Aug., 1922
THE JOURNAL OF INDUSTRIAL AND ENISINEERING CHEMISTRY
699
Nitrogen Fixation by the Cyanide Process1” By F.E. Bartell UNIVERSITY ow MICHIQAN, ANN ARBOR,MICHIOAX
An earlier paper has described the production of ammonia by the sodium cyanide method. The present paper deals with the study of the cyanizing end of the ammonia process. The results gioen in this paper were obtained with 4-in. batch retorts with a capacity of 8 Ibs. of briquets and with technical sized 8-in. continuous retorts. with a maximum cyanizing capacity of not less than 50 lbs. of cyanized briquets per hr. I n studying the progress of the cyanizing reaction in the 4-in. retorts, the retort was arbitrarily divided into fioe zones, heated to approximately the same temperature, and the extent of reaction in the zones was compared. The amount of cyanide formed in each zone was largely dependent on the concentration of carbon monoxide gas in that zone, a high concentration of the gas holding down cyanide formation. Some of the carbon used in cyanide formation came from decomposition of carbon monoxide. Iron used in the original mixture was oxidized to Fez03 and Fe804, during the process of making and drying the briquets. The reduction of this oxide during the cyanizing process was slight in the upper heat zones, even though it was
ATE in 1917 the author of this paper was assigned by the Nitrate Division of the Ordnance Department to study the Bucher process for the fixation of nitrogen, and to attempt the development of a process by which cyanide formed thereby could be converted into ammonia and ammonium nitrate, The latter part of this work has already been described in a previous paper.3 In that paper it was shown that the conversion of cyanide to ammonia could be satisfactorily accomplished. It was known, however, that the cost of cyanide production with the process as developed a t that time would be high; consequently a research was organized to study the cyanizing end of the ammonia p r o c e ~ s . ~This work was to be carried on simultaneously with the ammonia research. Experimental work on the cyanizing of briquets was begun in the laboratories of the Nitrogen Products Company at Greene, R. I., early in April 1918, and was continued until the middle of November of that year. In general, the plan of the investigation was along the following lines:
L
1-A study of different types of coke and a comparison of their reactivity and suitability for the production of cyanide. 2-A study of the effect of different compositions of briquet material. 3-Rate of reaction and chemical equilibria involved. Received February 28, 1922. This article is printed with the approval of the Chief of Ordnance, U. S . A., also with the approval of Mr. Edward E. Arnold, President of the Nitrogen Products Company. The author wishes to acknowledge the able assistance rendered by his associates in this work, all of whom should share equally with him any credit which may arise from it. The men in charge of the more important phases of this aork were: Mr. Donald V. Lowe (formerly Lieut., Ord. Dept., U. S. A.), Mr. H. E. Kaiser, Mr. 0. E. Madison, MI. Paul A. Keene, Mr. E. J. Bird, Mr. G . H. Loselle, and Mr. I.. A. S . Rapin (chemists and chemical engineers). a THIS JOURNAL, 14 (1922), 616. 4 During.1918, a 10-ton per day plant was built under the direction of the Bureau of Mines for the War Department. This plant was completed just about the time of the Armistice, so that little had been done along the line of obtaining data during actual operation of the process. Brown, THIS JOURNAL, 11 (19191, 1010. 1 2
in the presence of carbon and carbon monoxide at 1000” C. The repression, which is much as would be expected from a system higher in cwbon dioxide content, is being studied further. The paper also records large-scale work, in which 3- to 12-day runs were made. The work was done in 8-hr. shifts and yields were based on the performance of each shift. A normal production of 7 to 8 lbs. of sodium cyanide per retort per hr. was obtained, and this was finally increased to as high as 12.19 lbs. per retort per hr. Conseroative cost estimates showed that ammonia could be produced by this method for approximately 30 cents per Ib., which represented a cost of cyanide production in briquets of approximately 10 cents per Ib. Just before the Armistice, the method for the fixation of nitrogen as cyanide and the subsequent conoersion of cyanide info ammonia and nitrates had been deoeloped f a a point where positioe results were assured. A t that time preliminary plans were actively under way by the W a r Department for the construction of a plant for the fixation of nitrogen by this process.
4-Progress of reaction in technical sized units. &Application of above findings to commercial operation.
