Removal and Recovery of Arsenious Oxide from Flue Gases Huibert J. Wouterlood" and Keith McG. Bowling CSlRO Division of Process Technology, P.O. Box 136, North Ryde, New South Wales, 21 13, Australia
Removal of arsenious oxide vapor from hot flue gases has been studied on the laboratory scale. Inert materials such as glass beads, glass wool, and fly ash were found to be virtually ineffective; basic and acidic chemicals worked to a limited extent. Surface-active agents such as activated carbons, silica gel, and molecular sieve 13X absorbed As406vapor strongly. The amount of As406 absorbed a t saturation point ranged from 25 to 45% of the weight of the carbon. In particular, a relatively cheap activated carbon repeatedly absorbed As406 vapor from a simulated flue gas mixture at 200 "C and released it again upon heating to 400 "C in a nonoxidizing gas. These properties did not diminish significantly with the number of absorptionlregeneration cycles. (Up to 14 cycles were tested.) Carbon burnoff and pressure drop over the absorbent bed were also assesseld. When minerals are processed, the disposal of gangue and other extraneous material must take account of a variety of local processing and environmental factors. For instance, if the processing is performed a t high temperature (e.g., smelting), the waste gases may contain significant quantities of volatilized and particulate impurities that must be removed before reaching the chimney. Of special interest is the situation where particulate matter is separated a t an elevated temperature a t which other impurities are appreciably volatile. An example may be found in the smelting of an arseniccontaining sulfide ore, where an electrostatic precipitator traps dust from the flue gas stream at about 200 "C, but allows some adventitious arsenious oxide to $ass, owing to the significant vapor pressure of that substance. This paper reports experiments on ways in which arsenious oxide vapor, already a t low concentration, may be removed from a gas stream. A factor of paramount importance in the study and eventual control of arsenic emissions is the vapor pressure of As406,the species in which arsenious oxide occurs in the vapor state up to 800 "C ( I , 2). 'The saturated vapor pressure as a function of temperature h,as been measured by a number of investigators. Although earlier data show considerable variation, those presented by Behrens and Rosenblatt ( 3 ) agree closely with data given by Murray e t al. ( 4 ) . The first set of data is given by the equation: 6067 f 125 log p = 12.7858 f 0.319 - ~T where p = vapor pressure in torr (1 Torr = 133.3224 Pa) and T = temperature in kelvin. Likewise, for the second set: 6077 f 181 T Calculations based upon these equations show that a t 200 "C, the approximate temperature of a typical exhaust stack gas, the saturation vapor pressure of As406 is about 0.94 Torr, a t which its concentration in the gas would be 21.85 glNm3 (Nm3 = m3 a t standard temperature and pressure). Flue gas from a typical commercial smelter contains much less than this (=0.15 glNrn", if the smelter is fed with certain arsenical ores) but air pollution legislation commonly requires even lower concentrations (e.g., 0.02 glNm3). Smelter flue gas containing 0.15 glNm3 would have to be cooled to approximately 130 "C before it would be saturated with As406vapor and condensation could begin. Cooling would have to continue log p = 12.8248 f 0.048 -
0013-936X/79/0913-0093$01.00/0
@ 1979 American Chemical Society
to about 110 "C to reach a saturation vapor pressure corresponding to 20 mglNm3. Although in principle it would be possible to remove arsenious oxide by a condensation procedure, the reduction in temperature would seriously diminish the buoyancy of the plume from the stack. It would then be necessary to reheat the gas before discharge, which would be uneconomical. Inability to restrict arsenic emissions to acceptable levels can preclude the smelting of certain otherwise attractive ores. Attention was focused on alternative chemical and physical methods which would not require lowering of the flue gas temperature below 200 "C. Particular attention was given to methods which might be applied commercially. Since As406is amphoteric, chemical reactions with either basic or acidic reagents seemed feasible. However, laboratory tests confirmed that basic reagents react with sulfur dioxide in the gas and would make reagent consumption in a commercial smelter prohibitively high. Regeneration of reagents would be important, unless they are cheap enough to be discarded after a single application. Physical methods seemed attractive because arsenious oxide is a known catalyst poison which implies that it is strongly and selectively adsorbed on particular surfaces. The possibility arose, therefore, of selectively removing low concentrations of As406 from the vapor phase a t 200 "C in the presence of much greater concentrations of other gases, including sulfur dioxide. The solid materials used in the present investigation are loosely termed "absorbents". For the activated carbons absorption involves the crystallization of arsenious oxide in the pores, as observed microscopically. Two ways of employing these absorbents were contemplated: adding the absorbent as a fine powder to the gas stream, allowing it to absorb the vapors as it passes along the duct in suspension, and then removing the arsenic-laden dust by cyclones, electrostatic precipitators, or a combination of these; or, alternatively, passing the gas stream through a bed of absorbent particles. The second method was chosen for experimental convenience and because it would also permit the study of those gas-solid interactions which are of a chemical rather than an absorptive nature.
