INDUSTRIAL A N D ENGIXEERIXG CHE-MISTRY
956
reacting molecules together in close proximity. As the pressures in the writers' experiments are essentially the same as are used in the cracking processes mentioned previously, their results should be parallel to those obtained commercially. The method used requires only very small quantities of the materials. One cubic centimeter is far more than is needed for a considerable number of tests. The writers believe that this is an accurate method of studying cracking and yet it is extremely economical both of materials and time.
T'ol. 22, No. 9
Literature Cited (1) Edgar, IND. ENG.CAEY., 19, 144 (1927). ( 2 ) Hugel and Artichevitch, A n n . of. natl. combust. Jig., 3, 985 (1928). (3) Hugel and Szayna, Ibid., 1, 786, 817, 833 (1926). (4) Leslie and Potthoff, IND.ENG.CHEM.,13, 778 (1926). ( 5 ) McKee, U.S. District of Delaware, Equity Suit 571,p. 505 (June, 1926). (6) McKee and Parker, IND.END.C H E M . , 20, 1169 (1928). (7) Parks, Huffman, and Thomas, J . A m . Chem. Soc., 62, 1038 (1930). (8) Sachanen and Tilitcheyew, J . I n s f . Petroleum Tech., 14, 761 (1928); Ber., 62, 658 (1929). (9) Sokal, J . SOC.Chcm. Ind., 43, 283T (1924). (10) Zeitfuchs, IND.END.C I I E X . , 13, 79 (1926).
Reactions between Iron Sulfide, Sulfur Dioxide, and Iron Oxides in the Metallurgy of Copper' A. C. Halferdahl 25 ACACIAAvE.,
OTTAWA, ONT.
Data and calculations have been presented which and has been taken as repreH E p u r p o s e of this senting the free energy of iron indicate that magnetite is readily formed in smelting paper is to indicate operations. Once formed its reduction to ferrous oxide sulfide up to 1171" C., the the probable course of melting point. The free by ferrous sulfide requires temperatures of 1300" C. or certain reactions between energy at 1171" C. is - 16,670 higher. iron sulfide, iron oxides, and calories. Values of freeenergy A modification of present methods for treating copper sulfur dioxide which are imabove 1171' C. have been converter slag in reverberatory furnaces is suggested. portant in the pyrometalestimated by the relation bP' The function of coke in pyritic and semi-pyritic coplurgy of copper and lead. = A H - TAS, where AH is per smelting is defined in a somewhat different manner Essentially the reactions to change in heat content and than hitherto. be studied involve reductions AS is change in entropy. and oxidations, and to study the reactions the free-energy values of the substances entering AH for ferrous sulfide is -23,080 calories a t 25" C. (Interinto and appearing from the reactions must be known a t tem- national Critical Tables) from iron and rhombic sulfur. peratures of interest in metallurgy, say from 600" to 1400"C. To change rhombic sulfur to gaseous sulfur, SP, at 25" C. requires 29,690 calories. Therefore, the heat absorbed in the Ferrous Sulfide formation of ferrous sulfide from gaseous sulfur and iron at Jellinek and Zakowski (16) have measured equilibria be- 25" C. would be -37,925 calories. Bornemann and Hengstenberg (9) have measured the heat tween hydrogen and hydrogen sulfide over ferrous sulfide as capacity of ferrous sulfide up to 1200" C. The heat in iron follows: sulfide at 1100" C. was 195.03 calories per gram; a t 1150" C., TEMPERATURE RATIO 203.55 calories; and a t 1200" C., 265.92 calories. The c. H2S:Hz specific heat of ferrous sulfide from 1100" to 1150" C. appears 730 0.0032 910 0.007 to be 0.1701, which is assumed to hold up to 1170" C. Then 1100 0.013 the sensible heat a t 1171' C. would be 207.1 calories without Free-energy values for the reaction fusion. Assuming molten ferrous sulfide to have a specific heat of 0.18, the sensible heat from 1171 " to 1200" C. becomes FeS HZ = Fe H2S 5.2 calories, or 265.9 - 5.2 = 260.7 calories would be the heat were calculated by the equation AF = -RTlnK, where AF in ferrous sulfide after fusion, and the heat of fusion would be is the free-energy change, R is the gas constant (1.985 calo- 53.6 calories. By using specific-heat data of Eastman (5) ries), T is temperature absolute, and K is the equilibrium on sulfur [C,(S2) = 8.58 0.0003Tj and heat-capacity data constant. By combining with values of hydrogen sulfide as of Ralston ( S I ) on iron, the heat absorbed, AH, in the reaction given by Lewis and Randall (ZZ), free-energy values for Fe 1 / 2 S 2 = FeS was computed to be -35,360 calories before ferrous sulfide a t 730", 910," and 1100" C. were found to be fusion at 1171" C. The entropy change in formation would -21,170, -19,540, and -17,310 calories, respectively. be -12.9 units before fusion and -9.7 units after fusion. According to Loeb and Becker (25'). ferrous sulfide and iron The entropy of sulfur, 1/2S~,a t 1171' C. is calculated to be form a solid solution above 985" C. on the sulfide side con- 33.6 units and the entropy of iron, 20.5 units, is taken from taining about 2 per cent iron and 98 per cent ferrous sulfide, Ralston's thermal data. From these figures the entropy of and therefore the value of free energy of ferrous sulfide as ferrous sulfide was computed to be 44.5 units after fusion a t calculated a t 1100" C. cannot be considered as strictly accu- 1171" C. I n the usual manner, values of free energy of rate. However, a plot was made of the computed values for ferrous sulfide a t 1200", 1300°, and 1400" C. have been comthe three points against absolute temperature and a straight puted to be -16,230, -14,320, and -11,850 calories, reline was passed among the three points so as to come within spectively. 100 to 130 calories of any one point. The equation of this Oxides of Iron line was found to be: Equilibrium measurements in the reduction of ferrous A F = -31,820 10.49T (1) oxide and magnetite by carbon monoxide as observed by 1 Received May 24, 1930.