A method for the preparation of sodium cyanide by the Bucher process was described in detail in THIS JOURNAL in 1917.5 According to the author of that paper, the process was one which should offer practically no difficulties either chemically or mechanically, even when carried out on a large scale. It was pointed out, also, that the process “makes use of the crudest things such as coke, producer gas” -soda ash and also some iron which is supposed to function as a catalyst. It was further stated that we “can use producer gas just as well as nitrogen” and that the process could best be carried on a t a temperature of from 900” to 950” C. The reaction is shown by the following equation: Na2COs NZ = 2NaCN 3CO - 138500 cal. It was discovered early in our work that the process offered no serious difficulties chemically, but that much room remained for improvements mechanically. It was seen that owing to the reversibility of the various reactions, close correlation between the progress of the chemical reactions and mechanical treatment was necessary in order to obtain high yields. When properly treated the precise nature of the coke used made little difference in the maximum yield of sodium cyanide. The exact proportions of the carbon, soda ash, and iron in briquets was of far less importance than was the exact manner of bringing these constituents together and of their later treatment. The existing chemical equilibria, especially the equilibria dependent upon the concentration of the components of the gaseous phase, was of the greatest importance. I n a continuous process, the length of time of heating was highly important. Large retorts, externalIy heated, offered a serious problem, owing to the low thermal conductivity of the reactive mass coupled with the fact that the reaction is endothermic.
+
6
+
Bucher, THISJOURNAL, 9 (1917), 233.
THE JOURNAL OF INDUSTRIAL Ah-D EArGINEEIiISG CHE3fISTIZY
700
OUTLINEOF WORK Three lines of work were carried on simultaneously: I-In the chemical laboratory experiments were conducted in B I-in. iron retort about 3 ft. in length, with a heat zone of about 2 ft. This retort was heated in a small Fletcher xas furnace.
1-01. 14, S o . S
cent. Average = 29.93 per cent. (This reprpsented an average c,onversion of approxiinat,ely 82 per cent of the sodium carbonate origiudly prescnl.) These figures rcpresent just about. the limits of variability of the results obtained in the small scale work described in this papcr.
V x w s OP ~ B EXPB~MBNWL B CYANXEINO PI-~NI,GRBENE, X. I
scale work was carried on with 4-in. iron retorts ft. in length with a heat zone of approximately 3 ft. The capacity of these retorts was approximately 8 Ibs. of briquets. sxperiments with these reto& were carried on in a maMer closely conforming to actual operation of a technical sized unit. 3-Tpro technical sized units, quite similar to those in the Saltville plant, were installed, and all significant findings from the small scale work were applied in runs with these larger units.
Z-Small
4.5
Tho uiain object of the present paper is to outline the results obtained in the actual operation of a technical unit: accordingly, only results obtained in work under divisions 2 and 3 will be given. A more exact study of the complete chemical equilibria of the process (part 1) was not quite completed a t the time of the conclusion of the work at Greene. The author has since continued this work. A separate paper covering this phase will follow later.
SMALI. SCALEWORKWITH 4-IN. BATCHRETORTS In this small scde work, &in. retorts 4.5 ft. in length were used. These retorts were supported vertically in the furnace and were heated to a temperature of approximately 1050' C. hy means of oil burners. By a proper arrangement of bat3es a t the bottom of the furnace the hot gases mere deflected in such a way as to result in a fairly uniform distribution of heat throughout the furnace. In order to study intelligently the progress of a cyanizing reaction, i t was first necessary to know to what extent i t was possible to obtain duplicate results in different experiments. A series of experiments was planned in which operations were practically identical in each case. The first fifteen experiments with the 4-in. retorts gave a variability of sodium cyanide formed in the briquets ranging from 10.1 to 32.7 per cent. The average for the fifteen experiments was 16.7 per cent. This represents about the limit of variability in practice when the system is not under rigid control. After many runs, during which all factors, such as temperature control, rate of gas flow, method of placing material in retorts, uniformity of briquets, method of sampling, etc., were brought under control, results could be more closely duplicated. As an example, a series of five successive rnns gave 29.40, 31.05, 29.05, 32.15, 28.60 per