Experimental Apparatus. The laboratory-scale apparatus shown schematically in Figure 1 was used to carry out experiments on the absorption of As406 vapor from streams of air, nitrogen, or simulated smelter gases. The main part consisted of a horizontal glass reactor tube (33 mm i.d.) divided by a sintered glass disk. In one compartment, the contact chamber, the absorbent under investigation, in a granular form, was held in position by a small plug of glass wool. A glass boat containing As406 could be pushed into the second, or evaporation, chamber from a charging chamber to allow the arsenious oxide to vaporize. A slow stream of gas was passed over the boat, entraining the vapor. The arsenic-laden gas stream then entered the bed of absorbent through the porous glass disk. The glass reactor tube was located inside an electrically heated oven fitted with a temperature controller activated by a thermocouple. Except in the earliest experiments an additional, separately energized heating element was wrapped around the evaporator to increase evaporation as desired. From the contact chamber the gas stream passed into a Volume 13, Number 1, January 1979 93
CASISOLID
CONTROL
CONTACT CHAMBER
THERMOCOUPLE
GAS I N
CAS OUT IT0 WASHING TRAPS 1
COOLING WATER
11 '
CONDENSER
COOL1 NG CHAMBER
c I
~
OVEN
+% '
,
I
1
1
CHAMBER
:RLOCOUPLE
1
SINTERED GLASS D I S C
#
1 1
ABSORBENT
GLASS WOOL PLUG
1
330 rnm
~
1
-
~ 535_ mm1 - _
k------
725 mm
,
1 I
EVAPORATION CHAMBER
,
I
Figure 1. Schematic diagram of apparatus used for laboratory absorption experiments
cooling chamber fitted with a water-cooled cold-finger condenser. As a safety measure and to completely recover any As406 not trapped by the condenser, the gas was passed through two washbottles containing 2 N KOH solution. The apparatus for measuring the pressure drop across a packed bed of carbon consisted of a transparent plastic tube, 58 m m i.d., provided with a distributor plate. The pressure drop across the bed was obtained from two pressure taps, one above the bed and one below the distributor plate. Blank corrections were made by determining the pressure drop across the distributor plate over the same range of gas velocity, but without a bed in the tube. The bed depth was 305 mm. Carbon A (particle size 2-6 mm, as used in the absorption experiments) was used in these tests. The air flow rate was measured by a suitably calibrated rotameter. Materials. The arsenious oxide was of analytical reagent quality. The carrier gas used in most of the tests comprised 5% Son, 5%02,~ WC02, O and 80%N2,which simulated typical smelter stack gas. It was dust-free. Regeneration of absorbents was carried out with high-purity nitrogen. Four types of solid material were investigated: inert substances such as glass beads, glass wool, and pelletized fly ash (in some experiments a partial coating of As406was applied to glass beads to provide nucleation centers for the growth of further As406deposits); active substances such as activated carbons, silica gel, and molecular sieves; alkaline compounds, comprising analytical reagent grades of calcium carbonate, calcium hydroxide, and sodium carbonate; one acidic compound, viz. potassium bisulfate, applied as a coating on glass beads (this compound (mp 210 "C) was liquid a t the gas temperatures used). Procedure. A layer of the absorbent under study, 3 or 6 cm thick depending on the particular test, was placed in the contact chamber and the apparatus was assembled as shown. The tube-oven and auxiliary heater were then turned on, and an air flow of 0.5 L/min was maintained during the heating-up period in order to dry out the bed of absorbent. After temperature equilibrium was reached (with the contact chamber at -200 "C) and when no more water was evolved (after about 2 h) the air was replaced by the simulated flue gas mixture flowing at a predetermined rate (usually 0.25 L/min). Knowing the volume of the absorption zone, the superficial gas residence time could then be calculated. T o start a n experiment the glass boat containing a weighed quantity of As406(about 1 g) was pushed into the evaporation chamber. The test was 94
Environmental Science & Technology