T
+
+
+
+
+
IiVDUSTRIAL AND E.VGINEERIXG CHE-WXTRY
September, 1930
Eastman (6), Eastman and Evans ( 7 ) , and Garran (9) are given in Table I. One value above 1200" C. in the magnetite reduction given by Garran was omitted because this threw the subsequent calculations out. His figures for the CO2:CO ratios computed from the H20:Hz ratios were most probably based on measurements in the temperature range 500-600" C. to 1 0 0 ~ 1 1 0 0 C., " and therefore only these figures were chosen. The data of Tigerschiold (87) and of McCance (24) have been omitted, since they depend upon calculation of substantially the same data as were available to Eastman. Measurements in R e d u c t i o n of Ferrous Oxide a n d Magnetite
T a b l e I-Equilibrium
TEMP.
c.
Calories
F E R R O UOXIDE S 600 0 . 8 6 9 (hyj 700 0 . 6 7 8 (E & E) 700 0.690 (G) 700 0.687 (hy) 800 0 . 5 5 2 ( E & E) 800 0 . 5 1 6 ( G ) 800 0 . 5 6 8 (hy) 900 0 . 4 6 6 (E & E ) 900 0 . 4 3 5 (Gj 900 0 . 4 8 4 (hy) 1000 0 . 4 0 3 (E & E) 1000 0 . 3 8 6 (G) 1000 0 . 4 2 4 (hy) 1100 0.337 (G) 1200 0 . 3 1 0 ( G ) 1300, 0 . 2 9 0 ( G ) Deviation between calcd and observed values
--
Calories
49,530 47,890 47,850
47 xtm -- 46,240 46,290 - 46,190 -- 44,610 44.770
--
-
44;520 42,990 43,100 42,860 41,550 39,900 38,240
--
-
Obsd.
Calcd.
Calories
Calories
49,460 47,857 47,857
-- 48,744 - 48,644 46,832 - 46,992 - 46,99!) - 46,992
46:5;3 46,253 46,253 44,650 44.650 441650 43,046 43,046 43,046 41,442 39,839 38,235
---
47 8;17
--
-
-
62 190
47,007 45,340 45,484 45,27'9 43,6613 43,816 43,568 41,988 42,093 41,860 40,481 38,779 37,059
- 46,992 - 45,340 - 45,340 - 45,340 -- 43,687 43,687 - 43,687 - 42,035 - 42,035
-- 42,035 - 40,383 38,731 - 37,078
75 175
-- 193,990 -- 194,182 194,190 194,182 - 186.234 - 186.145 - 186:088 - 1861145
and observed values Average Maximum
68 200
70 190
by)-Garran's values computed from "best" hydrogen-steam equilibria by the method of Hofman ( 1 3 ) . by Eastman and Evans. (EI-Data by Eastman. (G)-Data by Garran.
(E & E)-Data
ORDER AXD METHOD O F CALCULATION-Thl? Values Of free energy for ferrous oxide designated as "observed" in Table I were computed by using values from the free-energy equations of Lewis and Randall (22) for carbon monoxide and carbon dioxide in one case (A), and also by using values from the free-energy equations of Eastman (4) for carbon monoxide and carbon dioxide in the other case (B), in combination with the free energy of the reduction reaction
FeO
+ CO = F e + C02
computed from the observed equilibrium constant. All the recorded observed values of free energy in each case were used in computing best average constants (by least squares) in straight-line equations, A F = A E T , where A F and T have their usual significance and A and B are constants. The equations obtained were:
+
(A) (B)
A F = -63,461 A F = -63,069
ferrous oxide in combination with the free energy of the reduction reaction Fea04f CO = 3Fe0
+ + 16.036T 16.523T
(2)
For magnetite similar equations for the free-energy change in formation have been computed by using the appropriate values of free energy of carbon monoxide, carbon dioxide, and
+ Cot
obtained from the equilibrium constant. The equations obtained were: (A) (B)
AF = -265,880 f 78.4047' A F = -264,346 80.362T
+
(3)
Calculation of the triple point in the dissociation of ferrous oxide according to the reaction 4Fe0 = Fe304
FREEENERGY(B)
(A) E Q U I L I B R I U M F R E EE N E R G Y RATIOCOI:CC Obsd. Calcd.
957
+ Fe
by Equations 2 and 3 gave 571 " C. in case (A) and 572" C. in case (B), and these are in agreement with 570" C. usually accepted for this temperature (8, 6, 8, S I , 34). Values of free energy of ferrous oxide computed by Equations 2 at 25" C. gave -58,680 calories in case (A) and -58,140 calories in case (B), a variation of 1210 and 670 calories, respectively, from -57,470 calories chosen by Ralston ( S I ) in a recent comprehensive review of the iron systems. Values for magnetite at 25" C. were -242,510 calories in case (A) and -240,390 calories in case (B), a variation of 20 and 2140 calories from -242,530 calories deduced by Ralston at 25" C. The values of free energy of magnetite in case (B) are always lower than those of case (A), and at 500' C. the difference amounts to 3000 calories while a t 1000" C. it is 4000 calories. Moreover, the free-energy values of magnetite in case (B) are considerably lower in all cases than values derived in a third law calculation made by Ralston. Eastman's equilibria for carbon monoxide and carbon dioxide are derived from indirect measurements made over the oxides of iron for the most part. He claims that the relative proportions of the gases in a sample withdrawn from direct equilibrium experiments are changed rapidly owing to catalytic action of a deposit of catalyst on the walls of the tube. Recent specific heat measurements by Roth and Bertram (33) lead to slightly larger values of free energy in a third lam calculation than Ralston deduced up to 700" C. Specific-heat data for magnetite do not extend beyond 800" C., and for the studies of this paper it was thought unwise to extrapolate some 600 degrees. Further comparisons are possible. The entropy change in the formation of magnetite a t 25" C. is -83.1 units (entropy of oxygen, '/202, taken at 24.5 units a t 25" C.). From the equation AF = AH - TAX, we may compute the heat absorbed in the formation of magnetite at 25" C.; that is, A H = -267,280 calories in case (A) which varies 1580 calories from Mixter's (26) value (and also Roth's, 52, same recent measurement) of -265,700 calories and by 3520 calories from Bertholet's (I) value of -270,800 calories. I n case (B) AH becomes -265,160 calories, a value in close agreement with that of Mixter and that of Roth. Ralston ( S I ) has averaged seven of Walden's (89) measurements on the dissociation pressure of pure ferric oxide with the two Sosman and Hostetter (3-5)measurements on a material containing 97.1 per cent ferric oxide, and has given the equation log
p
=
~
-23,550 + 13.83 T
as representing the average curve through these selected points. The temperature range of the chosen data is 1100" to 1400" C. The dissociation of ferric oxide proceeds according to the reaction 6Fe20a = 4Fe804 f 0,
INDUSTRIAL AND ENGTNEERING CHEMISTRY
958
We may then change the above equation into an expression for the free energy of this reaction and obtain: A F = -107,748
+ 63.551T
By combining this expression with Equation 3 for magnetite in this reaction, the free energy of ferric oxide is obtained: 2Fe
+
11/202
A F = -195,211 (B) A F = -194,122
(A)
= Fez08
+ 62.861T + 64.167T
(5)
