Subscripts
literature Cited
a = inlet air am = ambient air as = asphalt d = datum i = insulation nz = finite difference pozition n = finite difference time o = exit gas r = reactor vessel a = inner radius of insulation (reactor radim) 6 = outer radius of insulation 1 = timet 2 = time 1 At
(1) Am. SOC. Testing Materials, Philadelphia, Pa., “XSTM Standards on Bituminous Materials for Highway Construction,
Ac knowledgmeni
Work supported by National Science Foundation grant NSFG14249. Condensed from M.S.E. thesis submitted by D. B. Smith, University of Florida, 1962.
Waterproofing, and Roofing,” D-36-26, 1958. (2) Holmgren, J. D., Ph.D. Dissertation, University of Florida,
Gainesville, Fla., 1954. (3) Mickley, H. S., Sherwood, T. K., Reed, C. E., “Applied Mathematics in Chemical Engineering,” 2nd ed., McGrawHill, New York, 1957. (4) Smith, D. B., M.S.E. Thesis, University of Florida, Gainesville, Fla., 1962. (5) Traxler, R. N., Schweyer, H. E., OzI Gas J. 52, 158 (1953). RECEIVED for review October 12, 1962 ACCEPTED.4pril 10, 1963
+
Asphalt samples were supplied by the Texaco Corp.
FLUIDIZED ROASTING OF A R S E N O P Y R I T E S A Theoiy
of the Mechanism
of the Dearsenifcation Process
A N G E L V I A N , C O N R A D 0 I R I A R T E , A N D A N G E L
R O M E R O
Division of Industrial Research, Pzritas Espaiiolas, AUXINI, S. A . , Madrid, Spain
The roasting of arsenopyrites in a fluidized b e d offers great advantages, but the arsenic i s retained in the calcines, making them useless as raw material for siderurgical purposes. An introduction on the mechanism of roasting and elimination of arsenic i s followed b y a thermodynamic discussion of the reactions that may cause retention of arsenic in the calcines and the conclusion that the arsenic i s fixed b y the oxidized iron. This hypothesis i s confirmed b y the experimental results, which are compared with those of Gorling and Appler and of Heath and Holle. The proposed roasting method satisfactorily resolves the problem of arsenic fixation.
HE dead roasting of pyrites in a fluidized bed causes retenTtion in the calcines of almost all of the arsenic contained in the pyrites, so that the calcines cannot be used as iron ore. To overcome this difficulty, a two-stage roasting process has been proposed. In the first stage arsenic is removed in a n atmosphere of air deficiency, the arsenical compounds being drawn off with the roasting gases. In the second stage a great excess of air is blown in to obtain the calcines in the usual chemical form of ferric trioxide and to recover as much as possible of the sulfur contained in the raw ore. With respect to the first stage, three main techniques have been proposed: one by the Americans Heath and Holle (70), another by the Swedes Gorling and Appler (a), and a third by Vian (79, 20). A technique analogous to ours has been claimed by a well-known European firm. The Swedish method is considered as a single roasting stage; but it is mentioned here because of its similarity to the first stage of the other two processes. The difference in the Swedish, American, and Spanish techniques consists mainly in the air-pyrite ratio used for roasting. Gorling and Appler use air-pyrite ratios for flotation concentrates such that the roasted calcine is composed of
214
I B E C PROCESS DESIGN AND D E V E L O P M E N T
magnetite (Fes04) and is substantially desulfurized (S 0.57,); Heath and Holle use less air because in their opinion the dearsenication may be accomplished if the calcines are composed mainly of magnetite, even if they are accompanied by a certain amount of FeS. \Ve ourselves claim, o n the basis of a theoretical and experimental study of the process, that a still higher oxygen deficiency may be used. and we base it on the fact that the oxidation and further retention of the volatile arsenical compounds in the calcines are due to the production of iron oxides from the oxygen contained in the roasting gases, even when the amount of oxygen is low, and that, therefore, production of iron oxides must be carefully avoided. From our point of view, the amount of air used must be regulated to give mainly FeS in the solid calcines, so that only the labile sulfur is burnt in the first roasting stage. \Ye also think that the regulation of the amount of air must be based not only on stoichiometrical considerations, because the amount of labile sulfur contained in the ores is different for each type of ore, but also because labile sulfur combustion processes and solid calcine oxidation processes may occur simultaneously. depending on the temperature, grain size, and homogeneity of the fluidized bed.
