EP0508542B1

EP0508542B1 - Process for treating ore having recoverable metal values including arsenic containing components

Info

Publication number: EP0508542B1
Application number: EP92200989A
Authority: EP
Inventors: Anthony L. Hannaford; Marc K. Le Vier; Rene R. Fernandez; Gopalan Ramadorai; Arno Dr. Fitting; Gurudas Dr. Samant; Bodo Peinemann; Gebhard Bandel; Hans Kofalck
Original assignee: Metallgesellschaft AG
Current assignee: GEA Group AG
Priority date: 1991-04-12
Filing date: 1992-04-07
Publication date: 1998-06-24
Anticipated expiration: 2012-04-07
Also published as: DE69225993T2; BR9201355A; CA2065837A1; MY110328A; RU2079562C1; AU1482492A; EP0508542A2; EP0508542A3; DE69225993D1; CA2065837C; AU656952B2; ES2117028T3

Description

This invention relates to a process for treating ore particles containing gold and having arsenic-, carbon- and sulfur-containing components. The ore particles include refractory ores, ore concentrates and ore tailings.

Precious metals such as gold occur naturally in ores in different forms. Unfortunately, precious metal ores also frequently contain other materials which interfere with the recovery of these precious metal values, rendering these ores refractory to precious metal recovery. Furthermore, the precious metal content may be at a relatively low level. This low level content compounds the effect of the refractory nature of these ores. Many patents show attempts to deal with refractory components in precious metals recovery addressed to solving the arsenic contamination problems encountered when roasting precious metal and other metal ores having arsenic as an unwanted component present in the ore.

US-A-4 919 715 relates to the use of pure oxygen in roasting of refractory gold-bearing ores at temperatures between about 537°C and about 648°C. This patent fails to address the problem of arsenic volatilization, is silent on the arsenic content in the ore, and does not address in that context the optimizing of gold recovery from refractory sulfidic, carbonaceous ores or separation of cyanide-consuming components before recovery of gold from the ore. The disclosed method requires two fluid beds and stagewise roasting in these beds and the use of substantially pure oxygen (substantially pure oxygen being defined as at least about 80 % by weight).

For sulfide ores, any arsenic which is present is an undesired accompanying element and must be removed from the calcine and from the roaster gas. This is typically accomplished according to prior art by a so-called dearsenication roasting. The arsenic content of the material is volatilized in a roasting zone having a low oxygen content and enters the gaseous effluent as arsenic vapor or arsenic oxide vapor and arsenic sulfide vapor. In the gaseous effluent, arsenic and arsenic sulfides are oxidized to form arsenic oxide vapors under a relatively high oxygen partial pressure. The dustlike solids contained in the roaster gas are removed at a temperature exceeding the sublimation temperature of the arsenic oxides, which are subsequently separated at lower gas temperatures, or the solids and the arsenic oxides are jointly removed at lower gas temperatures. In the first case, contaminated arsenic oxides will be formed. In the second case, the arsenic which has been removed will be recycled in the process scheme.

The inventive process has the aim of roasting ores or refractory ores, ore concentrates or ore tailings of the type described herein for recovery of gold in an oxygen-enriched gaseous environment in order to minimize or eliminate arsenic volatilization, facilitate arsenic conversion to an insoluble, environmentally acceptable form immobilized in a waste product while reducing the effects of carbon- and sulfur-containing components on gold recovery. This problem is solved by the process defined in claim 1.

It is a desideratum to roast refractory gold ores in such a manner that cyanide leaching will result in a high yield of gold, will involve a low consumption of cyanide, and will assure economic environmetally acceptable disposal of arsenic-containing solids.

The solubility of the stable arsenates formed in the process is so low that these may be dumped without special precautionary measures. The water vapor content of the exhaust gas leaving the roasting reactor will result in a formation of stable arsenates having a particularly low solubility, e. g. such as scorodite or scorodite-like compounds.

