CA2183124A1 - Process for obtaining access to refractory gold and/or silver in arsenical ore - Google Patents

Abstract

A process for obtaining access to refractory gold and/or silver carried by arsenopyrites and/or arsenical pyrites comprises first heating dry ore concentrate containing arsenopyrites and/or arsenical pyrites at a temperature from 450°C to 850°C in an oxidizing atmosphere of sulfur devoid of free oxygen to volatile the larger part of arsenic from the starting ore and then roasting the non-volatile residue at a temperature of 850°C or less leaving a residue of iron oxide substrate from which gold may be retrieved by cyanide extraction.

Description

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TITLE OF THE INVENTION
A process for obtaining access to reFractory gold andlor silver in arsenical ore.

FIELD OF THE INVENTION
The present invention pertains to a process for removing n,f,d-,lo,i"ess in arsenical ore and, more particularly, to a process for obtaining access to refractory gold and/or silver carried by arsenopyrites and/or arsenical pyrites.

BACKGROUND OF THE INVENTION
Sulfide minerals are often ~so~: t~d with trace amounts of gold and silver. The recovery of these values from such ores is generally done by cyanide extraction applied on a conc~ dle of the sulfides obtained by flotation of the head ore.

With free gold dispersed in a sulfide, the cyanide extraction is a relatively simple operation that can achieve 95~h recovery of the ~old initially found in the head ore. However, under certain circumstances, the recovery can be as low as 5~h; the gold is then said to be "refractory", that is, i" l~ossible to recover with standard cyanide extraction.

Such refractory gold is often A~so,i~ d with arsenopyrite (FeAsS). In such cases, gold exists more or less as a solid solution in the crystal lattice of the arsenopyrite and only by the total destruction of said lattice can it be recovered by cyanide extraction.

Since all the elements involved in the stn~cture of arsenopyrite can be readily oxidized, the air roasting of the ore will destroy the arsenopyrite structure and will give access to gold as indicated by the following equation:

, .. , , . .. _ . _ .

2 ~3~

12 FeAsS + 29 ~2 > 4 Fe3O4 + 3 As40~ + 12 SO2 The air roasting of arsenopyrite has been often practised in the past in order to gain access to ~so~;-J~d gold. However, for envi, u"" ,entdl reasons, such a process is now nuled out.

Gold bearing arsenopyrite being of relatively common occurence, alternate means for the opening of the crystal lattice so as to gain access to gold has been coll~ider~d. One technique calls for the pressure oxydation of the arsenopyrite with pure oxygen rather than air (R.
M. G. S. Berezowski, D. R. Weir, "Pressure oxidation ~.retrbdl",~"l of refractory gold", Minerals and Metallurgical P~oces~i"g Vol. 1 (May 1984), pp. 1~).

With that process, since the reaction is performed in a pressure vessel at about 2 000 kPa with pure oxygen, there is no release of noxious arsenical compounds or sulfurous compounds. All of the arsenic ends up as insoluble ferric arsenate FeAsOI and all sulfur is discarded as sulfates.

However, the implu",~"ldlio" of this technique is rated as being expensive because of the pressure operation at 200~C under acidic conditions and the consumption of pure oxygen.

Another approach calls upon bacterial d~uldddtion of the arsenopyrite. This operation works but is rated as being slow and sensitive to ore ccm~o~ilio", traces of certain metals being inhibitors to bacterial action (P. B. Merchant, FIJllddmelltdl and Applied Bioh~ u"~ " Irgy, Elsevier Science Publishing, Amsterdam, 1986, pp. 53-76). Other techniques have been cc"~",, I ~ ~ such as nitrate oxydation but are believed not to have reached the industrial pilot stage.

It therefore appears that there is a need for a technology for the breakup of arsenopyrite that would be cc " ,,udtil~le with the envi. un" ,enlal requirements A~SO~;~tf d with arsenic and sulfur that would not incur the high capital and operation costs typical of pressure oxydation and that would s show an improved kinetic when compared to biological J~u~ dddliùl7.

SUMMARY OF THE INVENTION
The present invention relates to a process conceived in such a way as to first separate and recover most of the arsenic fraction of gold-bearing arsenopyrites in the form of sulfides of arsenic.