Also this variability represents just about the limits obtained in our best large scale work, The composition of the briquots used in practically all the experimental work described below was approximately: 50 per Cent c, 38 per cent Na~C03,and 12 per cent Fe, which upon oxidation of the iron became 48 per cent C , 36.5 per cent NalC03, and 16.5 per cent FeJO,. A petroleum coke of the following composition was used in making the briquets: Carbon 96.95 per cent; moisture = 0.56 per cent: volatile matter = 1.01 per cent; ash = 1.48 per cent. The length of the entire heat zone of the retort was 35 in. After each run the briquets were carefully removed from the retort so as to give as nearly ss possible five equal portions from this heat zone (7 in. each), as indicated in Fig. 1. An analysis made on a uniform sample from each portion. I n carrying out the experiments the same volume of briquets was used in each run; the briquets were added until they extended from the lower limit of Zone 5 to the upper t i t of Zone 1. By adopting this method a slight differ' ence in weight a t times resulted-due to the slight differences in physical properties of the briquets and tho manner of packing in the retort. The average weight of briquets introduced was approximately 7 lbs. The variation from this weight was probably in no case greater than about 2 per cent. In carrying out a run the briquets were placed in the retort, the burners turned on, and the furnace gradually brought to a temperature of approximately 1060° C . within a 30-min. period. An attempt was made to hold the temperature of the retort at 1050" C., but owing to passage of nitrogen gas through the retort and to heat radiation from the furnace, there was a temperature gradation within the retort of from approximately 1050" C . in Zone 4 to approximately 1020" C. in Zone 1. As soon as the furnaces reached the desired temperature the time was taken as the beginning of a run. A definite though small volume of nitrogen, which was paased through a pyrogauate solution to remove oxygen, was forced upward through the retort from the beginning of the beating. At the beginning of a run, a definite, larger volume was passed.
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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEiW8TRY
Aug., 1922
PROGRESS OF REACTION I n this study 160 individual runs were made. These runs were planned so as to cover time intervals of 0.5 hr., 1 hr., 1.5 hrs., and 2 hrs. For the discussion of results there are chosen for comparison averages of three successive runs made near the end of this series after all the variable factors were fairly well under control. These runs all appeared to be typical for the time interval in question. The average composition of briquets used in these different runs varied only slightly, The exact composition at the beginning and a t a given stage in the reaction is shown in Tables I and 11. TABLEI-COMPOSITION OF BRIQUETS BEFORE AND O F BRIQUETS--COMPOSITION After 1 Hr. Per cent Before ZONEof Cyanizing 1 2 3 4 0.37 3.12 0.26 Fe 0.11 0.18 14.07 13.14 11.74 9.03 FesOc 15.34 22.08 38.70 32.22 42.05 NazCOa 37.43 46.3 49.7 55.00 47.12 46.00 C 3.63 6.55 1.82 2.94 NaCN 0.00
TREATMENT .Composition Composite of .Eachin Retort Material
5 9.09 10.85 12.40 53.00 14.24
2.98 12.6 28.6 49.2 6.65
TABLE11-COMPOSITION OF ZONE MATERIAL B Y WEIGHT (IN 02.) AFTER CYANIZING Zone Zone No. Weight1 Fe FesOi NazCOs C NaCN 1 2 3 4 5
16; 5 25.3 17.5 16.3 25.7
0.05 0.07 0.06 0.52 2.31
2.37 2.92 2.46 2.15 2.89
the retort very slowly until the briquets were cool, after which the contents of the different heat zones were analyzed. Analysis was made for sodium cyanide, sodium cyanate, sodium ferrocyanide, sodium carbonate, free iron, and iron oxide. Carbon was determined by difference. REACTIONS-The principal chemical changes in this process are due to the following reactions?
+ 4C + Nz + 48, Pea04 + 4C 232 + 48 weight changes
Na& O !s weight changes 106
AFTER
.------
6.95 9.5 5.64 3.71 3.2
Total weight in retort sfter cyan29.00 3.01 12.7 izing 101.3 Weight before 41.2 0.12 16.9 cvanizina 110.0 Change in weight +S.7 4-2.89 -4.11 -12.2 1 The rather large differences in zone weights are runs, to indccuracies in apportioning zone volumes as removed from the retort.