The term "free oxide" in claim 1 indicates that said substance is not present as compound with arsenic or sulfur but in a form free of these. If calcium and magnesium as carbonates are available in a free form in the ore in a sufficient amount, it will be unnecessary to add said substances.

If iron compounds are present, even in a large excess, an addition will always be required, i. e. if below a ratio of 3.5 to 4.0 moles iron to a mole of arsenic, because a major part of the iron will always be included in compounds with arsenic or sulfur. The additives in the roasting reactor may consist of waste products, such as red mud from the alumina industry, filter salts and waste gypsum. Sulfates are particularly suitable. As seen from the data herein, iron compounds are preferred. The use of an additive is preferable because the additive, in particle form will then be present close to the ore particles and will be able to combine immediately with arsenic which may have been vaporized from the ore particles at the higher temperatures discussed herein.

The term "stable arsenates" designates those arsenates which have only a low solubility in rainwater when stored in a waste dump of an exhausted calcine.

Water required for the water vapor may be fed to the roasting reactor by a suitable addition of steam, as moisture or water in the ore, of crystallization in the additives or as a water of crystallization in a component in the ore. Depending on the SO₂ content, the exhaust gas may be processed for a production of sulfuric acid or may be scrubbed to remove the SO₂ or the SO₂ content may be liquified.

Preferably, the ore is roasted in the reactor in the form of fluidized solids, and more preferably, the ore circulates as fluidized solids in a circulating fluidized bed or in an ebullating fluidized bed (which has a circulation feature to it). Furthermore, a stationary fluidized bed having a defined upper surface may also be used. Further, a rotary kiln or a multiple-hearth furnace, may be employed, provided the proper reactions may be obtained.

The advantage afforded by the process in accordance with this invention resides in that the calcine which is produced has a very good leachability, with e. g. cyanide, resulting in a high yield of gold and in a low consumption of cyanide. Moreover, the arsenic is bound in the form of stable arsenates, which do not disturb the leaching and which have an extremely low solubility in rainwater such that these calcines may be dumped without a need for special precautionary measures or further treatment. The ores or concentrates may contain even up to 2 % of arsenic and more.

The temperature at which an undesirable molten phase is formed during roasting depends on the composition of the ore. A molten phase in or on the ore particle, even a partial molten phase, e. g. partial sintering is undesirable as metal recovery by leaching is undesirably affected. The percentages for the gases are stated in percent by volume.

In the event of a low arsenic content in an ore, the gas which is fed into the roasting reactor is adjusted to have a higher oxygen content. The reaction temperature is achieved by a feeding of hot gases and/or by an addition of fuel. If fuel is added, oxygen in the amount required for the combustion of fuel must be added. If a reaction temperature is low, the required heat is introduced by feeding of suitable hot gases and/or by a sufficient preheating of the charged materials.

Roasting, with two stage oxygen injection may be carried out particulary conveniently. The roasting in the lower portion of a circulating fluid bed reactor is carried out as the first stage. A fluidized gas contains an oxygen-containing atmosphere having an oxygen content below about 1 % by volume. The second oxygen injection during this roasting stage is carried out in the upper portion of the reactor with a supply of secondary gas and optionally even with a supply of tertiary gas having yet more oxygen injected in that phase at a corresponding higher oxygen content.

The candidate ores may have the following levels of arsenic, carbon and sulfur components on a percent by weight basis:

Arsenic	up to 1.0 % or higher
Carbon	2.5 % Maximum
Sulfur	5.0 % Maximum

(All percentages are on a weight-to-weight basis unless otherwise stated.)

A typical mineralogical analysis of these ores shows:

Quartz	60 - 85 Percent
Pyrite	1 - 10 Percent
Carbonate	0 - 30 Percent
Kaolinite	0 - 10 Percent
Fe_xO_y	0 - 5 Percent
Illite	0 - 5 Percent
Alunite	0 - 4 Percent
Barite	0 - 4 Percent

A typical chemical analysis of the ore shows an average composition as follows:

Arsenic	0.2 Percent
Sulfur (Total)	4.0 Percent
Carbon (Total)	1.0 Percent
Iron	3.5 Percent
Zinc	0.08 Percent
Strontium	0.03 Percent
Gold	4.3 g/t

This ore, if so treated, typically shows gold recovery of less than 10 percent by simple cyanidation and less than 20 percent by simple carbon-in-leach cyanidation.