The process of the present invention therefore comprises the steps of:
heating dry ore conce"l,dte cor,ldir,i"g arsenopyrites and/or arsenical pyrites at a temperature from 450~C to 850~C in an oxidizing d~, ,u~h~ of sulfur devoid of free oxygen to volatize a high pe~ ~ Itd~ae of 1S arsenic from tne ore conce"t, dle and leave a non-volatile residue including ferrous sulfides and remaining arsenic sulfide; and roasting the non-volatile residue at a temperature of 850~C or less leaving a residue of iron oxide substrate from which gold may be retrieved by cyanide extraction.

The trace amount of arsenic remaining in the ore is then eliminated as ferric arsenide and the residual sulfur content of the ore after removal of arsenic is ~d-,~ru,,,,ed into calcium sulfite. 7-he end product afterthese two ope,dlions is a gold-bearing iron oxide and alu,n;"
readily dl "t" ,Jdble to cyanidation in order to collect gold.

2s In one preferred fomm the process of the present invention includes the following steps:

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(a) the vr 'i ' " ' , of arsenic in a sulfur-rich mildy oxiding dtlllospllelt:, this step removing the larger part of arsenic from the starting ore;
(b) the air oxidation of the residual material from the vol-t~ iorl step so as to complete the removal of arsenic and the oxydation of remaining sulfur and iron;
and (c) the acid scrubbing of arsenious oxide by an acid solution followed by basic scrubbing of SO2 by a basic solution, the arsenious oxide being oxidized to arsenic oxide and p~ .;'.,t~,d as FeAsO4.

The main steps of this process are illustrated by the following equations:
1 ) 2 FeAsS + [S]~~2 FeS + As2S3 ~ S~
2) 2 FeS (As2S3)air--~Fe203 + 2 SO2 + (As20,) 3) As20, +2 [O] > As20s Fe2(SO4)3 + 2 H3AsO4- ~ 2 FeAsO4 + 3 H2SO4 SO2 + Ca(OH)2 > CaS03 + H20 The i"" I~",e"ldliu" of those steps leads to a residual iron 20 oxide that lends itself to easy and efficient gold recovery by cyanide extraction, a yield in the range of 95~/0 being observed with an arsenopyrite ore which otherwise was not dl I ,andal;,le to cyanidation.

Further scope of a" ' ' ' 'y of the present invention will become apparent from the detailed desu i~,lion given he, ~i"dfler. It should 25 be u".le,~lood, however, that this detailed dtsc,i~,lion, while indicating preferred e" Ibodi,, ,e, lia of the invention, is given by way of illustration only, since various changes and Illodiri~dlions within the spirit and scope of the invention will become apparent to those skilled in the art.

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BRIEF DESCRIPTION OF THE DRAWINQ
Figure 1 is a block diagram detailing the various steps of the process.

DETAILED DESCR.. I ION OF THE INVENTION
The conditiu"s required for an optimal ~oxl~loil ~ion of the invention will now be described while referring to figure 1.

It should first be noted that the rep,~sentdtio,) of the arsenopyrites by the formula "FeAsS" is a useful ~ " 11 but the actual ores are generally much more complex than such an abstract structure might suggest. Very often, the arsenopyrite is r-- -' ' i with pyrite (FeS2).
Pending on the type of ore, sulfides of lead (galena) or zinc (sphalerite) or sulfides of base metals, such as copper and nickel, can also be present. The following table gives a list of some of the volatile sulfides of arsenic that can be encountered in ~ 50~ -on with arsenopyrites.

rablQl Vo/atiles sulfides of arsenic Name Forrnula Boiling T~ (~C) Realgar As2S, 565 Orpiment As2S~ 707 Sulfur S" 445 The c~"ce, Itl dle obtained by flotation of the head ore is in the range of 80-90% in sulfides, the balance being made up of various silicates or al-" "i" " ' ~. This non-sulfurated fraction is carried along through the 25 process.

~~ - 6 - 2 1 ~

It is known that arsenic sulfides are volatile entities as shown in Table 1. Upon thermal treatment, in the absence of oxygen, it is possible to remove these volatiles cu~ uon~l ~ts. However, in doing so, there must be enough sulfur present so as to make sure that the arsenic sulfides are 5 volatilized in the form of the polysulfide As2S3 S~ where "x" is of the order of 3 or more. To achieve this proper molecular ratio of sulfur to arsenic, pyrite, either already present or added to the ore, can play an important role by acting as a source of sulfur by the following reaction:
FeS2 > FeS + S

lo If the ore is still below a 311 ratio of S to As, even while taking into account the sulfur made available by decon l,uosiLion of the pyrite, then elemental sulfur can be added so as to have the right ,ul UpOI lion of these elements.

