Recovery of Cyanide from Waste Cyanide Solutions by Ion Exchange

I

ERIC

GOLDBLATT

Stilfontein

Gold Mining Co., Transvaal, Union of South Africa

Recovery of Cyanide from Waste Cyanide Solutions by Ion Exchange Pilot Plant Studies This system achieves the ultimate in efficient, economical plant operation. Permanently fouled ion exch.ange resin, which would otherwise be discarded from the uranium plant, is used to recover cyanide, water, and complexed base metals from the effluent of the gold works. Furthermore, this same effluent can substitute for caustic soda in removing temporary poisons from the uranium plant and thus reduce operational costs

L

i B s R . k T O R y \\-or& a t Stilfontein ( t i ) has shoiim the feasibilit!, of appl!,ing ion exchange for the recovery of c)-anide from Lvaste cyanide effluents. particularly those of the gold mining industry. 'I'he results icere sufficiently encouraging to warrant further investigation on a pilot plant scale. Ho\\-evrr. until rrcently certain aspects of the investigation could not be disclosed for reasons of security under the South .4frican .%turric Energy .Ict. In 1954- the Stilfontein Gold hIining C o . became a joint uranium producer \ \ ith several other smaller irines \\hic!i pumped their ore pulp 10 the Stilfontein uranium plant for treatmcnt ( I ) . \Vith the pulp there !$.as a considerable amount of solution containing cyanide \\-hich had to be combincd \vith Stilfontein's cyanide waste mlurion. complesed \vi:h iron(I1) sulfate, and disposed of along \vith the pulp residue. Not only \\-as there a loss of potentially recoverable cyanide lvith the additional csprnse of its neutralization, hut also a loss of 500.000 gallons of water daily. I t !vas the unsatisfactory economics of such a combined loss that initiated the original investigation into cyanide recovery by thr use of ion exchange resins. T h e reco\w)- and concentration of uranium at Stilfontein requires the use of a strong base ion exchange resin ( R o h m & Haas Amherlite IRA-400). \Vhile resin is in the uranium plant. its capacity for uranium is considerably reduced by the presence of certain compounds in the leach liquor from which the uranium is extracted rvhich are poisonous to strong base anion exchange resins. Some of these poisons are temporary, such as silica and the polythionates which can he removed by a 5Fc caustic soda regenerant.

Adsorption Circuit----

Elution Circuit H2S04

Fresh Acid

Waste Cyanide Solution I

I I I

I

Cation column IR.120

+ I

I

Return to Gold Plant

Lime Slurry Concentrated Cyanide Waste Metal/NaC1 Solution Figure 1. Free cyanide from the primary column effluent i s cornplexed with deliberately precipitated copper(1) cyanide in the conditioned columns (SIand S2), and effluent from them can then b e returned to the gold plant as comparatively fresh water VOL. 51, NO. 3

MARCH 1959

241

Pilot Plant Construction

OOO!

j

I

o-Free NaCN A-complexed NaCN X- NeCNS

Z

s

iI

i

Figure 2. Adsorption o f the primary column (as shown b y these typical curves) is complete when thiocyanate in the effluent begins to d r o p with an accompanying increase in cyanide Influent, 0.001 0 pound o f sodium cyanide and 0.0009 pound o f sodium thiocyanate

per gollon

Hoivever, one of the cobalt cyanide complexes. derived from the acidification of the original cyanided ore pulp. is a permanent poison to the resin (3. 8). Over a period of t\vo years the resin's capacity for uranium is so reduced by gradual build-up of the cobalt complex that its further use in the plant is uneconomical and a large quantity of used resin becomes available for other uses Recovery Process

The pilot plant studies on cyanide recovery are based on Ivork carried out on the used resin from the uranium plant. I n compliance bvith gold plant practice. all references to cyanide, Lvhether complexed or free, are sodium cyanide as XaCN. In addition, all gallons are in Imperial units, and Lveights are avoirdupois including that of gold. 'The principle of ion exchange cyanide recovery is to absorb the metal cyanide complexes present in gold plant waste cyanide solurion (Table I) on a column of the strong base anion exchange resin in the sulfate form. The effluent from

Table I. Composition o f Waste Cyanide Solution from the Gold Reduction Plant oiiiponrnt.~