The free energy at 25" C. is - 176,570 calories in case (A) and -174,990 calories in case (B). The entropy change in formation of ferric oxide a t 25" C. is -65.87 units, and the heat absorbed becomes -176,570 (298 X -65.87) = -196,210 calories in case (A), which varies by 4010 calories from Mixter's (26) measurement of - 192,200 calories, 1490 calories from LeChatelier's (21) value of - 197,700 calories, and by 1390 calories from Roth's (32) recent determination of -197,600 calories. I n case (B) the value of AH becomes - 194,110 calories. I n subsequent calculations it was found that Ralston's (31) values of free energy of magnetite, and ferrous and ferric oxides at higher temperatures when used with the above deduced values of ferrous sulfide, gave results which are not in accord with well-known facts, even when it is assumed that the free-energy values of iron sulfide are 30 per cent in error. The reasons for these facts appear to be that in reductions by ferrous sulfide the errors present in the free-energy values are multiplied several times in calculations of some reactions and subsequent subtractions give anomalous figures. On t,he other hand, it was found that the practical conclusions of this paper were not changed whether the equations of Lewis and Randall or of Eastman for the free energy of the oxides of carbon were used in deducing the free energy values of the oxides of iron. There seems to be a little better agreement on the whole with various known data, with figures computed by equations in case (A), and these have been used in the sections about to follow. The reader may easily substitute the values of equations of case (B) if he prefers. It would appear that no ultimate high degree of accuracy in such expressions is possible since the solid phases are in general not pure but are complicated by solid solutions which vary slightly in composition with temperature. Furthermore, heat effects in the thermal transformations of iron and its oxides are given no weight in such equations, although their effect is probably small.
+
Sulfur Oxides
The free-energy equations of Lewis and Randall (22) for sulfur dioxide and sulfur trioxide have been used in these studies: 1/&
+ O2 = SO2, A F = SO2
+ '/zO2
+ 2.75TlnT - 0.0028T2 + 0.00000031T3 + 0.9T AF = -22,600 + 21.36T
-83,260
= SOa,
Using the data above given for free energy of ferrous mlfide, the oxides of iron, and sulfur oxides, estimates of the free energy of a reaction involving these substances at a specified temperature can be made. A negative value of the free energy indicates that the reaction is possible, whereas a positive value indicates that the reaction will not occur unless the products as formed are diluted considerably. The sign and magnitude of the free energy are thus measures of the tendency of a reaction to occur, or of its driving force. Before taking up the detail of a series of reactions which were considered, several applications of free-energy calculations to actual smelting operations will be studied.
Vol. 22, No. 9
Application to Smelting Operations
The smelting of copper calcines is accomplished in reverberatory furnaces. Calcines usually contain a percentage of magnetite which is formed during roasting operations. Since magnetite melts a t 1538" C., a temperature above that attained in a reverberatory, it must be reduced and slagged, or it may accumulate in the furnace bottom as inconvenient bottom accretions. Tammann and Batz (36) found that ferric oxide and magnetite reacted with silica only after losing oxygen. Ferrous sulfide is the one possible reducing agent present which can react with magnetite to give ferrous oxide which can be slagged with silica. Some magnetite can be carried out mechanically in the slag, but it is usually observed that s l a g high in magnetite are also likely to be higher in copper. Wartman and Oldright (40) have called attention to the possible presence of copper ferrite in slags. Copper ferrite melts at 1458"C., which is as high as, or higher than, the usual highest temperature of a reverberatory furnace, and if present it may be expected to be associated with magnetite. The reduction of magnetite by ferrous sulfide is usually r e p resented as taking place according to the equation: 3Fe304
+ FeS = lOFeO + SO2
Starting with magnetite and ferrous sulfide, it hardly seems possible to obtain sulfur dioxide except as represented by this reaction. According to free-energy calculations given subsequently, this reaction would give ferrous oxide and one atmosphere of sulfur dioxide at about 1300" C. or above. Wartman and Oldright showed experimentally in platinum boats that reduction of magnetite by ferrous sulfide took place to a small extent at 1000" C. in a stream of nitrogen and that reduction was more rapid at higher temperatures and also with a larger amount of ferrous sulfide present. These investigators performed an experiment which tended to show that reduction would not occur a t 1200" C. a t depth in a bath of matte or slag because the partial pressure of the gaseous products would drive the reaction in the reverse direction. They observed elimination of relatively small amounts of sulfur trioxide and of elemental sulfur in the heating of ferrous sulfide with magnetite. A study of accretions in the bottom of a copper reverberatory furnace is instructive (Table 11). Magnetite accumulates to a considerable extent towards the front end of the furnace where the temperature is lower (about 1250" C.). The free-energy calculations on the reduction equation are in agreement with this fact. Keller (17) has recorded the presence of magnetite in both blast furnace and reverberatory mattes, and with mattes of a grade so that the magnetite is specifically heavier, the magnetite will sink through the bath of matte where the temperature is not sufficient to cause reduction. It would appear from free-energy calculations (in a later section of this paper) on the reaction 6Fe0
+ SO2 = 2Fe304 + '/z&
that, if ferrous oxide existed at or below 1300" C., sulfur dioxide would oxidize it to magnetite unless it combined with adjacent silica to form slag. According to the calculations, elemental sulfup should reduce magnetite to ferrous oxide a t or above 1350" C. Temperatures above 1300" C. -are indicated as necessary for the reduction of magnetite by ferrous sulfide also. The mechanics of skimming might account for the large proportion of magnetite in the bottom accumulations in the front end of the usual side-fed reverberatory furnace. Furthermore, but little magnetite was noted before the adoption of side feeding of reverberatories and the advent of flotation. On the other hand, the grade of matte in the bottom accumulations in the hot zone is noted at about 47 per cent copper, but is of higher grade, about 60 per cent, in the
INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1930
front end of the furnace. Cuprous sulfide, CuZS, is heavier than magnetite. A bath of matte at a temperature somewhat above that prevailing a t the front end of the furnace (1250" C.) should, according to the calculations, lead to reduction of magnetite, unless the matte is of very high grade. It seems logical to conclude that reaction between matte and magnetite up to a grade of about 60 per cent copper takes place at about 1250" C. if time enough is allowed and also perhaps if a certain
I
Feet from firing end
I-
CUZS FeS
Fer04 Spinel (250.AhOd Slan
1400° C.
