IKDUSTRIAL AND ENGINEERING CHEMISTRY Podbielniak, Oil Gas J.,28, 3 s (Jan. 17, 1929): Ibid., 29, 235 (Oot. 2, 1930). Podbielniak, Ibid., 29, 22 (May 14, 1931). Thiele, Ber., 33, 666 (1900). U. 9. Bur. Mines, Bull. 197 (1926). Wheeler and Wood, J. Chem. Soc., 1819 (1930). Williams-Gardner, Fuel, 4, 430 (1925).
Vol. 24, No. 3
(25) Williamson, "In a Persian Oil Field," p. 70, Benn, 1927. ENQ.CHBM.,8, 674 (1916). (26) Zanetti, J. IND. (27) Zanetti and Leslie, Ibid., 8, 777 (1916).
RECEIVED October 31, 1931. Presented as part of the Symposium on "The Utilization of Gaseous Hydrocarbons" before the Division of Petroleum Chemistry at the 82nd Meeting of the American Chemical Society. Buffalo, N. Y., August 31 t o September 4 1931.
Thermal Decomposition of Alunite S. C. OGBURN,JR.,AND H. B. STERE,Department of Chemical Engineering, Bucknell University, Lewisburg, Pa. LUNITE as a possible Cameron and Cullen (4)obThe progressive stages of decomposition of commercial raw material tain potassium alum by digesUtah alunite, effecled by heating the material for chemical i n d u s t r y tion of the ore with sulfuric acid at constant temperatures and over a temperawas brought into the field of aca t 110" C., and r e c o v e r the ture range f r o m 100" to 850" C., have been tive investigation in 1912 when product in a way similar to the obseraed through the medium of chemical analyButler and Gale ( 2 ) published process of Tilley. results of their investigation of Detwiller (9) prepares a nitric sis. Combined water is liberated at 460" C., the chemical and geological naacid solution of the alumina and and complete decomposition of the aluminum ture of the a l u n i t e d e p o s i t s potassium while the silica resulfate to f o r m alumina results at 800" C. found near Marysvale, Utah. m a i n s insoluble. Soluble Potassium sulfate is not decomposed by heating A report by Waggaman (12) in n i t r a t e s and s u l f a t e s are reat this temperature and has been leached f r o m covered. the same year discussed "Alunite Blough and McIntosh ( 1 ) sin&s a S o u r c e of P o t a s h " and the calcined material and recovered as 99.26 ter a mixture of finely ground showed that potassium sulfate per cent pure potassium sulfate. The residue alunite and sodium chloride a t could be leached from alunite after leaching consists mainly of aluminum 800" C. and leach out soluble which had been calcined a t about oxide containing small amounts qf silica, iron salts, after which the sodium sul700" C. F o l l o w i n g these reoxide, and magnesia. fate and insoluble alumina are ports, several patents were issued heated toaether a t temDeratures on methods of recovering Dotash less than 1500" C. to form soluble so&m aluminaie. and other compounds from alunite. Although the greater number of patents include decomposiA process by Tilley ( 1 1 ) recovers the potash as potash alum by digesting the crushed ore with sulfuric acid a t 90" C. for tion of the ore by heating, no literature has been found t o 24 to 48 hours. The undissolved ore residue is converted to show either the rate or completeness of decomposition at aluminum sulfate by roasting, and is recovered as such. given temperatures. The work presented herewith was done to determine by Moldenke (10) has proposed a similar treatment, except that calcination is carried out in steps in a circulating atmosphere means of chemical analyses the course of the thermal decomposition of Marysvale alunite. of sulfur dioxide and oxygen. Chappell ( 7 ) treats the alunite with sulfuric acid and, after precipitating the aluminum as hydrate with ammonium CHEMICAL KATUREOF ALUNITE hydroxide, produces mixed sulfates of ammonium and potassium to be used for fertilizer. In other patents (6, 6) Alunite is a naturally occurring, hydrated double sulfate of he recommends slow calcination a t 750" to 1000" C. in a aluminum and potassium of the approximate composition strong current of air, and calcination in two steps, first a t represented by K20~3A1203~4S03.6Hz0.Impurities, such as 600" C. and finally a t 900" t o 1000" C., after which the hot silica and salts of iron, magnesium, and sodium, are usually material is dumped into water to leach out potassium sulfate. present in small and varying amounts.
A
TABLE SOURC'E
I . A~ALYSES OF ALUNITE FROM
SiOn
81208
son
%
%
%
K10 %
38.6 35.52 38.34 36.54 35.24 37.92 38.93 38.50
11.4 10.21 10 46 9.71 3.27 6.77 4.26 4.48
37.0 Theoretical (8) 0:95 38.38 Marysvale. UtahC 0.22 37.18 Marysvale Utah ( 2 ) 34.30 5 . 2 8 Marysvale: Utah (PI 42.35 2.54 Rico Mts. Colo. ( 2 ) 37.66 1.79 Rico RIts.: Colo. ( 2 ) 0.50 39.03 Silverton Colo (2) 38.05 2.64 Tres Ceriitos, Calif. ( a Water removed at 105' C. h Water of combination removed at 460' C . c .4lunite used in this investigation.