On the other hand, gold recovery by using the process of the present invention yields from about 75 percent to about 90 percent (and even higher) gold recovery.

According to an embodiment of this invention, the roasting treatment is preceded by a first roasting stage, in which the roasting is effected at temperatures which are between 450°C and 900°C, preferably below 575°C, and below the temperature at which a molten phase is formed of an ore material and in an oxygen-containing atmosphere having an oxygen content below 1 % by volume. Such roasting assures vaporization and an immediate reaction of the arsenic with the additive. The additives and the water vapor need not be present in the first roasting stage but are preferably added already in the first roasting stage.

According to a preferred feature the water vapor content of the gas in the roasting reactor ranges from about 0.5 % to 10 % by weight. Arsenates having a particularly low solubility such as scorodites will be formed if the water vapor content is in that range.

The advantages set forth herein-before will be achieved even with ores which contain about 1 % to 2 % of arsenic if the roasting is effected by two stage oxygen injection. Roasting in two stages will produce particularly good results with ores which contain less than about 1 % of arsenic although equivalent results will also be obtained by proper use of arsenic immobilizing additives and oxygen content in the roasting gas.

According to a desired feature, provided that no molten phase forms on or within the ore particle, the roasting is effected at temperatures of 500°C to 750°C. In case the formation of a molten phase may reliably be avoided, and the heat consumption may be low, the arsenic will effectively be bound and immobilized and the calcine will have a good leachability.

According to a preferred feature, the substances are added in a particle size below 1 mm into the roasting reactor. That particle size will result in an effective contact and binding of arsenic present in the ore material. According to a preferred feature 80 % of these substances are added in a particle size of 10 to 50 µm. The ore is comminuted, or ground, before roasting.

Next, the ground ore is roasted in an oxygen-enriched gaseous atmosphere wherein the carbon and sulfur content is substantially completely oxidized from an initial roaster feed to a final calcine content as follows (% are percentages by weight):

Ninety-eight percent or greater of the sulfur content and 90 percent or greater of the carbon content are respectively oxidized during roasting. For extraction of gold from these refractory ores, an important consideration is the completeness of the oxidation of the carbon and sulfur values. Final carbon values at 0.05 % to 0.1 % provide good results. The same applies to sulfide sulfur levels, with final sulfide values at 0.05 % to 0.1 % providing good results.

Typically, greater than about 95 % of the arsenic is fixed in the calcine by the presence of a e.g. proper amount of iron. If desired, additional iron may be added to facilitate this conversion to an insoluble form. By having greater than a ratio from about 3.5 : 1 and e.g. 4 : 1 of iron to arsenic (molar ratio), ferricarsenate compounds formed during roasting render the arsenic in a fixed form in the calcine. Further, the ferricarsenate compound is insoluble in the subsequent leaching and from the tailings in dump storage after the gold values are extracted. Consequently, not only are the arsenic values not volatilized by the process of the present invention by retaining them in the calcine in a non-volatile form, but also these arsenic values can be retained in a form which is insoluble to the leaching and insoluble over a long period while in a dump.

In another aspect of the invention and especially when volatilized arsenic compounds are formed at higher temperatures and thereafter converted to resoluble compounds, higher temperatures are used. However, for the arsenic sequesteration without arsenic volatilization and/or solubilization, sintering is to be avoided, i.e. molten phase formation should also be prevented since molten phase silicates formed, upon even partial sintering, make the precious metal content of the ore less amenable to recovery. Further, the reaction temperatures in the reactor apparatus must be sufficiently high to optimize the oxidation reaction, particularly the oxidation of carbon- and sulfur-containing components and formation of e.g. ferricarsenate compounds. It has been found that a reaction temperature in the reaction apparatus for the oxygen-enriched gaseous atmosphere of from about 475°C to about 600°C is desirable, while a preferred temperature range is from about 500°C to about 575°C.