6.66 12.06 8.71 8.75 13.61
0.47 0.75 0.63 1.17 3.69
49.79
6.71
51.8 -2.01
0.00 +6.71
due, in this set of the materials were
I n this series of experiments heating was stopped at the end of the 1-hr. period. Nitrogen gas was passed through
701
=
2NaCN
=
+
3CO (1) 98 (+ 56 loss from
retort) = 3Fe 4CO (2) = 168 (+ 112 loss from retort)
+
In order to follow the changes which occurred within the retort during the reaction we will take the average of three typical runs in which a complete analysis of material present was made after a 1-hr. heating period. During a number of the runs, similar in all respects to those above described, three small (l/~-in.)iron tubes were inserted down through the center of the retort in such a manner that samples of gas could be removed slowly from the reaction zones a t different intervals during an experiment. The tubes extended down to the approximate middle of Zones 2, 3, and 4. The gases represented approximately the gaseous mixture coming from Zones 3,4, and 5, respectively. It was found that the gas composition varied somewhat during different stages of a run, also that the composition was quite different during similar stages of different runs. The carbon monoxide and carbon dioxide content of the gas did not check closely with known equilibrium values for these two gases at this temperature.? This work was not carried far enough to enable us to obtain data which could be regarded as giving absolutely characteristic equilibrium concentrations for the different stages of a run. An average of values obtained during one typical run (after 1 hr. of heating) gave the results recorded in Table 111, which were representative of those obtained in a fairly large number of runs. TABLE111-COMPOSITIONOP GASIN ZONBS3, 4, GAS TOP MIDDLE 27.1 41.6 N
co
71.3 1.6
COa
58.0
1.4
AND
5
BOTTOM 61.2 38.8 0.0
Suppose we assume that the reactions occurred as in Equations l and 2, we will take then, as a basis for calculation, the changes within the system which would be necessary to account for the appearance of the 6.71 08. of NaCN and the 2.89 oz. of Fe. REACTION 1 6 . 7 X 48/98 = 3.29 oz. carbon necessary 6 . 7 X 106/98 = 7.25 02. NazCOa necessary 6 . 7 X 56/98 = 3 . 8 3 02. total loss due t o reaction or 7.25 (NanC03 used) 3.29 (C used) 6.71 (NaCN formed) = 3.83 total loss due t o Reaction 1
-
+
REACTION 2 2.89 X 48/168 = 0.82 oz. carbon necessary 2.89 X 232/168 = 4.00 oz. Pea04 reduced 2.89X112/168=1.93 02. totalloss due to Reaction 2 or 4.00 (FesO4 reduced) 0.82 (Cused) -2.89 (Fe formed) = 1.93 total loss due to Reaction 2
+
Comparing now the changes calculated with those actually found we have the following: I t is probable that 6 Calculations are based upon iron oxide as FesO4. much of the iron was present as FezOa; this has, however, little bearing on our calculations, inasmuch as the conversion factors are so similar (FesO! 1.38 against FenOa 1.43). 7 Rhead and Wheeler, J . Chem. Soc., 99 (1911), 1140.
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THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
702
FOUND
CUANOE Narc08
CALCULATED
12.2
c
7.25 4.11
2.01 4.16
FesOc
4.00
DXFFERENCS 4 . 9 5 (loss) 2.10 0.16 (gain) (slight loss)
The results of these calculations show a loss of sodium carbonate, a gain of carbon, and practically no change in total iron. (The slight gain indicated is within the limit of experimental error in this determination.)
Vol. 14, No. 8
TABLEIV-RBLATION OF CYANIDE CONVERSION TO GASCOMPOSITION Conversion of Carbonate to CO in Cyanide1 Gas Mixture Per cent Per cent 11.1 71.5 26.0 55.0
58.0 8R R
* The amount of NazCOs and NaCN present at the end of an experiment was taken as the basis of calculation in the determination of per cent conversion of NazCOs to NaCN. Losses from the retort during the experiment were not included in this calculation. These results indicate that the equilibrium data obtained by Ferguson and Manning do apply directly to equilibrium conditions as they exist in a large system in which there are present all the materials which take part in the cyanide reaction. They bring out clearly the fact that producer gas when used at 1000' C. or less would be far less satisfactory than pure nitrogen in this process. These results show further that in B process using a continuous vertical retort, the greatest conversion of carbonate to cyanide must a t all times occur in that part of the heat zone in which nitrogen first enters, i. e., in that part in which the partial pressure of carbon monoxide is lowest.