%
%
52.7 37.8 4.7 3.4 0.5
57.3 28.8 8.9 2.8 0.5
I
I
130OoC. 30
45
52.5 19.9 19.9 3.5 3.6
20.6 13.9 40.6 6.9 17.5
I
1275'C.
+ FeS = 1OFeO + SO2
Calculations (see later section) for several other reactions would indicate that both elemental sulfur and sulfur trioxide would be quite possible constituents of the gaseous products of the reduction of magnetite. It is well established in the basic converting of copper mattes that the temperature during the slagging period ordinarily must not be allowed to rise above 1250" or 1300" C. or the protective coating of magnetite on the magnesite brick will disappear. Reduction by iron sulfide and slagging of the ferrous oxide formed will result in the presence of sufficient silica. The calculations accord with these facts. Higher temperatures may be employed when the slag is quite low in silica without removing the magnetite coating, however. The cleaning of copper converter slags carrying copper and magnetite is of interest at this point. I n this process the magnetite should be reduced, and to accomplish this reduction the slag should be poured into the furnace (treatment in a reverberatory calcine smelting furnace is here considered) where the temperature is above 1300" C. The presence of equally hot iron sulfidelow-grade matte-to mix very intimately with the hot slag should give good opportunity for reduction of the magnetite according to the calculations. Copper ferrite, if present, would probably be associated with the magnetite and it also should be reduced. I n the case of slags from the converting of copper-nickel mattes, nickel ferrite (NiOFezOs), might be present. Veil (58) reports preparation of chromites and ferrites of nickel and cobalt. Nickel ferrite would probably be more difficult to reduce and sulfidize than copper ferrite. The addition of raw cold pyrite in a stream with the slag can hardly be expected to be as effective as hot low-grade matte for these reductions. The driving off of the free atom of sulfur requires heat, as does the fusion of the iron sulfide. This procedure requires high-grade heat energy and cools the slag. As indicated in the calculations, elemental sulfur has a reducing action on magnetite above 1300" C. Unless the converter slag is kept in the hottest part of the furnace for some time, the reduction of the magnetite will not be completed and concurrently copper ferrite or nickel ferrite present in the converter slag would also escape reduction. The liberation of the volatile sulfur should agitate the bath and it may sulfidize metallic copper present in the converter
12500 80
%
amount of agitation occurs so as to permit easier liberation of gaseous products of reduction. The furnace mattes tapped usually were 35 to 40 per cent copper. Time and heat are required to complete such a reaction as 3Fe304
slag. I n any case intimacy of contact is important, and if pyrite is used it should be introduced below the slag stream to get the benefit of the agitation. Calculations show that the sulfur may act on ferrous oxide below 1300" C. to give magnetite and ferrous sulfide. The ferrous oxide resulting from the reduction must be combined with silica. The tendency of ferrous oxide to unite with free clean hot quartz (silica) would seem to be much greater than for it to unite with silica in the
on B o t t o m of No. 2 Reverberatory Furnace at Anaconda, M o n t . , 1916
Table 11-Accumulations Temperature
959
20.6 11.2 33.0 9.4 23.4
29.5 16.0 39.0 5.6 8 3
90
105
c. 115
125
13.5
%
%
%
%
%
%
32.0 9.1
24.5 11.8 46.8 7.5 7.7
23.3 11.3 55.9 4.4 2.4
19.3 12.7 58.0 3.9 7.4
21.9 9.4 61.8 2.5 2.8
11.5 4.7 59.7 7.4 16.6
62.0 4.7 1.8
form of silicates. If silica is fed during the pouring of converter slag, such silica should preferably be free silica and if possible preheated. It may be observed that when converter slag is cooled, crushed, and mixed with the calcine charge going to a reverberatory, the resulting slags are lower in copper than when no converter slag, either liquid or solid, is on the charge. Iron silicates are very fluid and permit of better settlement of matte prills than slags relatively high in magnetite. The fact of greater fluidity in the absence of magnetite has been brought out by McLellan (85) in a recent paper. The reduction of magnetite absorbs considerable heat, and since the reduction by ferrous sulfide only becomes effective above 1300" C. it is logical that the reduction would be performed more quickly and completely in that part of the furnace where the thermal head is the greatest. Proposed Modified Method of Treating Converter Slags
The foregoing considerations have resulted in the proposal of a method for treating converter slag which introduces a few different steps than the methods now in use. Figure 1 represents a longitudinal section of a reverberatory smelting furnace.
