduced material is occurring. That t,he characteristics of tlie reduction in experiments 6 and 7 vere ent,irely different could he seen hy examination of the reduction product. Run 6 a t 832" C. gave particles of niaterial that contained a hard core of partially reduced polyhalite within a shell of conipletely reduced polylialite, whereas each particle from run i a t 882" C. was a uniform mass of partially reduced jmlylialite, soniewliat porous and entirely different in appearance from the product of run 6. Apparently the upper temperature limit of reduction in this particular kiln was near 880" C., since the adhesion to the walls became quite pronoiinced io rim i .
F~~~~~ 2. T E ~ ~ D~~~~~~~~~~~ ~ R A ~IN ~ ~ T ~ ~ op , ROTARY FURXACE USXDFOR REDUCING POLYHILITE
~
~
FEBRUARY, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
The possible degree of reduction attainable a t 832" C. is illustrated in experiments 8 and 9. Experiment 8 was the same as 2 except that a greater excess of gas was used, whereas in experiment 9 the time of retention was increased by decreasing the slope of the kiln. The contamination by iron, chromium, and nickel from the chrome-nickel steel kiln, to be ,(l+S,HZO noted in the succeeding discussion, had no appreciable increased catalytic effect over that due to the iron naturally occurring in the polyhalite. This is shown by the fact that the reduction rate in the alundum-lined kiln (run 12) was practically the same as that in the chrome-nickel \I steel kiln. "I The inlet gas in experiments 1, 2, 3, and 4 FWJRP: 3. showsa ratherlow value for the hydrogen-carbon monoxide ratio in a d d i t i o n t o a p p r e c i a b l e amounts of nitrogen and carbon dioxide. The indications are that air leaked into the converter (as shown by the complete failure of the converter after experiment 4) because of oxidation of the black iron pipe from the outside surface. For the remaining experiments (5 to 9, inclusive) a new converter was used, with the result that the composition of the inlet gas was normal and no nitrogen was present. For the temperatures given-that is, below 880" C.-no free sulfur was formed by the oxidation-reduction reaction except possibly under conditions of extreme oxidation. When free sulfur was found in the analysis, it was due to a small amount of air leaking in as a result of a too rapid rate of sampling. This is shown in runs 5 to 9, inclusive, with the new converter, where in run 8 (the only case in which nitrogen appeared) there was accompanying elemental sulfur. As a check on the percentage oxidation of the gases, a comparative calculation was made on the per cent reduction of the solids. Both sets of values, calculated respectively from the gases and from the solids, are given in Table VI. Some of the checks are not very close, but considering that the sampling of the gas is practically instantaneous and the time of retention of the gases is of the order of 0.1 minute, not much more is to be expected. Variations in the rate of flow and composition of the gases would have a pronounced effect, particularly for experiments 1 to 4, inclusive, where a defective converter was used.
Composition of Product On calculating the percentage potassium in the reduction product from its calcium content and the ratio of potassium and calcium in the original raw polyhalite, the results shown in Table V I are obtained. By comparing the calculated values of potassium with those determined, i t is seen that they either agree closely or else the determined values are slightly higher, indicating that the volatilization of potassium during reduction of polyhalite a t temperatures below 880" C. is negligible. On the reduction of magnesium sulfate in the presence of water vapor, magnesium oxide is formed. For complete reduction under the conditions in this work i t may be assumed that the calcium and potassium would be equivalent to the remaining sulfur. Table VI1 shows the comparative compositions of the rotary kiln reduction, together with some reductions of the simple constituents of polyhalite in porcelain boats. I n runs 13 and 14 the product was cooled in nitrogen to some temperature below a red heat and then withdrawn to a stoppered bottle. Since the slight excess of total sulfur over sulfide ion seemed to indicate some oxidation, the remaining boat reductions were cooled to room temperature in nitrogen. Comparison of runs 14 and 18 shows that some oxidation did occur in the case of the former.