usually terminated after 1 h and the reactor tube withdrawn from the oven for observation while still hot. This caused the evaporation chamber to cool rapidly and the evolution of As406vapor ceased. If any arsenic had passed beyond the bed material, as indicated by a very thin, but readily visible, white deposit on the cold-finger condenser and/or crystals on the walls of the contact chamber beyond the bed, the downstream walls of this chamber were washed first with 2 N KOH solution and then with water. The absorbent was removed and divided into a front and a rear portion, as seen from the direction of gas flow. The portions were weighed and analyzed separately. The boat containing residual arsenious oxide was removed from the evaporation chamber and weighed, the amount of As406 evaporated being obtained by difference. The carbon absorbents were regenerated by passing a stream of nitrogen (flow rate 0.25 L/min) at 400 "C through the apparatus until As406 was no longer deposited on the downstream surfaces. This usually took 24 h. In one instance, regeneration was effected by extracting the absorbed arsenic with hot 2 N KOH solution for 15 min. Absorbents other than carbon were not regenerated. The influence of moisture in the gas upon the arsenic removal process was investigated in two experiments with carbon A by using dry gas in one case and, in the other, by saturating the gas mixture with water a t room temperature before introducing it into the apparatus. Analytical Methods. Carbon samples and deposits of As406 on parts of the apparatus were extracted with 2 N NaOH. Qualitative tests were carried out either by a modified Gutzeit method, or by means of the precipitation of Ag3As0.7. In the modified Gutzeit method, reduction, instead of being carried out in an acid medium (with Zn and H z S O ~ )was , done directly in a n alkaline medium by adding a few grains of aluminum to the test tube containing the solution. The test tube was stoppered by a rubber bung carrying a capillary glass tube containing a few crystals of AgN03. The presence of AsH3 was demonstrated by the yellow, then black discoloration of the crystals. It was shown that SO2 in the gas mixture did not interfere with this test. In the other test, the alkaline solution was acidified with 2 N H2S04(with litmus paper as indicator) and boiled to remove SO*. After cooling, a few drops of 5% AgNO3 solution were added. NH40H (2 N)was then added dropwise until the
Table 1. Results of One-Hour Absorption Tests (Bed Thickness, 6 cm; Gas Residence Time, 12 s; Gas Flow Rate, 0.25 L/min) wt AS406
Of
absorbent
absorbent, g
pelletized fly ash CaC03 Ca(W2 Na2C03 KHS04 active carbon A active carbon E active carbon F brown-coal char silica gel silica gelC mol. sieve 13X a
evap., 9
0.2270 0.2920 0.2707 0.1112 0.2235 0.1522 0.1597 0.6242 0.3182 0.2761 0.8371 0.1742
22.0996 35.2872 19.2931 22.6269 9.8184 20.0863 18.9870 18.2650 33.5655 28.8610 31.4915 34.0643
Equals (weight clf As406 absorbed/weight of carbon) X 100.
% of A s 4 0 6 evap. trapped in contact chamber front rear
26.67 2.74 51.80 50.10 56.94 53.68 100.69 95.43 12.15 64.10 92.73 87.12
12.15 2.73 36.30 33.02 31.97 43.04 Ob Ob
12.26 1.18 10.66 O b
total total uptake, a
AS406
removed, %
breakthrough
Yo
38.83 5.47 88.10 83.12 88.92 96.72 100.69 95.43 24.41 65.27 103.38 87.12
0.40 0.05 1.24 0.41 2.02 0.73 0.85 3.26 0.23 0.62 2.75 0.45
yes Yes no yes slight no no no yes no slight no
Not analyzed; qualitative A s test was negative. Duration of test = 3 h
Table II. Performance of Several Activated Carbons in Prolonged Laboratory Trials (Carbon Bed Thickness, 3 cm; Gas Residence Time, 6 s; Gas Flow Rate, 0.25 L/min) imported carbons, high absorptive act., higher priced
carbon from Victorian brown coal, medium absorptive act., modest price
carbon identification surface area, m 2 / g breakthrough time approx, h A s 4 0 6 evap., g evap. rate, mg/min As406 concn in gas, mg/L wt of carbon, g As406 uptake, a YO gas mixture a
A 900 13 2.8707 3.66 14.64 1 1.0793 25.9 dry
1000 12 3.5460 4.93 19.72 11.5199 30.8
C 1100 17.5 4.5167 4.28 17.12 10.9012 41.4
D 1200 15.75 4.7824 5.06 20.24 10.5221 45.5
dry
dry
dry
0
A 900 10.75 2.6459 4.10 16.40 10.7801 24.5 wet
Weight of A s 4 0 6 absorbed at breakthrough, expressed as percentage of the weight of carbon
yellow color (or precipitate) was formed. The precipitate is soluble in excess ammonia. No further tests have been carried out to assess the relative merits of the two methods. Quantitative (determinationswere carried out as follows. A sample of 0.15-1 g was twice extracted for 20 min with 20 mL of boiling 2 N NaOH. The extract was neutralized by adding 9 mL of concentrated HCl followed by 1-2 g of NaHCO3. Under these conditions, As(II1) was titrated with 0.1 N iodine solution with starch as indicator. For the determination of total arsenic, 2 mL of 3 M K I solution and 100-150 mL of concentrated HCl were then added to reduce As(1V) with I- in approximately 4 N acid and excess iodide. After cooling for 5 min in a water bath, the liberated iodine was titrated with 0.05 N thiosulfate solution.