Cameron (3) calcines the ore a t 500" C., causing a restricted evolution of sulfur oxides. The soluble aluminum compounds .are leached and alum separated. The latter is then calcined a t 750" C., forming potassium sulfate and alumina, which is digested with hot water to yield a solution of the potash salt.
V.4RIOCS SOURCES
Nag0
FerOs
MgO
Ha0'
%
%
%
%
0:56 0.33 0.56 4.02 2.12 4.14 2.78
...
...
0.13
0 84
...
0.38
Trace Trace
...
0.23
...
Trace
0 :i 4 0.09 0.11 0.13 0.06
Nil
FLOf,
% 13.0 13 0% 12.90 13.08 11.99 13.03 13.35u.b 11.92
Table I shows the composition of representative samples of raw alunite from different localities and from different mines in the same locality. The natural product is practically insoluble in water, but, after calcination a t temperatures which produce partial decomposition of the original structure, potassium sulfate and
I N D U S T R I A L A N D E N GI N E E R I N G C H E M I S T R 1-
hlarch, 193'7
aluminum sulfate may be leached from the material. From a consideration of the analysis of alunite used in this work, it is to be noted that there is an insufficient amount of 803 present to combine with all of the alumina,. potassium oxide, and sodium oxide t o form sulfates. Hence it would not be possible to recover all of these oxides as sulfates without addition of sulfuric acid or its equivalent in 803. Since the interest in this investigation was centered on the thermal changes, such possible intermediate products were not investigated.
EFFECTO F TEVPERATURE ON EXTENT OF ~ECOiVPOSITION A study of the thermal decomposition of alunite must necessarily include both the effect of temperature of calcination and the time of heating a t selected temperatures. Uniformity of sampling and careful temperature control are recognized prerequisites.
Temp.
-
289
from the material and the percentage of volatile matter at given temperatures are shown in Figure 1. The decomposition of alunite is accomplished in two stages, the first being approximately completed a t 460" C. with complete removal of water of combination; the second stage is approximately completed by heating to 800" C., a t which temperature oxides of sulfur are removed. Analyses of the evolved gases in this latter stage of decomposition have shown approximately 90 per cent SOa and 10 per cent SO,.
RATEOF DECOMPOSITIOK AT CONSTANT TEMPERATURES To study the rate of removal of combined water near the end of the first stage of decomposition, duplicate samples were heated a t a constant temperature of 460" C. The samples were weighed a t regular intervals, and the percentage of water removed was calculated. The time rate of removal of water under these conditions is shown diagrammatically in Figure 2. Water was liberated continuously as the time of heating was increased, until ultimately the entire six molecules of combined water were removed. Since removal of water is so regular, it is evident that no intermediate hydrates form during calcination of the alunite. Curve I (Figure 1) and Figure 2 both show this regularity in decomposition.
0 OC.
FIGURE1. EFFECTOF TEMPERATURE ON DECOMPOSITION
Curve I. Percentage volatile matter at given temperatures Curve 11. Percentage SO8 removed at given temperatures
The material used in this work was crushed to pass a 40mesh sieve. The calcination was effected in an electric muffle furnace with variable temperature control actuated by a platinum-platinum rhodium thermocouple as the thermo-unit. The accuracy of control was =t8"C. a t 1000" C. To determine the effect of the temperature of calcination, duplicate samples of 1.0000 gram of alunite were weighed into platinum crucibles and placed in the furnace in which the temperature was maintained constant. The saniples were removed ,periodically, cooled in desiccators, and weighed quickly. Heating was continued until no further loss in weight of the samples was noted. I n this manner the total volatile matter removed from alunite a t definite ternperatures between 110" and 780" C. was determined. Xew samples were used for each temperature of calcination studied. a 14
g 12
$10 +. m
28
P C 4
d
2 0
4
8 12 T i n e of iieating
16 - Hours
20
FIGURE2. RATE OF REMOVAL OF WATERAT 460" C.
These calcined samples were analyzed for so3 content, since, in the temperature ranges used, 803 and water are the major volatile constituents. The percentage of SO3 removed
4
I
, j
~-
I
The rate of decomposition of a 1-inch bed of alunite has been determined by heating the material a t a constant temperature of 805" C. and removing samples a t definite time intervals. These samples were analyzed and the percentage of SO3 removed over a period of 4 hours was computed. The rate of removal of SO3 under these conditions is shown in Figure 3. After 4 hours of heating, the so3 remaining (16.78 per cent) was slightly in excess of that required for combination with the potassium oxide and sodium oxide present to form sulfates. Later experiments conducted on a larger scale have confirmed these data and show that prolonged heating is essential to complete decomposition of the aluminum sulfate when the material is heated in a stationary bed. It is seen from Figure 3 that decomposition of a 1-inch bed of alunite is practically completed in 4 hours of heating a t 805" C. To determine the nature of the compounds remaining after such treatment and the possibility of recovering potassium sulfate from the calcined material, it was necessary to prepare a larger quantity than previously used in these studies. About 12 pounds of calcined material were prepared by heat-