229
ELEMENTARY FLOW DIAQRAM FOR REDUCTION OF POLYHALITE
As might be expected, the reductions of the simple constituents show that carbon dioxide was absorbed only by the potassium, and that this absorption did not result in the loss of any sulfur. Where a constant excess of reducing agent was used to keep the partial pressure of water low, mixtures of magnesium and potassium did not lose all of the sulfur equivalent to the magnesium. Apparently a polysulfide of potassium was formed, as shown in run 17. Under these conditions the reduction of calcium sulfate produced essentially calcium sulfide, but with a higher partial pressure of water and a sufficiently high temperature (as in run 10) considerable calcium oxide was formed. The greater value for excess sulfur found in runs 1 , 2 , and 4 to 9, as compared with runs 11 and 12, is evidently due to the presence of sulfides of iron, chromium, and nickel derived from the surface of the 18-8 alloy rotary kiln. The fact that no carbon dioxide was absorbed in runs 3 and 4 shows that all the potassium was still present as potassium sulfate. From this information it is possible to make an approximate calculation of the composition of the reduction product, as given in Table VIII. The result of this calculation shows that the ease of reduction of the sulfates decreases in the order magnesium, calcium, and potassium. With properly controlled conditions these constituents may be selectively reduced. TABLEVIII. RunNo.
CALCULATED CONSTITUENTS OF PARTIALLY REDUCED POLYHALITE
Cas01
MgSO4
7
3 4
3.35 3.16
1.26 0.78
K z S O ~ Cas MgO Moles per 1000 grams 1.85 0.73 0.60 2.00 1.33 1.27
K1S 0.00 0.00
Industrial Application The preceding experiments show that a t temperatures above 800" C . the rate of reaction is sufficiently rapid t o make industrial application feasible. It is also probable that the reaction can be carried out in a rotary kiln. Other possible forms of apparatus would be the shaft kiln, the multiplehearth kiln, and a flash reduction kiln operating on the same principle as a flash roaster (4) where the finely divided solids would be allowed to react while suspended in the heated reducing gases. Whatever specific type of equipment is employed, the complete process of reduction will consist of three major operations-the preheating and conversion of the natural gas, the reaction between the gas and solid a t the proper temperature, and the preheating of the unreduced solid. In preheating the gases and solids, there are chances of recovering waste heat. Thus the by-product gases from the reduction reaction may be used to preheat the incoming
INDUSTRIAL AND ENGINEERING CHEMISTRY
230
TABLEIx. CALCULATED QUANTITYd K D
COST O F
GAS USED
FOR
REDUCTION O F POLYHALITE
4H Ex 0.50 0.50 0.50 5
E2 0.10 0.20 0.30
S
for Reduction
0.50 0.50 0.50
0 0 0
I
Debit
VI
VZ
6800 6800 6800
7130 7130 7130
Va
12,800 6,400 4,270
.
VP
v 6
v 6
Net Cu. Ft. per Ton
1272 1272 1272
3210 3210 3210
22,200 15,800 13,900
---Credit-
0 0 0
VOL. 28, NO. 2
Cost of Gas for Reduction a t Assumed Prices per 1000 Cu. Ft. Pes ton Pes ton polyhalite" equivalent K20 $0 02
$0 10
$0 02
$0.10
0.44 0.32 0.28
2.22 1.68 1.39
2.84 2.06 1.78
14.21 10.11 8.89
Theoretical composition.
solids, and the gases of combustion from the converter may have part of their heat utilized in a recuperator serving to preheat the air used in the combustion. The extent to which these waste heats may be utilized is limited by the particular type of apparatus chosen. A heat balance for the complete reduction process is shown in Figure 3. To insure an adequate estimate of the heat requirements, a temperature of 900" C. has been assumed. For the heats of reaction involved, those for the methane conversion reaction and the polyhalite reaction have been discussed previously. Storch (14) gives a value for the heat of reaction in the thermal decomposition of polyhalite between 298" and 400" C. as AH = +66,000 calories. The specific heats of methane, water vapor, carbon monoxide, and hydrogen are the same as those used in calculating the methane conversion reaction, while that of carbon dioxide is from Lewis and Randall (9). As the available data from 0" to 400" C. for the heat capacity of calcium sulfate (8) showed an approximate linear variation, an extrapolation was made to 900" C. Heat capacities are given (8) for magnesium sulfate only a t 61" C. and for potassium sulfate only a t 100" C. As a rough approximation, the variation with temperature for these latter two substances was drawn parallel to the calcium sulfate. The heat capacity of the water of crystallization of polyhalite was taken as equivalent to ice. Detailed calculations for the efficiency of heat transfer, the heat losses from exterior surfaces of the apparatus, and the efficieney of the heat supplied by combustion, which includes losses due to cold air and gas and to incomplete combustion, were not made. Instead, a n over-all heat efficiency, which includes these three factors, and to which were assigned values found with similar types of apparatus in industry, was introduced. From this information, the following have been calculated : VI = heating gas for gas preheating and con(1 8) ' version El Va = gas for reduction reaction = (1 8 ) 4760 Va = equivalent combustion for solid preheat = 1280 E2 V4 = equivalent combustion for heat of reduc- - -AH (cal.) tion reaction 150.2 V s = equivalent combustion in sensible heat by-product gases = 7690 V e = equivalent combustion in combustion of excess gas from reduction = S(6420) where X = per cent excess of gas over that theoretically required for reduction of polyhalite X 10-2 El = per cent heat efficiency of converter X 10-2 EO = per cent heat efficiency of reduction apparatus X
+ -
+
10-2
The quantities, 8, when originally in heat energy units, were converted directly into equivalent volumes of methane, utilizing the net heat of combustion of methane under standard conditions which is 898 B. t. u. per cubic foot saturated with water vapor a t 60" C. and 30 inches of mercury (16). Methane composes the major portion of natural gas. The other constituents, the higher hydrocarbons, only raise the B. t. u. content and reduction capacity.