Results Table I shows the results of 1-h absorption tests. These indicated that inert and chemically reactive absorbents were less efficacious than active types. After encouraging results had been obtained with several activated carbons, tests of 12-18 h duration were conducted on four commercial varieties to determine both their absorptive capacity for arsenious oxide, and the time that elapsed before As406 broke through the bed. Alternative tests with carbon A were conducted with dry and moist gal;. The results are given in Table 11. The As4O6 uptake is the weight of it collected by the carbon, expressed as a percentage of the weight of carbon used. Further experiments were carried out to investigate the
Table 111. Influence of Linear Velocity on Arsenic Removal from Dry Gas Stream carbon identification bed thickness, c m flow rate, L/min residence time, s linear velocity, cm/s breakthrough time, approx., h As406 evap., g evap. rate, mg/min As406 concn in gas, mg/L wt of carbon, g A s 4 0 6 uptake, %
A 3 0.125 12 0.25 9.5 3.1277 5.49 43.92 10.0091 31.3
A 6 0.250 12 0.5 13.5 5.8925 7.23 28.92 19.8116 29.7
A 12 0.500 12 1.o 22.5 13.1635 9.75 19.50 39.041 1 33.7
effect of gas velocity on the uptake of As406 a t a constant residence time of 12 s. This was achieved by varying the bed thickness and flow rate. The results are given in Table 111. The regeneration of an activated carbon was investigated by loading a sample of carbon D with As406 until breakthrough occurred and then regenerating it at 400 "C as described above. At the end of the loading as well as the regenerating operation, a 1-g sample of absorbent was taken for chemical analysis. This loading/regeneration cycle was repeated once more on the residual carbon. The results are given in Table IV. The experiment was repeated with a sample of carbon A Volume 13, Number 1, January 1979
95
kPa
I
cm H,O
m
Y
4--40
Y LL
E"
3--9
B Ern
2
-- 20
z3
1
-- 10
LL
FLUIDlZATlOlu
ONSET OF BED LIFT
m
Y b@
OF
CYCLES
00
Figure 2. Effect of successive cycles of loading and regeneration of carbon A (medium activity) on its ability to absorb As406 (A) Amount of As406evaporated and taken as absorbed; (E) amount of nonarsenious material (SO2,C02, etc.) adsorbed (total weight increase less curve A); (C) amount of material retained after regeneration(weight of sample less initial weight of carbon)
0 2
I
I
1
L
0.4
06
08
IO
SUPERFICIAL A I R VELOCITY AT ROOM TEMPERATURE
1
mrs I
Figure 3. Pressure drop through absorbent bed of carbon A as a function of superficial air velocity at room temperature
Table V. Carbon Burnoff (Carbon A) a
Table IV. Regeneration of a Highly Active Carbon (Carbon D) a
wt increase after heatlng In oxldlzlng gas mixture,
As406 content of absorbent (chem anal.),
run
Oh
materials retained after further heating In N2, %
1 2
2.35 2.01
1.34 0.83
%
after after after after a
1st loading until breakthrough 1st regeneration 2nd loading until breakthrough 2nd regeneration