I hr
0
TIME IN
ffbr
2 hr
H W S
FIG. 2
DISTRIBUTION OF KaCN
AND
Na2C03
A study of all data shows that at the end of the first halfhour period by far the greatest amount of cyanide (see curve, Fig. 2) had been formed in the bottom of Zone 5 and a progressively decreasing amount in each of the other zones. A t the end of the 1-hr. period the same order maintained. At t h e end of the 1.5-hr. period Zone 4 had the greatest amount of cyanide, Zone 5 next, then progressively decreasing amounts in Zones 3, 2, and 1, respectively. At the end of the 2-hr. period the order of per cent cyanide in the different zones was 4> 3> 5> 2> 1. From the data it is clearly seen that cyanide formed in the lower zones was distilled upward as the reaction progressed. The total amount of cyanide in the retort at the end of the 2-hr. period was very nearly the same as a t the end Qf the 1.5-hr. period. During this last time interval the rate of distillation from the retort was practically the same as the rate of formation within the reaction zone of the retort. Although considerable cyanide was distilled up through the reaction zone, the percentage of cyanide in Zones 1 and 2, the two top zones, was a t all times low. This was undoubtedly due to the fairly high concentration of CO gas which caused a reversal of the main reaction Na2CO3 4C Ne = 2NaCN 3C0, which resulted a t all times in a low concentration of sodium cyanide in the upper zones. The work of Ferguson and Manning8 had shown that the presence of carbon monoxide gas has a marked influence on this equilibrium. In their work different amounts of carbon monoxide gas were added to nitrogen, then brought in contact with sodium carbonate, or with sodium cyanide, and the final equilibrium in each case noted. In Fig. 4,Curves 1and 2 show the conversion of cyanide obtained by Ferguson and Manning a t temperatures of 1000' and 946' C. Curve 3 is the calculated conversion of sodium carbonate to sodium cyanide, obtained a t the end of the 1-hr. period as described above, plotted against the average per cent carbon monoxide found in the gas. It should be pointed out that while Curve 3 does not represent true equilibrium conditions, it does, on the other hand, give a good indication of the extent of the reversibility of the main reaction in practice. JOURNAL,
+
DISTRIBUTION OF IRON AND IRON OXIDE The mixture before briquetting was approximately 50 per cent C, 38 per cent NazCOa, 12 per cent Fe. After addition of water, briquetting, and drying, it was found that practically all the iron had been oxidized and that the composition of the briquets was approximately 48 per cent C, 36.5 per cent Na2C02, 16.5 per cent Fe304. During the cyanizing process some of the iron oxide was reduced. This reduction was fairly active in the lower heat zones, but in the upper heat zones, 1 and 2, the extent of reduction was exceedingly low, even a t the end of the 2-hr. period.
+ +
+
THIS
DISTRIBUTION OF CARBON During an experiment the concentration of carbon increased in the lower zones. This was due largely to the fact that sodium cyanide (and possibly some sodium carbonate per se) had distilled upward from these zones. The total carbon which disappeared was less than the carbon required for the reactions which have taken place within the retort. This apparent gain in carbon was undoubtedly due to the reaction 2CO = COZ C. The equilibrium conditions of carbon monoxide would demand some carbon increase; further, the analysis of the gas from the reaction zone showed the presence of carbon dioxide.
11 (1919), 946.
+
I
1s
2
TIM€ N MWRJ
FIG. 3
The equilibrium responsible for the low reduction of iron oxide in the upper heat zones has not as yet been definitely determined. It is now under investigation in this laboratory.
APPLICATION OF RESULTS The three main points, brought out in this small scale
T H E JOURNAL O F INDUSTRIAL AND ENGINEERING CHEMISTRY
Aug., 1922
work, which, it seemed, would find direct application in our technical work, were: 1-The partial pressure of carbon monoxide must be kept as low as possible in order to prevent reversibility of the main reaction. This might be accomplished by keeping the rate of nitrogen flow high enough to overcome (by out-speeding) the effect of the main reaction. 2-I,oss of sodium cyanide by volatilization from the lower heat zones should be avoided. This could be accomplished by regulating the length of the retort, the rate of heat conduction through the briquets, and the discharge rate in such a manner as to get a maximum of fixation with a minimum of volatilization. This condition would correspond, in our 4411. batch retort, to a discharge a t the end of the 1.5-hr. period. 3-Temperatures a t least as high as 1050' C. should be employed. This temperature is somewhat higher than has previously been used.
SHIFT No. 66 66 67 68
708
Pounds of Briquets Cvanized
Pounds of Briquets Cyanized Der Hr.
Per cent NaCN in Briouets
Pounds NaCN pea Retort Der Hr.