The net volume of gas in cubic feet required for the reduction of one ton of polyhalite is therefore volume A for the converter plus B for the reduction apparatus, where: A
=
(VI
+ V2);
B
(Va
+
8 4
- Vs - Ve,)
When B is negative, it is taken as equal to zero, since an excess of heat in the reduction unit cannot conveniently be transferred to the converter unit. ' Volumes and costs of gas are summarized in Table IX. The rotary kiln used in the cement industry has a heat efficiency of approximately 20 per cent (11). The heat efficiency of the converter is taken as 50 per cent for a conservative estimate. As shown by Table VIII, an excess of 50 per cent over the theoretical amount of gas required for reduction should be adequate. For comparison, values based on efficiencies of 10 and 30 per cent for the reduction unit have been included in Table IX. During 1929 in Texas, natural gas used for carbon black manufacture had a value of 2.5 cents per thousand cubic feet ( 7 ) . The range of cost of gas given in Table IX is believed to cover probable conditions in the Texas and New Mexico regions.
Acknowledgment The writers wish to express their thanks to H. H. Storch of the Pittsburgh Station of the U. S. Bureau of Mines for supplying the natural gas used in this investigation, and to C. M . Davis and F. Spille of this station for the construction of the equipment.
Literature Cited (1) Clarke, L., Davidson, J. M., and Storch, H. H., Bur. Mines, Rept. Investigations 3061 (1931). (2) Fraas, F., and Partridge, E. P., IND. ENQ.CHEW,24, 1028-32 (1932). (3) Fraas, F., and Partridge, E. P., IND. ENQ.CHEM.,Anal. Ed., 7, 198-9 (1935). (4) Freeman, H., Chem. & Met. Eng., 38, 334-6 (1931). (5) Friend, J. N., "Text-Book of Inorganic Chemistry," Vol. IX, Pt. 2, pp. 135-6, London, Charles Griffin & Co., 1921. (6) Hawk, C. O., Golden, P. L., Storch, H. H., and Fieldner, A. C., IND. ENQ.CHEM.,24, 23-7 (1932). (7) Hopkins, G. R., and Backus, H., Bur. Mines, Minera2 Resources of the U . S., 1929, Pt. 11, 319-40. (8) International Critical Tables, Vol. V, pp. 99-100, New York, McGraw-Hill Book Co., 1929. (9) Lewis, G. N., and Randall, M., "Thermodynamics," p. 80, New York, McGraw-Hill Book Co., 1932. (10) I b i d . , p. 574. (11) Martin, G., Rock Products, 33, No. 16, 49-50 (1930). (12) Mellor, J. W., "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. XI, p. 432, New York and London, Longmans, Green and Co., 1931. (13) Partridge, E. P., and Fraas, F., U. S. Patent 1,975,798 (Oat. 9, 1934). (14) Storch, H. H., IND. ENQ.CIIEM.,22, 934-41 (1930). (15) Storch, H. H., J. Am. Chem. SOC.,53, 1266-9 (1931). (16) U. S. Bur. Standards and U. S. Bur. Mines, Table of Heating Values for Gases, April, 1933. R ~ C ~ I V August ED 29, 1935. Presented before the Division of Industrial and Engineering Chemistry at the 8 9 t h Meeting of the American Chemical Society, New York, N. Y.,April 22 to 26, 1935. Published b y permiasion of the Director, U. 5. Bureau of Mines. (Not subject to copyright.)