20.44 4.57 23.77 4.70
See Table II. Temperature of absorption, 200 O C ; temperature of desorption,
400 O C .
(medium activity, modest cost). After each loading or regeneration procedure the carbon was weighed but samples for chemical analysis were not taken, as this would have rapidly depleted the supply of absorbent. The results are plotted in Figure 2. I t was further found that 96% of the absorbed As406could be removed from activated carbon by hot alkali extraction. The regenerated carbon was not further tested for absorptive capacity, because this was not considered a promising method for regenerating carbon economically or conveniently on any future commercial scale. As the use of purified flue gas was visualized for regeneration in the full-scale plant, the possibility of carbon burnoff in this gas a t 400 "C was investigated. Two runs were carried out in which a sample of carbon A was first dried in nitrogen a t 400 "C until no more moisture was evolved (-1.5 h). The sample was cooled and weighed to determine the loss of moisture. The same simulated flue gas mixture as was used in the arsenic absorption experiments was then admitted for 1.5 h (at 400 "C) and the sample was weighed again. A weight increase was found rather than a weight loss from burnoff. Thereafter, the carbon was further heated in a nitrogen stream for 3.5 h a t the same temperature and weighed again. The results are given in Table V. The behavior of a packed bed of carbon A was investigated by measuring the pressure drop as a function of gas flow rate. The results are given in Figure 3, which defines the conditions under which packed and fluidized beds can be operated.
Discussion Arsenic mass balances were incomplete in most of the runs shown in Table I. When breakthrough occurred, the arsenic caught in the condenser and wet traps could not be recovered. 96
Environmental Science & Technology
a
See Table II
Where breakthrough was minimal, the missing arsenic was probably caught in the glass wool plug, but the plugs were not analyzed for arsenic. A fresh glass wool plug was used for each test. The absorption tests using inert materials showed that some condensation occurred on their surfaces even though the carrier gas was not saturated with As406.This indicated that the surfaces were not acting simply as collectors for condensation, but that chemisorption or chemical reaction was occurring. The effectiveness of these surfaces in promoting condensation decreased in the order: glass wool or beads coated with As4O6;uncoated glass wool or beads; fly ash derived from coal combustion and pelletized with either cement or sodium silicate. Since the fly ash could not be regenerated even when heated in an air stream for 16 h at 250 "C, it seemed likely that the natural or added alkaline constituents had chemically bound the arsenic. Of the chemicals tested for their ability to trap As406 (Table I), calcium carbonate showed a low efficiency and poor absorptive capacity. Moreover, the same amounts of arsenic were found in the two halves of the contact bed, indicating that the whole charge was saturated with As406before the end of the 1-h test. In contrast, calcium hydroxide, sodium carbonate, and potassium bisulfate each trapped 80-90% of the arsenic to which they were exposed. In all three cases more arsenic was collected in the front half of the contact bed than in the rear. This indicated a greater strength of bonding between As406 molecules and these reagents than was the case with the more inert surfaces. The sample of brown-coal char (surface area 650 m2/g) included in the present tests was relatively ineffective as an absorbent for As406. As it had not been subjected to any special activation treatment, brown-coal char should not be dismissed from further studies. All the surface-active absorbents tested (activated carbon, silica gel, and a molecular sieve) showed a strong affinity for As406 and were very effective in trapping it, predominantly in the first part of the bed (see Table I). Despite the fact that the same conditions applied in each
run, Table I shows that the amount of As406 evaporated varied considerably. This may have been due to irregular sintering of the As406 in the boat. As shown in Table 11, all four commercial, activated carbons were found to have trapped substantial amounts of A s 4 0 6 before breakthrough occurred. The amount of As406 absorbed a t saturation point ranged from 24.54 to 45.45%.Absorption increased with surface area of the carbons, although not in a linear fashion; the effect was probably due to differences in pore-size distribution between samples (i.e., accessibility of internal surface area to the gas). With a wet gas mixture the absorptive capacity of the carbon seemed to be somewhat less than with dry gas, as is evident by comparing the first and second columns of Table 11. The results in Table I11 show that over the range 0.25-1 cm/s, changes in linear velocity a t constant residence time do not discernibly affect the uptake of A s ~ O A ~ .comparison of column 2 of this table with the first column of Table 11, however, shows that for the same linear velocity but different gas residence times, the uptake is less for the shorter residence time. This is to be expected even over a much wider range of velocities because, a t higher velocities, gas turbulence would be increased, which would improve contact between the gas and the external carbon surface. Presumably, mass transfer by diffusion to the large absorption surfaces within the pellet would not be affected. Table IV shows that As406 absorbed on commercial, activated