The interest of obtaining the arsenic in the form of a polysulfide is related to ecu"o,ni~al and envi,u"",e"Ldl cunside,~lion~. Arsenic polysulfides are esse, lli.llly non-soluble entities with no vapor pressure at room temperature. Therefore, they can be stored or retumed underground without creating env;.u"",enl~l hazards. Also, these sulfides can be used readily as a source of arsenic in the manufacture of arsenical derivatives 20 used as pesticirl~s, preservatives for wood and similar products.

The vol ';' ~ on of arsenic sulfide is done in a reverbatory type of fumace, in the absence of air. A relatively shallow layer of ore which has been previously dried is heated at a temperature of from 400 to 8ûO~C
and the arsenical vapors are driven off the furnace by the vapors of excess 25 sulfur which acts as a carrier. Therefore, only initial purging with nitrogen at the start-up of the fumace is required.

Although air-free, the al"~osph~,e in the fumace is mildly oxidizing because of the presence of excess sulfut. It is important that it never goes reductive since there would result a production of arsine (AsH3), an exceedinyly toxic gas with the smallest trace of water.

rhe volatiles from this treatment are directed to a w, nl~:n;ldl;Vn chamber where arsenic sulfides are cundt:llsed on large surfaces by intemal cooling. Since the volume of gas circulation in the fumace is small, coming mainly from the vo~ ;oll of sulfur, the speed of circulation in the condenser will be very slow, allowing adequate e,~,l,,",yes with the lo condel,si"g surfaces and solid growth rather than fine particle deposits.Periodically, the cond~naed material is removed by scrapping. It will be readily noted that, from the boiling point of sulfur (445~C) and arsenic trisulfide (707~C), it is possible to separate the excess sulfur from the arsenic sulfide by the adjustment of the le",pe, mre of the c~"~denai"g surfaces.
This collected sulfur could be recycled to the system.

The boiling temperature reported for arsenic sulfide (AS2SJ) is 707~C at dllllOS~h dric pressure. At a lower temperature, say 500~C, there exists already a siynirl~nl vapor pressure of arsenic sulfide but the rate of v(~' "" " , will be rather slow, particularly iF the material is not swept by a 2 o gas in order to displace the vapors of the sulfide.

If the temperature of the treatment is in the range of the boiling temperature of the arsenic sulfide, then the rate of v~- I "' 1 will be much faster, and the large volume of gas thus generated will sweep out the sulfide.

This is why a temperature range of 700-750~C is preferred although some vr' "" " ~ already takes place at 450~C.

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Above the preferred range of 700-750~C, additional temperature expands further the gaseous sulfide, thus speeding its evacuation, but a significant increase in energy consumption is involved.

The non-volatile fraction at the end of the v~ of 5 arsenic compounds is esse:"t;a.'y ferrous sulfide (FeS) with the non-sulfurated products initially present in the dried co"cel Itl elle.

Arsenical compounds are substantially removed, to the extent of 95~fO or more, by vol~ l;on. The relatively small amount remaining is handled in the course of the second step of the process.

o The non-volatile fraction is then oxidized with air, in a closedrotary kiln, so as to transform all the sulfur into sulfur dioxide and the traceamounts of arsenic into arsenious oxide, Asz03.

It must be noted that this oxidation reaction is an t:~oth~ll"ic process. It is important to keep an adequate control over the temperature durin~ this roasting. If maximum oxidation speed is allowed by unlimited access of air (dead buming), the resulting product will show significant sintering and will not perform correctly when submitted to cyanide treatment ,~. for the recovery of gold; it will still be somewhat refractory. But if air access to the roaster is controlled in such a way that the temperature durins the roasting does not exceeds 850~C, then the resulting ferric oxide has a very open structure and the recovery of sold from this burned fraction reaches 95o~o The non~n.l~llsed sases from the distillation of srsenic sulfides along with the flues from the roasting of ferrous sulfide are directed first to a scrubbing tower operated under slightly acidic conditions, by ~ ' 2 ~

addition of some sulfuric acid, so as to prevent significant solution of S02 (pH around 2.5).