n b+-*-+-c //--
In the case'of side-fed furnaces it is proposed to maintain a lower grade bath of matte at (a) in the hottest part of the furnace independently of the matte pool usually maintained a t present. The converter slag would be poured into this hot part of the furnace from such a height-through a hole in the roof, for example-as to give a plunging action and thereby insure that the heavier converter slag will penetrate through any layer of slag into the matte bath below. Matte from this pool would be tapped at (b) from time to time as accumulated. The slag could flow away to the other end of the furnace, as it does in present practice, or it could be tapped intermittently at a point suitably higher than (b), as at ( c ) . Siliceous flux or calcines would also be charged to flux the ferrous oxide formed. I n the case of furnaces where the calcines are center-charged into a deep pool of matte, the matte could perhaps all be tapped a t (b) and all of the slag as at present, thus observing a countercurrent flow
INDUSTRIAL AND E,YGISEERISG CHEMISTRY
960
of matte and slag in the furnace. Otherwise the procedures would be the same as for side-fed furnaces. These procedures insure intimate contact of the converter slag with the matte at a higher temperature and also give more time of contact. Furthermore, the increased liquidity of the slag due to the higher temperature for a longer period of time permits of more complete settling of the matte prills. Laist (19) has pointed out that center-charged furnaces-charging calcines into a deep pool of matte-give cleaner slags than side-charged furnaces. I n the light of the foregoing discussion, this may be due to the fact that the slag remains longer in a relatively quiescent state in contact with the matte and at a higher average temperature. Application to Copper Blast-Furnace Smelting
I n addition to the series of reactions which have to be considered in reverberatory smelting, blast-furnace smelting involves reactions with carbon and its oxides. A certain minimum heat development is necessary, and this figure has been variously estimated at from 575 to 725 calories per unit of charge. Coke has been viewed in the light of thermal insurance in some smelting operations, and again but part of the coke has been considered as such insurance, depending upon the kind and thermal value of the ore charged. Generally the amount of coke needed on the charge increases with the amount of combined silica and with a decrease in the amount of sulfur and iron as sulfide. It has been stated that pyritic smelting has been carried on for periods without the use of coke; yet in most cases some coke is used. Statements have been made repeatedly that the function of the coke in pyritic smelting is to serve as a preheater of the charge in the shaft of the furnace-that the reaction
so2 + c
=
coz + 1/2sz
takes place with positive thermal effect (thermochemicslly) in the absence of oxygen from the air-and, according to discussion by Peters (28, 29) and Hofman and Hayward (14) on pyritic smelting, this seems to be a tenet of faith in such work. The writer does not dispute that such a reaction takes place, but it is subrhitted, and evidence to be here given substantiates the claim, that this reaction is undesirable and results in wastage of fuel. Certain instances of operation are also cited which further substantiate this statement. The following reactions have been considered:
(1) (2) (3) (4) (5) (6) (7) (8) (9)
+ + + c + cor = 2 c o c + = coz C + '/sOa = CO Fer04 + CO = 3 F e 0 + Cos Pea04 + C = 3 F e 0 4- CO 2FeaO4 + C 6 F e 0 + COz so2 c = 1/zsz cos so2 + 2 c = 1 / 2 s 2 + 2 c o sot + 2 c o = ' / a s 2 2c02 0 2
=
FREE-ENERGY VALUES 1400' C. CaI ori es -26,100 -40,200 -21,800 69,900 -30,400 -10.600 4,300 -29,600 -94,250 -94,220 -45,000 61,900 240 7,500 4,100 37,100 3,800 -44,600
600° C. Calories
+
+ +
-
--
I n reactions 1to 3 it will be seen that the reduction by carbon and carbon monoxide of sulfur dioxide can take place at all temperatures considered. The tendency to the formation of carbon monoxide increases with rise of temperature, but carbon monoxide would hardly be present in the furnace gases a t any temperature unless there was a deficiency of sulfur dioxide. Let us consider reactions 1,4, 6, and 6. At all temperatures considered, reaction 5 results in the largest free-energy change per mol of carbon and also in the largest thermal effect. Available heat is heat available for doing useful work-in the furnace for smelting the charge properly. Yet in the copper blast furnace4he coke is used partly to perform a reaction which results hi the production of less available heat than if
Vol. 22, No. 9
burned by oxygen of the air. It is a well-known principle of energetics that low-grade heat may do the work of lowcr temperatures, but here a high-grade fuel, capable of developing much more heat at a higher temperature, actually is found to perform in part the function of lower grade heat. Furthermore, such extra preheating of the charge is undesirable in that the top of the charge becomes hotter than is necessary and much heat is shot up the stack. The place to burn carbon is lower down in the furnace with air oxygen from the tuyeres in order to secure maximum availability of the heat energy developed and of course a higher degree of availability of the heat should result in a cut in the fuel requirements. Sulfur dioxide may act on iron sulfide to give magnetite and whatever magnetite is formed in the shaft of the furnace or at the focus or tuyi.re zone has to be reduced for the most part. But above 700" C. only carbon or carbon monoxide has a reducing effect on magnetite until a temperature of 1300" C. is reached. Then iron sulfide reduces magnetite and this reduction has to be rapid since only at or just a little above the tuyere zone can it be effective. All chemical reactions require time for their completion. If an excessive amount of magnetite forms in the furnace, at or just above the tuyeres, the comparatively feeble reducing action of iron sulfide at the tuyere level during the brief time that the descending materials are passing could not reduce all the magnetite or keep the iron oxide reduced to keep the furnace open; a magnetite mush results and the furnace freezes up. It is common knowledge that free silica combines with such bases as ferrous oxide much more readily than with combined silica, especially magnesium and aluminum silicates. More heat and longer time are required to effect combination in such instances. With silicates present and insufficient available heat, magnetite is certain to form. I n other words, conditions are such that a concentration of excess magnetite can occur and this contingency requires more reducing action-i. e., coke and ferrous sulfide. Theoretically, then, in pyritic work, if too much combined silica happens to accumulate in the furnace, or is charged to the furnace, the iron will not slag properly. It forms magnetite and whatever slag is made will analyze high in iron because of its high content of magnetite. The remedy is more reduction and more heat; that is, coke or fuel at the tuyilres and the corrective charge should consist of coke, low-grade matte, and limerock to help flux the silicates. Ferrous sulfide is more efficient than pyrite since less energy is needed to melt i t than p y r i t d 0 0 kilowatt-hours for matte and 700 kilowatthours for pyrite per ton. The hot rising gases will probably give sufficient preheat to the charge. The Japanese practice (15) of introducing coal through the tuyhres of the furnace has enabled running of the furnaces with from 3 to 4 per cent of fuel. The furnace charge is never hot on top. The charge ran 4.1 per cent copper and the matte, 24 per cent copper. At one time a charge containing 2.3 per cent copper, 17.6 per cent barium sulfate, 5 per cent aluminum oxide, 24 per cent silica, 19.6 per cent iron, 6.2 per cent zinc, and 19.6 per cent sulfur was usep. The matte assayed 30 per cent copper, and 2.8 per cent of coal was introduced through the tuyeres. I n such practice the highest thermal value of the fuel is obtained and the hot gases rising from the tuyere zone after doing work there serve to preheat the descending charge. Lathe (201, in semi-pyritic smelting of a low-grade coppernickel sulfide ore at Nickelton, Ontario, found that the chief requisites for low coke consumption were to keep the heat 13w down in the furnace and the tonnage up. By lowering the charge column in the furnace periodically every 12 to 16 hours and then refilling, the top was kept cool. This should cause a larger percentage of the coke charged to reach lower levels in the furnace. The focus should be hotter if this is true and the tonnage should go up. However, with a full fur-
September, 1930
INDUSTRIAL A N D ENGIXEERING CHE-PIISTRY
nace the heat persisted in rising in the shaft. The gases coming off the top of the charge were very white. This is thought by the present writer to be due to finely divided sulfur, and as soon as the temperature rises above the boiling point of sulfur (444.5" C.) this white color should disappear. The same phenomenon occurs during the blowing-in period of a pyritic furnace. Before the advent of large furnace units, prehezting of the blast was found advisable in pyritic smelting to keep the hearth hot enough for operation (18, 27). When the furnaces were increased in size it was found that hot blast could be dispensed with, not bwause hot blast was no longer beneficial to operation, but because hot blast was no longer indispensable and was found t o be not economical. The greatly increased furnace activity, higher blast pressures, and faster driving overcame losses of heat due to radiation and cooling water. Hot blast increases the availability of heat at the tuysres. Harris (11) relates experiences in what he terms "arsenidic" smelting. An ore containing a large amount of sulfarsenide yielded an intermediate arsenide of iron and other metals which was smelted much as pyritic ores are smelted, the arsenic being volatilized and a speiss made. It was found that with low coke, magnetite formed and often choked the furnace. The addition of free silica permitted slagging of iron with lower coke on the charge. Potts (SO) has recently recorded experiences in smelting Rio Tinto ores and states: In former years, the coke was reduced to under one per cent and runs were made lasting up to 35 days in which the furnaces smelted on an average well over 400 tons per day, using absolutely no coke or other extraneous fuel This form of smelting has been abandoned for many years because i t was necessary that the pick of the export ore, running 48 per cent sulfur, should constitute 60 per cent to 65 per cent of the total charge: also a n extremely basic slag running about 60 per cent Ft.0 and from 0.60 to 0.65 per cent Cu was unavoidably made, so that the apparent saving in fuel was considerably offset by slag losses.
Potts states further that in this locality free silica (quartz) is very scarce. It is the view of the present writer that to generate the heat necessary to carry out as slag the silicates charged to the furnace, it became necessary to oxidize much iron. This bessemerizing action was comparable with that on low-grade mattes in modern copper converters, where if sufficient silicates are charged the converter is cooled off and excessive temperatures due to oxidation of the large amount of iron and sulfur are avoided. Too much cooling off with silicate in a pyrite furnace is disastrous. Slags of 60 per cent FeO and 25 per cent silica should contain magnetite and should be somewhat viscous. At Rio Tinto coke on the charge is 3.3 per cent and the present slag carries more silica and less iron. It appears that there is an essential balance between iron oxidized, quantity of silicates charged, and coke in each pyritic or serni-pyritic smelting operation. Hamilton ( I O ) related in 1905 that at Anaconda, Mont., ashes from the rererberatories was sent over jigs and the coke saved. This coke was found to be of very little value when burned under boilers, but when briquetted along with slimes from the concentrator, i t was found that one ton of coke in briquettes saved more than one and one-half tons of coke added in the usual way to the blast furn,aces, doing pyritic smelting.
If this observation is correct, it would point to the fact that the pieces of coke (probably small) were protected from the action of the furnace gases in the furnace shaft by the enclosing slimes and that the coke thus became available for developing heat of a higher degree of availability lower down in the furnace where heat is most needed. More intimate mixing with the fuel should also be of some benefit.
96 1
Hayward (12) has indicated the advisability of a short column and coke in big chunks rather than small pieces in lead blast-furnace work. Larger coke and a shorter column give less opportunity for gaseous action on the fuel in the upper portions of the shaft and should conserve the coke for use a t the tuy6res where its thermal and reducing effects are greatest. If the views presented here are correct, then it would seem that the essential feature in copper blast-furnace work, whether pyritic, semi-pyritic, or reduction smelting, is a hot focus. It corresponds to a hot hearth in the iron blast furnace, and in both cases availability of hearth heat is the chief factor in fast sure running. Furthermore, it would appear that calculation of the heat balance of the focus or tuyi.re zone of the copper blast furnace, rather than an overall heat balance, should be made if important information on the operation is to be obtained. Discussion of Smelting Reactions
A very large number of reactions can be written and the number given here is not exhaustive. Some of those given in the literature are hardly possible thermodynamically, and others which are thermodynamically possible probably play a small part in actual smelting since other reactions are dominant. Mutual partial solubility of the phases in the reactions, being unknown, has been ignored and this no doubt introduces some error in the figures given. What follows should be considered in a qualitative way only. The reduction of magnetite by ferrous sulfide is usually represented as taking place according to the equation: 3FetOa
+ FeS = l0FeO + SOz
At 600" C. this reaction has a free-energy value of +52,200 calories and a decrease takes place with rise in temperature. At about 1325" C. the free energy becomes zero, or a t about this temperature the pressure of sulfur dioxide would be one atmosphere. At 1400" C. the free energy is computed &s -5300 calories. If the reaction is carried out in a stream of inert gas, sulfur dioxide would be evolved at temperatures lower than 1325" C., since the partial pressure of the sulfur dioxide gas would be lowered. Another reduction of magnetite is sometimes stated: FeaOa
+ FeS = 4Fe0 + l/2S2