carbon can be efficiently desorbed by heating the absorbent in a nonoxidizing gas to a temperature higher than that used during ,absorption.A slow stream of inert gas passing over the carbon during thermal desorption helps carry the evolved As406 t o a condensation chamber. Too much carrier gas is undesirable because it would interfere with the complete condensation of As4O6 vapor. In industrial application a dust-free flue gas would make a good carrier. On a sample of carbon A the loading and regenerating were carried out repeatedly. I t can be seen from Figure 2 that the amounts of As406 absorbed (curve A) vary to some degree, but do not diminish significantly with time. This variation in performance between successive cycles could be due to differences in carbon packing and hence in the flow paths available to the arsenic-laden gas. Figure 2 also shows the difference between the increase in weight of the sample and the amount of As406 entering from the evaporator (curve B). This must have been caused by the simultaneous adsorption of SOz and COz present in the gas mixture. After an initial rise the amount of adsorbate decreased slowly. The third curve (C) shows the material remaining after regeneration. (Even after 24 h some As406 was still being releasled.) After the 14th regeneration the weight of the carbon plus retained material had become less than the dry weight of the carbon a t the beginning of the test, owing to the cumulative effect of losses due to repeated handling of the sample for weighing. The run was therefore terminated. In the two burnoff tests. carbon showed an increase in weight rather than a loss when heated in the sulfur-bearing, oxidizing gas used as a carrier in the arsenic absorption experiments. This showed that significant absorption of substances other thlan arsenic compounds occurred. Further heating in a nitrogen stream a t the same temperature did not remove the absorbed as the Table V show. If any carbon loss by combustion did occur, it was masked by the-uptake of the-nonarsenious compounds from the gas. The results of the pressure drop tests ( ~3) show i that ~ the bed began to lift a t an air velocity of about 45 cm/s, incipient fluidization occurring a t about 73 cm/s. I t is unusual
that the slope of the line before bed lift is less than that between bed lift and fluidization, but no explanation can be offered at this stage. The results of this study indicated that physical absorption of arsenious oxide by carbon offered the most promising basis for removing this compound from flue gases on an industrial scale. Accordingly, a pilot-scale test rig was designed and constructed, and is currently being operated on an arsenicbearing gaseous effluent generated in an industrial plant. The design of this rig and provisional projections to an even larger scale have been based on the following assumptions. (a) Intimate contact between the gas stream and the carbon absorbent in deep beds would be a t least as effective as in the thin layers used in the laboratory experiments, provided that the same gas residence time is allowed. (b) Higher gas velocities through deep beds would not significantly affect the rate and completeness of arsenic removal prior to breakthrough. (c) The gas velocity should be kept low enough to prevent fluidization and elutriation of fines. (d) The ratio of bed depth to bed cross-section should be minimized, within reasonable limits fixed by constructional requirements, in order to minimize the pressure drop and hence the power requirements for gas circulation. Preliminary test-rig results confirm that suitable grades of activated carbon are satisfactory reagents for removing As406 from flue gases and for enabling the arsenic to be recovered as a byproduct. Further results will be reported later. Conclusions Surface-active materials, particularly activated carbons, showed a strong affinity for As406 and were very effective in trapping it from a gas stream a t temperatures of about 200 "C. * The amount of As4(& absorbed at saturation point ranged from approximately 25 to 45%of the weight of the carbon. Absorption increased with increasing total surface area of the carbon, but not proportionally. The distribution of pore sizes evidently played some part in determining the amount of arsenic absorbed. As406 could be efficiently desorbed by heating the absorbent to a higher temperature in a nonoxidizing gas. Absorption and desorption could be repeated many times without any significant decrease in the absorptive capacity of the carbon. Although regenerating the carbon by means of potassium hydroxide solution was feasible, thermal desorption appears to be the more practical method. It does not require chemicals, it can easily be carried out without disturbing the bed of absorbent (unlike leaching, washing, or drying operations), and it yields As406 as a solid byproduct instead of producing a liquid effluent. The work described formed part of the program of the Division of Process Technology, a unit of the Minerals Research Laboratories, CSIRO. Acknowledgmmt The authors wish to thank their colleagues, Mr. R. J. Cosstick and staff, for analytical services. Literature Cited (1) Rllt7, H , Z Phqs C'hem ( L e i p i g ) , 19,417-24 (1896) ( 2 ) Maxwell, 1, R , Hendrlcks, S B , Demlng, L. S , J Chpm p h y s , 5.626-37 (1937) (3; 'Behrens, R.G., Rosenblatt, G. M., J . Chem. Therrnodynarn., 4, 175-90 (1972). (4)~Murray, R. F., PUPP,C., Can. J . Chern., 52,557-63 ~ J. J., Pottie, ~ (1974).
Received for rpuieu August 1 , 1977. Accepted August 23, 1978.
Volume 13, Number 1, January 1979
97