NaCN (total) NaCNS Zn Ni cu

co

Au

242

Lh.!100 Gal. 0.10-0.16 0.07-0.10 0.01-0.03 0.001-0.015 0.001-0.010 0.0001-0.001 0.00001-0.00004

this column is then passed to another column of similar dimensions Tvhere free cyanide is cornplexed with copper(1) cyanide that has deliberately been precipitated ivithin the second columns' resin beads. A column of resin containing precipitated copper(1) cyanide is referred to as being "conditioned." Effluent from this second column, \\,hich contains thiocyanate and carbonate anions but is otherit-ise free of deleterious substances, can be returned to t h e plant as comparatively fresh ivater. The columns are eluted with 1.07; sulfuric acid solution rvhich decomposes the metal complexes in the first column and the copper cyanide complex in the second conditioned column. During elution of the first column, the eluate containing various metal sulfates and hydrocyanic acid is passed directly through a column of cationic resin of the sulfonic acid type (Rohm Kr Haas IR-120) in the sodium form to remove metal ions. The eluate, free of metal ions. is than suitable for re-use in the gold plant. If it is necessary ro increase cyanide concentration further. the eluate is reacidified to its former 1.054 sulfuric acid strength and used to eluate the second copper(1) cyanide conditioned column. In thc latter instance it is not necessary to pass the eluate again through the cation column, as copper(1) cyanide from the breakdoivn of the copper complex is reprrcipitated Lvithin the resin beads. Metals are elutcd from the cation column b!. a loc< sodium chloride solution.

INDUSTRIAL AND ENGINEERING CHEMISTRY

Columns for the pilot plant (Figure 1) ivere constructed from 17.5-inch rubberlined piping and were 6.0 feet long, with 0.75-inch stainless steel outlets attached to the top and bottom rubber lined plates. These in turn were bolted to the flanges of the columns. Inside each column over the bottom outlet, was a perforated? 4-inch-diameter plastic disk mounted on 0.25-inch legs so that a free flow of solution was achieved through and around its sides. The disk and the bottom of each column Fvere covered to a depth of 3 inches with 1-inch stones. 3 inches Lvith 0.5-inch stones?3 inches ivirh fine-screened gravel, and finally resin to a depth of 24 inches. The fresh and used acid tanks had a capacity of 500 gallons each, and the lOYc sodium chloride and concentrated cyanide ranks were of 200-gallon capacity. All tanks tvere rubber lined. with 0.75-inch stainless steel outlets a t the base. Both the fresh and used acid tanks \Yere sealed. except for U-trap vents a t the top of each Lvhich contained a lime slurry to prevent loss of hydrogen cyanide by evaporation from the acidified contents. Columns and tanks were inrerconnected with 0.5-inch polythene tubing. Elboivs, T-pieces, and diaphragm valve connectors were of 0.5-inch Kralastic piping. Saunders diaphragm 0.5inch valves of mild steel ivere used with the columns and tanks \\hen 1,070 sulfuric acid was used. Diaphragm valves on the cation column, sodium chloride tank, and pump bvere Saunders plastic 0.5-inch valves. Gorman-Rupp Mansfield Line 8B12 pumps were used for pumping sulfuric acid and sodium chloride elutriants. Each of the primary (PI and P2) and secondary (Si and $2) columns were loaded with used IRA-400 resin from the uranium plant; fresh IR-120 was used in the cation column. The resin depth of 24 inches allowed a volume of 3.3 cubic reet of resin or 20.6 gallons for each column. Adsorption flow rate \vas 3.5 gallons per minute (0.17 bed volume per minute); elution flow rate. lvhich was also the adsorption flow rate of the cation column, was 1.0 gallon per minute (0.05 bed volume per minute). Conditioning. Maximum conditiony \vas ing of secondary columns Siand S achieved by the follou ing procedure : Commercial copper sulfate \vas dissolved in water to give an approximately 2.5YG solution, and a strong solution of commercial caustic soda was added to precipitate the hydroxide. The addition of caustic soda !vas continued until the Cambridge-blue precipitate just turned to a deeper or oxford-blue color. A strong solution of sodium cyanide \vas then added until the precipitate

C Y A N I D E RECOVERY

3'0T I

Water Wash'

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-NaCN

- - - - - NaCNS

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006,

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001.

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'C1

2oc

300

ELUATE GALLONS

400

5w

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Figure 3. Elution of cyanide from primary column with 1 .O% sulfuric acid

\

Effluent i s passed directly to cation exchange column to remove metallic

0

ions

dissolved. The cuprocyanide solution (approximatel!. 30 gallons) was then pumped into a secondary column and allowed to percolate very slo\vly through the resin bed for a period of 12 to 18 hours. The bed \vas then thoroughly washed \vith water and eluted with 1.0% sulfuric acid solution until the cyanide concentration of the eluate dropped to less than 1.0 gram per liter of sodium cyanide. The column, after washing by downflow: \vas ready for adsorption of free cyanide. Plant Operational Procedure

The main objects of the investigation with the pilot plant were to recover and concentrate cyanide by ion exchange and to achieve this by the use of the uranium-plant used resin (Amberlite IRX-100). Procedure for operating this pilot plant iyas based on a previous report \vhich continued a step by step procedure for continuous operation (6). Adsorption curves determined with the pilot plant for the primary and secondary columns follo\v closely those obtained on the laboratory columns; Figure 2 illustrates the adsorption curve for a primary column. Exhaustion of the primary column was estimated to be