Mechanism of Roasting and of Arsenic Removal
first reaction, which is generally assumed, is not possible because, as will be shown in detail, the equilibrium
Whatever amount of air is used, each pyrite grain introduced into a roasting fluidized bed first experiences a heating, placing it a t the thermal level of the bed (about 800" C.), then immediately begins distilling off the labile sulfur, accompanied by the arsenical compounds. Most of the arsenic in the pyrite is in the form of mispickel, FeAsS (4, 77), which decomposes by heating as follows:
+ As A s ~ S+ Z 2FeAs + 2FeS
Fe.4sS
4FeAsS -+
FeS
[ . ~ S ~ O S ] 1/2 C (AsnOs)g~f ( 0 2 )
is unfavorable with respect to the [ A S ~ O above ~ ] , temperatures, a littlelessthan 1100" K. (827' C.). Even at lower temperatures, it is not possible to fix arsenic as [ A S Z O ~under ] ~ the actual conditions a t which roasting furnaces are operated, because of the extremely low arsenic content of all forms, which is reached whatever the arsenic content of the starting pyrites may be. T o show this, consider a n example in which a highly arsenical pyrite is roasted (S = 48%; Fe = 46%; As = 0.5%) to produce roaster gas with 5.3 volume yo 0 2 and 12 volume % Son, corresponding stoichiometrically to 0.013270 AsIOL. At the maximum total pressure (most favorable for arsenic fixation) of the gases in the roasting bed, which may vary from about 1.O to 1.2 atm. through the bed,
(1) (2)
according to these reactions, T h e formation of As and .4S& or others, has been experimentally verified by us by heating pyrites in an inert medium. The formation of iron arsenide must be assumed, since when pyrites are heated in an inert medium, the residue always contains some arsenic. At increasing temperature:; the amount of residue is less, possibly because the arsenide is decomposed by the labile sulfur :
+ 2S2
2FeAs
+
ASS?
+ 2FeS
Po:
Substance
0.053 X 1.2
6.36 X =
atm.
1.58 X
atm.
the thermodynamic possibility of forming JAS~OS], exists only when %os
+ 2'4s * As&
=
Since
(4)
X POZ 3
K p
For this example, K , 6 8 X 10-4, which, as is shown in Equation 14, occurs when T 6 860" K. Even in the hypothetical case that the oxygen concentration in the roaster gases were twice that of the example and that of the arsenious anhydride ten times greater, one finds T 6 900" K. Therefore, under the conditions existing in the roasting furnace, arsenic cannot be fixed as pentoxide solely by the action of the oxygen contained in the roaster gases, even in the case of pyrites of maximum arsenic content. In contrast, the oxidation AsT3 + As+s and the fixation of A s i 5 as ferric arsenate are possible in all cases by means of the iron oxides, whenever these are present in the roasting bed. Experimental thermogravimetrical runs show that this compound is stable under a nitrogen stream and a t temperatures below 900' C. It does not comp!etely dissociate below 950' C. Figure 1 shows the pyrolysis line for a sample composed of 1 gram of hydrated pure ferric arsenate heated in a thermobalance under a nitrogen stream. Eecause of the complete lack of thermochemical data for [FeAs04],and other substances of interest here, we have had to evaluate a number of heats and entropies of formation by several methods, using the data available (76). The results are collected in Table I, in which the authors' values are underlined. Methods of calculation are discussed in a later section.