In that tower, any arsenical material carried over from the distillation to the roasting is dissolved as trivalent arsenious oxide and then 5 pl t~ Jitdl~:d by addition of a ferric salt after oxydation to arsenic oxide to give ferric arsenate. Ferric arsenate is a very stable and completely insoluble salt of arsenic that can be disposed in the environment without problem.

Under acidic conditions, the S02 resulting from the roasting operation will not be dissolved because of the low pH in the first scrubber but 10 will be carried over to a ser,ond scrubbing tower operated with a lime slurry.
Under such conditions, SO2 will be ,u~ as calcium sulfite, an insoluble salt.

The roasted ore can then be submitted to standard cyanide extraction for gold and the residual solids disposed of along with the ferric 15 arsenate and calcium sulfite.

Hence, the present invention leads to the l~ rulllldliui- of a refractory arsenopyrite into a material from which gold can be reclaimed without problem. This process operates at dl~w .phe(ic pressure and allows the recovery of most of the arsenic as a source of useful products rather 20 than waste and its operation is entirely cc",~,dlible with e~;.u~ e~ldl regulations ~l "_e" ,i"g arsenic and sulfur.

EXAMPLE

Kassandra arsenical material containing 40% Fe, 41% S, 12~h As, 0.8% Zn, û.08% Cu, 0.54% Pb, 25.5 g/t Au and 20 g/t Ag is extremely 25 refractory since a cyanide extraction with 0.5 Kg NaCN/t of ore, a standard ~ ' Z.l~312~

cyanide procedure operated at pH 10.5, gave a recovery of 7~h of the gold present.

By:,, ' ' ~ of the present invention, the material was first treated at 730~C in the absence of air so as to let the volatile arsenic sulfides 5 evaporate.

The residual material was then oxidized by a slow air stream, the temperature in the reacting mass being at 830-850CC.

After cooling, the residual product was submitted to cyanide extraction using 0.5 Kg/t of cyanide per treated ore at pH 10.5.

o The recovery of gold from the initial sample was 95~f~.

The arsenic wl Idt~ c les were treated with hydrogen peroxide and the arsenic ,u, ~cil.itdt~d by addition of ferric sulfate to give the expected ferric arsenate.

Although the invention has been described above with respect with one specific fomm and one example, it will be evident to a person skilled in the art that it may be modified and refined in various ways. It is therefore wished to have it understood that the present invention should not be limited in scope, except by the terms of the following claims.

Claims (12)

1. A process for obtaining access to refractory gold and/or silver carried by arsenopyrites and/or arsenical pyrites comprising the steps of:
heating dry ore concentrate containing arsenopyrites and/or arsenical pyrites at a temperature from 450°C to 850°C in an oxidizing atmosphere of sulfur devoid of free oxygen to volatize a high percentage of arsenic from said ore concentrate and leave a non-volatile residue including ferrous sulfides and remaining arsenic sulfide; and roasting said non-volatile residue at a temperature of 850°C
or less leaving a residue of iron oxide substrate from which gold may be retrieved by cyanide extraction.

2. A process as defined in claim 1, wherein said roasting step is conducted in an air oxydizing atmosphere.

3. A process as defined in claim 2, further comprising the step of condensing said high percentage of arsenic volatized to recover arsenic polysulfide and an excess of elemental sulfur.

4. A process as defined in claim 3, wherein non condensed fumes from said condensing step are directed to said roasting step.

5. A process as defined in claim 1, wherein flues obtained from said roasting step are circulated to an acid scrubber to dissolve arsenious oxides.

6. A process as defined in claim 1, wherein said sulfur oxides obtained from said roasting step are further circulated to a second scrubber to transform said sulfur oxides into insolubles sulfites or sulfates ofcalcium.

7. A process as defined in claim 5, wherein said arsenious oxides are oxydized to arsenic oxides and precipitated as ferric arsenate.

8. A process as defined in claim 1, wherein said ore concentrate is air dried, prior to said volatization step, at a temperature of about 100°C to 150°C.

9. A process as defined in claim 1, wherein said volatization step is done by reverbatory heating a shallow layer of ore concentrate.

10. A process as defined in claim 1, wherein said roasting step is carried out in a temperature controlled rotary kiln.

11. A process as defined in claim 1, wherein air access to said roasting step is controlled to limit the roasting temperature to 850°C or less.

12. A process as defined in claim 6, wherein the second scrubbing step is operated with a lime slurry.