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I

100

2W

1

300

400

ELUATE G A L L O N S

Figure 4. Elution o f cyanide from secondary column with 1 .O% sulfuric acid First portion o f effluent returned t o gold ploni. Second portion recycled t o used and fresh elute tanks

complete when the thiocyanate content of the effluent began to drop \vith a corresponding increase of cyanide. This was an indication that the column was saturated with complexes, and thiocyanate was beginning to pass straight through the resin bed. Completion of a secondary column adsorption cycle was taken on the first trace of cyanide breakthrough. In the original laboratory work, nickel was evident in the primary column effluent throughout the adsorption cycle. Since then the Stilfonrein plant has adopted a fairly large water byash for its pulp filters, resulting in a higher dilurion of the waste cyanide solution than previously experienced. .4t this lo\ver concentration. nickel cyanide is completely adsorbed by the resin and only appears in the effluent when the resin bed is almost saturated with the metal complexes. Therefore, as a rapid means of determining column exhaustion it has been found preferable to estimate a spot test (5) the completion of the adsorption cycle by the appear-

ance of nickel in the effluent. Elution curves with 1.O%;, sulfuric acid are shown for the primary and secondary columns (Figures 3 and 4). As the eluates from the primary columns were passed directly into the cation resin column to prevent metals entering the used acid tank. a further curve (Figure 5) shows the eluate after passage through the cation column. One cation resin column of 3.3 cubic feet of IR-120 had sufficient capacity for metals from the elution of two primary columns. Therefore, for calculation purpo3es. a "run" includes the adsorption and elution of two primary columns, one secondary column cycle that may have taken place during that period, and one cation column adsorption and elution cycle (Table 11). For studying the economics of the cyanide recovery process. it had been decided originally that metals adsorbed by the cation column were waste products to be removed from the column as economically as possible. Elution of the cation column by the VOL. 5 1 , NO. 3

MARCH 1959

243

I I

0

ELUATE GALLONS

Figure

5.

Primary column eluate after passage through

First portion o f cyanide bearing effluent returned t o used ocld tank

conventional methods with 10% sodium chloride elutriant w.ould not only require operational procedure equivalent to the rest of the pilot plant alone but also the quantity of elutriant to ensure maximum recovery would prove expensive. Therefore, a method similar to that used for regenerating resin columns for chromium recovery from plating solution was adopted (7). T h e cation column, upon exhaustion after the passage of the eluates from the two primary column elutions. {vas not required for a further 30 hours or so. Consequently, this period of time allo\ved a prolonged contact of elutriant with the resin. Cation Column Elution. T h e column was drained of superfluous liquid equivalent to the volume above the resin bed, replaced with 30 gallons of 10% sodium chloride solution, and a further 10 gallons of column liquor was removed. T h e resin was allowed to stand overnight in contact \i.ith the brine solution, and on the following day it was slowly drained to waste. Such lengthy contact time of the sodium chloride elutriant with the loaded resin effectively removed most of the metals with a minimum use of regenThe resin, after backwashing erant. thoroughly with water, was prepared for the next cycle by draining the column until only six inches of water remained above the level of the resin bed. Eluate Distribution. From the elution curves (Figures 4 and j), the follo~ving procedure was adopted for the eluate distribution from the primary and secondary columns.

244

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Figure 6. This primary column i s fouled with copper(1) cyanide, and free cyanide breakthrough is delayed because of complexing as in a conditioned column. Treatment with iron(ll1) sulfate regenerates the primary column Primary Columns (from Figure

Source Fresh acid tank

Bed

1011s

Vol.

40 200

1.9 Waste 9.7 Used acid tank via cation column 10.7 Return to fresh acid tank

220

Water Total

Secondary Columns (from Figure 4)

5)

Gal-

120

5.8

Destination

Freshacid tank

580

Figure 5 sholvs that M’ater to fresh acid tank is quite high in acid because of the removal of bisulfate ion from the resin bed. This leaves most of the resin in the sulfate form. which at rhe same time recovers acid (2).

Gal-

Red

Source

loiih

Used acid tank

40 90

1 01. 1.9 4.4 1.0

20

Water Total

110 260

-

5.3

De-t iiiatioii

Waste Concd. cyaniae tank Return to used acid tank Fresh acidtank

As iiith elution of the primal!, coluinn. arid recovery \vas high because of the removal of the bisulfate ion from the resin. T h e above procedure does not allo\t, for a complete solution balance. and

Table (I. Cyanide Recovery Resin capacity was lowered b y the presence o f coppedl) cyanide

Hun No. 27 28 29 30 31

Influent, Gal. 35,280 25,200 21,420 17,850 15,540

32 33 34 35 36

32,970 27,500 20,000 20,580 19,000

INDUSTRIAL AND ENGINEERING CHEMISTRY

__

_-

Total X a C N Available, Lh. 39.1 28.9 29.4 24.8 20.7

NaCX Eluted Der I