GENERAL47956

• Permit No: M1988112
• IBM Index Class Name: General Documents
• Doc Name: THE RECOVERY OF CYANIDE FROM SLURRIES
• Media Type: D
• Archive: No

Pege 5
The tdlowing sections d the paper torus on the chemistry, performance, appiicatlons, and economics of
the cyanide recovery from sdutions end slurries, wRh emphasis on the cyanide recovery system proposed
for the Gdden Cross Mine.
PROCESS DESCRIPTION AND CHEMISTRY -
Cyanide recovery processes utilize the vdatUlty of HCN at a lowered pH (I.e. Figure 2) to strip tree
from sdution or slurry and recover h In usable form (I.e. NaCN or Ca(CN),). The vdatilRy of tree rr
cyanide (HCN) Is related to fts Henry's_l~~+CAnstant, which Is affected by several factors Including v
Is Iiauid film limfted. and
to
psi are required to create t
_. __.~.____._.Y,
There are several factors which affect the rate and extent of HCN removal Irom slurries or sdutions through
air stripping. These (actors Incude:
1. The pH of the soution
2. The form of cyanide
3. The concentration of cyanide
4. The temperature of the slurry or solution
5. The pressure maintained within the recovery system
6. The air to liquid ratio
7. The mechanical dispersion equipment
8. The viscosity of the sdutlon or siurry
9. The liquid tc air contact period
A great deal of Information is available regarding the effects of these design parameters on the recovery of
cyanide Irom clarHied sdutions, but limfted data is available concerning the treatment of taiings slurries.
The current cyanide recovery processes are upgraded versions of the original Mills Crewe Process
previously discussed. A schematic of the generalized cyanide recovery process is presented on Figure 3.
._ The process is conducted In three stages. The first stage known as acidification Invdves lowering of the ~ ~
~' ~ waster nH In rha range of tS-8.5 with the use of concentrated mineral acid. The acid employed most
commonly Is sulfuric acid, due to fts relatively low cost and ease of availability. In addition, the handling,
storage, and feeding of sulfuric acki Is quite common in industry: -The potential problems associated with
sulfuric acid Include an Increase in sulfate and total dissdved solids concentrations and the potential for;.
precipitation of calcium sulfate or gypsum In oversaturated sdutions. The pH'of the sdutlon or slurry is -
_ , ..monitored In-situ continuously. The aciditiratloasten must tie endosed to oreyent escape of HCN gas and --
requires about 10 to 20 minutes to complete. The reduction of HCN during the acidification step Is about I
t0 to 15 percent of the total cyanide concentration.
In accordance wfth the stability of the particular complexes
recovered. For the recovery of free cyanide from
and Zn(CN),I. 8 DH In the range of 4.5 to 8.5 Is employed. For recovery of weak add Clssoclable~wAU)
cyanide, the pH is lowered to about 4.0. For rernoyai of Iron complexed~anlde the pH Is_ lowered to_ less
than P.0.-ln this case, an Insduble metal Iron cyanide complex forms, and precipitates from solution. The
es approach Is to utilize a near neutral or basic pH when possible to minimize precipitation Problems and
an Increase in total dissdved solids. _ - -- -
From the ackificatlon stage, the acidified solution or slurry containing HGN passes Into the cyanide stripping
or vdatillzat(on stage, which consists Of either open or packed to•.vers, a series of complete rnfx reactors,