181.6 199.2 214.4 265.6
22.7 24.9 26.8 33.2
35.04 36.08 32.14 31.66
7.95 8.99 8.63 10.47
During another shift of 6 hrs. (No. 71), later in this run, 353.5 Ibs. of briquets having a sodium cyanide content of 20 per cent were discharged a t the rate of 58.9 Ibs. per hr. This gave a production of 12.19 lbs. sodium cyanide per retort per hr. The run was terminated a t this point owing to mechanical difficulties in the plant.
LARGESCALEWORK Two 8-in. cyanizing retorts 10 ft. in length, of a type similar to those installed in the Saltville plant, were set up and put into operation. A mixer, briquetter, and dryer of sufficient capacity to care for the two cyanizers were also installed. We a t first experienced difficulty in proeuring, economically, nitrogen for this work. A simple but surprisingly efficient method was finally worked out whereby nitrogen was obtained by passing air over heated coke. This was a type of low temperature combustion, giving the reaction C 0 N (air) --tCOz N. A retort was constructed of such size (8 in. x 6 ft.) that when air a t such a rate as would furnish the required amount of nitrogen (5 to 6 cu. ft. per min.) was passed through the coke, the heat of reaction was just enough to cause the reaction to be self-sustaining (temperature approximately 478" C.). This retort was mounted vertically within a furnace which could be heated by means of an oil burner. The burner was used in starting up the reaction and in bringing up the temperature in case any drop of temperature occurred within the retort. The carbon monoxide formed was removed by a circulating caustic scrubber, and nitrogen of better than 99 per cent purity was obtained (the presence of more than 1 per cent oxygen decreases the nitrogen fixation markedly). This process was run for days a t a time and caused but little trouble. When a cyanizing run was started it was carried on continuously in 8-hr. shifts. The calculated cyanide yield, etc., was based on the average for each 8-hr. period. The first attempt to cyanize continuously was made on May 27, 1918. Owing to the inexperience of the men and difficulties with the apparatus, the highest fixation recorded during this run was 17.89 per cent sodium cyanide in briquets. On July 16 another run was started. This run continued for about 12 days, during which time it was possible thoroughly to acquaint the men with their various duties and to correlate chemical and mechanical control. During one shift cyanide in briquets reached 23.17 per cent. The average for the entire period was about 15 per cent. Following this a number of shorter rbns of 3 to 4 days each were made. Over one 40-hr. period of five shifts, briquets with an average of 25 per cent sodium cyanide content were obtained. This gave a cyanide output between 7 and 8 lbs. per retort per hr. This seemed a normal output to expect, inasmuch as we were able to reach and hold this figure. Later the principles stated under small scale work were more exactingly applied, and every care was exercised to regulate mechanical control so as to conform to rate of chemical reaction. In the last run made, the following results were obtained:
+
+ +
I
I
(0
u-,-.-_.'.
I
\\
\\
s
\-..
\
Based on 7 Lhs. NaCN per Based on 8 Lbs. NaCN per Hr 20 Per cent NaCN Hr 23 Per cent NaCN 85 Pyr cent NHs. Plant tb 95 i;er cent "8. Plant io Cost $2,700,000 cost $2,100,000 3-yr. 5-yr. 15 Per 3-yr. 5-yr. 1.5 Per Amort. Amort. cent Amort. Amort. cent 0.0324 0.0284
.... .............. ........... ............. ............ .............
........
....
........
.... ....
Coke reactions Coke loss and dis0.0022 0,0019 card1 NazCOs loss and disca;dl 0.0036 0.003 Fe loss and dis_.~ card'. 0.0018 0.0016 Nitrogen 0.004 0.003 Steam.. 0.0014 0.0012 Replacement of retorts and equipment, cyanizing. 0.0663 0.0526 Replacement costs, steaming 0.0105 . . . . . . . . 0.0054 Fuel 0.0166 0.0181 Power.. 0.0123 0.0107 Labor.. 0.0644 0.0613 Total. excludinz _. fixeh charges;:: 0.2204 0.1876 Fixed charges $0.25 0.15 0:iiiS O l i 4 4 5 0.1166 O:bSi6 Total per Ib. "8, including fixed 0.47 0.37 0.33 0.38 0.30 0.275 charges.. 1 Discard due t o accumulation of ash during a continuous cyanizingsteaming-cyanizing process. ~
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......
....
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... ... ... ... ... ... ... ....
....
.... .... ....
.... .... .... .... ....