The free energy would be +22,200 calories at 600" C. and -900 calories a t 1400" C. At above about 1375" C. sulfur would be evolved by reduction of magnetite by ferrous sulfide according to this reaction, unless a vigorous stream of gases were passing to lower the already low partial pressure of sulfur vapor. Below 1300" C., free sulfur with ferrous oxide could give iron sulfide and magnetite. Wohler, Martin, and Schmidt (41) passed sulfur dioxide over iron sulfide a t 900" C. and obtained magnetite and elemental sulfur. They stated the reaction to be: 3FeS
+ 2S02 = Fes04 + 21/2S2
The writer has secured upwards of 80 per cent elimination of sulfur from iron sulfide at 790" C. in "4 hour in a stream of sulfur dioxide. At 600" C. the free energy appears to be +6800 calories and decreases to +5800 calories a t 1400" C. According to this statement, a considerable excess of pure sulfur dioxide would be required. The reverse reaction could perhaps take place in a furnace in the absence of free oxygen. On the other hand, 2FeS
+ SO2 = 2Fe0 + 11/zS2
appears to show a free-energy value of +14,500 calories at 600" C. and of +2400 calories a t 1400" C.; that is, there is an increasing tendency for the reaction to occur with rise in
INDUSTRIAL AND ENGINEERING CHEMISTRY
962
temperature. Excess of sulfur dioxide theoretically would complete this reaction. But ferrous oxide would appear to reduce sulfur dioxide:
+ SOZ = 2FesO1 +
6Fe0
l/&
The free energy a t 600" C. would be -30,000 calories, a t 1400" C., +4300 calories. At about 1300" C. the free energy is zero. This indicates that up to about 1300" C. sulfur dioxide oxidizes ferrous oxide with production of elemental sulfur and magnetite, and that sulfur would only reduce magnetite to ferrous oxide above 1300" C. If we consider the last two reactions simultaneously, the equilibria appear to be such that elemental sulfur and magnetite would form at 7001000" C. Moreover, the reverse of the reaction of Wohler and his co-workers should cease at about 800" C. by reason of the effect of the last two stated reactions according to the equilibrium calculations. Complete utilization of sulfur dioxide should not be possible and dilution with an inert gas should limit the formation of elemental sulfur. Oxidation of ferrous oxide to ferric oxide by sulfur dioxide would be expressed: 4Fe0 SO2 = 2Fez03 '/zSZ
+
+ SO2 = 3Fez03 + FeS
The free energy appears to be -29,300 calories a t 600" C. and becomes positive between 1000" and 1100" C. The value at 1400" C. is +29,400 calories. This indicates that below 1000" C. ferric oxide could form by action of sulfur dioxide on ferrous oxide. The oxidation to magnetite is more vigorous, however. Direct reduction of ferric oxide by iron sulfide to ferrous oxide could occur above 1100" C., but, as shown later, reduction to magnetite is more likely to take place iirst. Ferric oxide is reduced by sulfur to magnetite: 6Fez03 '/SZ = 4FerOa SO1
+
+
Here the free energy is -15,900 calories a t 600" C. and becomes -52,600 calories at 1400" C. We may write: 4FeS SO2 = 2Fet0 2l/ZS,
+
+
I n this case the free energy is +14,400 calories at 600" C. and +32,600 calories at 1200" C. Reduction of ferric oxide to magnetite is more likely-that is:
+
10FezOa FeS = 7Fe301
+ SOZ
Here the free energy is -24,200 calories at 600" C. and -82,300 calories at 1400" C. We may also write: 4Fez03
+ FeS = 3Fe304 + '/zSZ
This has a free energy of -8300 calories a t 600" C. and of -29,700 calories a t 1400" C. Elimination of sulfur dioxide rather than sulfur is more likely. Ferrous sulfide does not reduce ferrous oxide to iron: 2Fe0 FeS = SO2 3Fe
+
+
The free energy is +53,000 calories at 600" C. and is +23,500 calories a t 1400" C. Sulfur dioxide will oxidize iron to magnetite and to ferric oxide: 3Fe 2S02 = Fe304 SZ
+
+
The free energy is -61,100 calories at 600" C. and -26,700 calories a t 1400" C.
+ 3S02 = 2Fez03 +
11/2S2
In this case the free energy is -76,200 calories a t 600" C. and -18,100 calories at 1400" C. The action of ferrous sulfide on magnetite to give sulfur trioxide is usually expressed: FeS
+ 4Fe304 = 13Fe0 + SO3
Here the free energy is +97,300 calories a t 600" C. and is +35,100 calories at 1400" C. The reaction as stated would require high dilution of the sulfur trioxide in order to take place a t all. Sulfur trioxide is eliminated in small amounts, however, and other reactions may be possible. For example:
+ l/zSz=. 6Fe3O4+ SO3
9Fez03
The free energy is +6300 calories at 600" C., becomes negative just above 700" C., and is -38,700 calories at 1400" C, That is, sulfur trioxide can form above 700" C. from ferric oxide and sulfur. Ferric oxide and ferrous sulfide will not give ferrous oxide and sulfur trioxide:
+
The free energy is -14,700 calories at 600" C. and becomes positive at about 900-950" C. and is +20,400 calories at 1400" C. That is, below 900" C. ferric oxide mould not be reduced by elemental sulfur to ferrous oxide. The oxidation of ferrous oxide may also be expressed: 7Fe0
4Fe
Vol. 22, No. 9
4Fe203
+ FeS = 9Fe0 + So3
The free energy is +52,100 calories at 600" C. and +9500 calories at 1400" C. To obtain sulfur trioxide, it is suggested that ferric oxide is first formed as an intermediate product and that elemental sulfur acts on it to give sulfur trioxide above 700" C., remembering that ferrous sulfide slowly gives off sulfur, leaving iron dissolved in ferrous sulfide. The presence of adjacent free silica should facilitate reduction of magnetite by ferrous sulfide at a lower temperature than indicated, since ferrous oxide combines readily with free silica. Practical experience seems to indicate definitely, however, that silicates will not combine nearly so readily with ferrous oxide and therefore, in the presence of silicates, reduction of magnetite would require more time and a greater thermal head. The free-energy values will accrue a t equilibrium and the attainment of complete equilibrium requires time. It may be stated that in smelting reactions equilibrium is perhaps not attained. Nevertheless, equilibrium calculations should prove helpful in indicating the direction a reaction will run if the temperature is changed. Moreover, the further a system is from equilibrium, in general, the greater is its rate of reaction at a given temperature. Literature Cited (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)
(18) (19) (20) (21) (22) (23) (24) (25)
Bertholet, Ann. chim. phys., 18, 119 (1881). Bornemann and Hengstenberg, Metall Erz, 17, 314, 339 (1920). Chaudron, Ann. chim., 16, 221 (1921). Eastman, Bur. Mines, Circ. 6126 (1929). Eastman, Bur. Mines, Tech. Paper 446 (1929). Eastman, J . A m . Chem. Sac., 44, 975 (1922,. Eastman and Evans, Ibid., 46, 88s (1924). Ferguson, J. Wash. Acad. Sci., 18, 275 (1923). Garran, Trans. Faraday Sac., 24, 201 (1928). Hamilton, J . Can. Min. Inst., 8, 349 (1905). Harris, Eng. Min. J . Press, 118, 214 (1924). Hayward, Ibid., 127, 336 (1929). Hofman, 2. Elektrochem., 81, 172 (1925). Hofman and Hayward, "Metallurgy of Copper," McGraw-Hil!, 1924. Ikeda, Trans. Am. Inst. Mining Met. E n g . , 49, 123 (1923). Jellinek and Zakowski, Z . anorg. allgem. Chem., 142, 1 (1925). Keller, Eng. Min. J., Nov. 16, 1895, p. 465. Koch, Ibid., 77, 796, 1035 (1904). Laist, Mining Met., 11, 33 (1930). Lathe, Trans. Can. Inst. Mining Met., 28, 174 (1925). LeChatelier, Comfit. rend., 120, 623 (1895). Lewis and Randall, "Thermodynamics," McGraw-Hill, 1923. Loeb and Becker, Z . anorg. Chem., 77, 301 (1912). McCance, Trans. Faraday Sac., 21, 176 (1925). McLellan, Am. Inst. Mining Met. Eng., Tech. Pub. 806, Class D , Nonferrous Metallurgy, No. 25 (1930).