Tvhich is displaced toward the right side because the amount of sulfur is very high; this is due to the high dissociation pressure of the so-called "labile sulfur:" relative to that of the other two components. If the medium surrounding the grain is inert, the labile sulfur, the arsenic, and the arsenic sulfide distill off the bed. If the air feed contains only enough oxygen to burn the labile sulfur, this will burn and give a SO2 medium, protecting the arsenic compounds produced by distillation from further reactions. Therefore, these arsenic compounds leave the bed in the ;gaseous form of As$?, with SO,. But if the amount of oxygen is slightly higher than that stoichiometrically equivalent 1.0 the labile sulfur: or even someivhat lower (because the mixing of reagents in the bed is not perfect), the oxidation of the As2S2 vapor and the solid FeS (produced by pyrite when sulfur is liberated) becomes possible, forming h 2 0 3 (vapor), FeaOl (solid), and SO2 (gas). As203 and SO2 leave the bed with the roasting gases. O n the other hand, it is known from experiment that the arsenic is fixed in quinqueva!ent form. This must involve either oxidation of A13203 to A s 2 0 j in the vapor phase by the oxygen of the air and further reaction of the pentoxide xvith iron sulfide or the iron oxides, or, alternatively, the direct heterogeneous reaction of A s z 0 3 with the iron oxides and the gaseous oxygen so that the arsenic is fixed as arsenate. The
Table 1.
=
P A ~= ~ o0.000132 ~ X 1.2
(3)
so that the reaction is enhanced with temperature (78) Therefore, when the grain is heated in the roasting bed, S2, As, and ,4szS2 are liberated in a primary reaction. in relative amounts given by the gaseous equilibrium:
SL
(5)
Standard Heat of Formation, Entropy, and Heat Capacity Function of Implicated Substances
Hma, Kcal. / M o l e 0
- 63.7 -267 -196.5 -284.1 -218.6 - 225 -485
SO,
Cal./Mole ' K . 49 01
13.4 35 21 5 107 4 25 2 23 65
C, us. T , Cal./Mole O K., ' K. 8 27 0 258 X - 1 877 X 1 0 5 T 2 12 62 4- 1 492 X lO+T - 0 762 X 105T-2 41 17 f 18 82 X lO-3T - 9 795 X 1 0 6 T 2 24 72 16 04 Y 10-3T - 4 234 X 105T-2 39 88 A 6 54 X I O - 3 T 35 06 -k 11 034 X 10-3T - 9 302 X 1ojT-2 26 8 978 Y~10-3T - 4 734 X 1 0 5 T 2 _ 51 f_ 59 25 22 96 Y 10-3TT9 485 X 1 o j F 2 -
+ + +
VOL. 2
NO. 3 J U L Y 1 9 6 3
215
40
30
F Y
r
.-m f
20
b
i
2
10
0 -
First let us consider the case in which only FesOl is present in the calcines; arsenic is fixed as ferric arsenate according to the reaction 1/2 (As408)
+ 7/6
( 0 2 )
+ 2/3 [FesO4] s 2 [FeAsOJ
(6)
even if the waste oxygen in the roasting gases is very low. For this reaction we show later that: log K , = -13.61
+ 0.2 X
T
+ 28.22 X l o 3 T-'
-
2.02 log T
(7)
The equilibrium constant is about lo9 a t 1000' K. and l o 4 a t 1200' K. Let us now consider the case of roasting a pyrite such as the one cited in the preceding example, and using 10% oxygen in excess (the maximum amount of oxygen from the technological point of view) for the production of calcines essentially consisting of FesOl. The percentage of As406 in the roasting volume % and that of 0 2 gases is then about 1.64 X is about 2 volume %. In such a case) and for total pressure of 1 atm. in the furnace, P A s 4 0 6 1.64 X atm.; POZ
-
-
2 X lop2 atm., and as K, [FeAsOI J c requires that
=
7,6,
PE40, x PO? 1
the formation of