September, 1930 (26) (27) (28) (29) (30) (31) (32) (33) 434)
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
hlixter, Am. J . Sci , 40, 23 (1916). Peters, Eng. Mzn. J., 77, 881 (1904). Peters, “Principles of Copper Smelting,” McGraw-Hill, 1907. Peters, “Practice of Copper Smelting,” McGraw-Hill, 1907. Potts, Inst. Mining Met., Bull. 299 (August 15, 1929). Ralston, Bur. Mines, Bull. 298 (1929). Roth, Z . angew. Chem., 42, 981 (1929). Roth and Bertram, Z Elektrochem., 36, 297 (1929). Schenk, Slahl Ezsen, 46, 655 (1926).
963
Sosman and Hostetter, J. Am. Chem. SOL.,38, 807 (1916). Tammann and Batz, Z. anorg. allgem. Chem., 161, 136 (1926). Tigerschiold, Jernkontorets Ann., 107, 67 (1923). Veil, Compt. rend., 188, 330 (1929). Walden, J. Am. Chem. SOL.,30, 1360 (1908). Wartman and Oldright, Bur. Mines, Repfs. Investigations 2901 (1928). (41) Wohler, Martin, and Schmidt, Z . anorg. allgem. Chem., 127, 273 (1923).
(35) (36) (37) (38) (39) (40)
Extinction of Ethylene Dichloride Flames with Carbon Dioxide’ G. W. Jones and R. E. Kennedy PITTSBURGH EXPERIMENT STATION, u. s. BUREAUO F MINES, PITTSBURGH, PA.
T
HE production of ethylene dichloride has greatly increased during the past few years, owing largely to its increased use as a solvent for oils, fats, and waxes, and as a fumigant and insecticide. For the latter uses the ethylene dichloride, which is a liquid with boiling point 83.5 O C., is vaporized into closed spaces in the proper conccntrations t o kill the vermin. Ethylene dichloride vapor is combustible and its extensive use as a fumigant arid insecticide introduces haAards due to possibility of explosions, especially when used in large enclosed spaces. Considerable damage might result should an explosion take place during fumigation. At the request of the Carbide and Carbon Chemicals Corporation, an investigation was made by the United States Bureau of Mines to determine the limits of inflammability of this vapor when mixed with air and the amounts of carbon dioxide whicbh must be added to the ethylene dichloride vapor to render it non-inflammable in all proportions when mixed with air.
it is desired to destroy-that is, they absorb the ethylene dichloride at a faster rate and are more quickly killed. Results
The areas of inflammability of ethylene dichloride, carbon dioxide, and air mixtures are shown graphically in Figure 1. It also shows how the limits of inflammability of ethylene dichloride are narrowed by the addition of carbon dioxide. I n this curve the values for “Carbon Dioxide in Atmosphere” refer t o the percentage of carbon dioxide in the atmosphere before the addition of the ethylene dichloride. For example, if carbon dioxide is added t o a closed space containing air until the “atmosphere” contains 21 per cent carbon dioxide, then
Procedure Samples of ethylene dichloride were submitted to the bureau by the firm mentioned and tests made in the inflammability apparatus described in a previous publication (2’). Previous tests have shown that results obtained in this apparatus are very close to those obtained in large volumes and are practically comparable as to conditions with those in which ethylene dichloride 1s used in practice. The explosion tube, mounted vertically, was 4 feet long and 2 inches in diameter. Flames were propagated from the bottom to the top. As ethylene dichloride does not have sufficient vapor pressure at ordinary laboratory temperatures to give higher limit mixtures (air saturated with ethylene dichloride at laboratory temperatures is explosive), it was necessary to conduct the tests at elevated temperatures. A temperature of 100” C. was chosen to insure complete vaporization of the ethylene dichloride a t the upper-limit mixtures. The determined limits of inflammability of ethylene dichloride in air (dried with CaC12) r e r e 5.8 per cent lower and 15.9 per cent higher by volume at 100’ C. The lower limit at 22’ C. (laboratory temperature) was 6.2 per cent by volume. Tests were made to determine the amounts of carbon dioxide which must be added to ethylene dichloride mixtures to render them non-inflammable. Carbon dioxide has a twofold value: It reduces the explosibility of ethylene dichloride mixtures; and it has been shown by Cotton and Young (1) that it stimulates the respiratory processes of the vermin which 1 Received May 13, 1930. Published by permission of the Director, U. S Bureau of Mines (Not subject to copyright.)
I
! I
I
l
~
l
I
i
1
I
I
~ ‘ I i i i L I
1
Figure 1-Limits of I n f l a m m a b i l i t y of E t h y l e n e Dichloride i n Mixtures of Air a n d Carbon Dioxide
ethylene dichloride can be added in any proportions without danger of explosion. The dotted line in Figure 1 indicates the mixtures containing oxygen and ethylene dichloride in the proportions to give complete combustion. As the preferred practice is to mix the carbon dioxide with the ethylene dichloride before introduction into a closed space, the amounts of carbon dioxide which must be added are better shown in Figure 2, which gives the volumes of carbon