During roasting, the situation with respect to arsenic has more subtle ramifications since certain of its intermediate oxides, such as arsenic trioxide (As₂O₃) (boiling point 465°C), volatilize at elevated temperatures as do certain of its sulfides, such as As₂S₂ (boiling point 565°C), and As₂S₅ (sublimates at 500°C). The focus, therefore, is on the formation of insoluble compounds with the substances recited above, such as ferricarsenates compounds, e. g. scorodite, to avoid the volatilization problem and to keep arsenic values out of the process off-gas and keep these in a highly insoluble state. This control is one of the desirable results that the present invention achieves by a combination of steps including the reaction conditions, oxygen content, roasting residence time, iron content, stepwise oxygen injection, etc.

The gaseous atmosphere in which the gold ore is roasted is an oxygen-enriched gaseous atmosphere, such as oxygen-enriched air, having a total initial oxygen content, after enrichment, of less than about 65 percent (by volume), and desirably from about 25 percent (by volume) to about 60 percent (by volume); industrially a range of oxygen of 35 % to 55 % by volume is indicated for the process.

The ground ore is roasted preferably as fluidized solids in the oxygen-enriched gaseous environment. In most instances the ore concentrates will have sufficient oxidizable content that there will be an autothermal oxidation reaction during roasting. In those instances where there is not sufficient oxidizable content, such as ore which does not support an autothermal reaction, additional oxidizable content is provided by adding a comburant so that there will be a thermal reaction during roasting. Typically a low ignition point fuel is added, e. g. coal or butane/propane. Hence, desirably the ignition point should be that of propane or below.

As another embodiment an ebullating bed may be used with the overflow from the ebullating bed being constantly circulated. The reaction velocity may be lower in an ebullating fluid bed. Efficiency and control over the oxidation and reaction conditions are improved by circulating the ore as fluidized solids. An advantage of a circulating fluid bed or an ebullating fluid bed is the precise control of the bed temperature.

According to a preferred feature the roasting is performed in a circulating fluidized bed. The fluidized bed system consists of a fluidized bed reactor, a recycling cyclone and a recycling line. The suspension discharged from the fluidized bed reactor is fed to the recycling cyclone(s) of the circulating fluidized bed and substantially all solids are removed from the suspension in said cyclone(s). The solids which have been removed are returned to the fluidized bed reactor in such a manner that the solids circulated in the circulating fluidized bed systems amount to at least four times the weight of solids contained in the fluidized bed reactor.

The residence time of the ore in the oxygen-enriched gaseous atmosphere should be from about 8 to 12 minutes or more, but constrained by practical design considerations such as vessel size; pump size etc. It should be understood that residence time is a function of ore mineralogy. Control of residence time at temperature also controls silicate melting which is to be avoided. High porosity and low sintering is desirable for the subsequent leaching of gold.

Following roasting, the precious metal values are recovered from the thus-roasted ore, or calcine, by leaching. Such leaching techniques are known in the art and are described in general in U.S. patents 4 902 345 and 4 923 510. The roasted gold ore may be subjected to an oxygen or chlorine treatment after roasting and prior to leaching. This treatment may be in the form of bubbling gaseous oxygen or chlorine through a suspension or a slurry of the thus-roasted ore either in a bath at ambient pressure or in a closed vessel at ambient or elevated pressure prior to leaching the ore.

Embodiments and details of the process are described together with the drawing.

Fig. 1 and 4: show flow schemes of process variations and
Fig. 2 and 3: show diagrams with limits of arsenate formation, depending on temperature and partial pressure of oxygen, both vertical axis showing log p(O₂) in bar.

In Fig. 1 an embodiment showing a schematic industrial application of the process is illustrated. A circulating fluid bed (CFB) reactor (100) is fed from an ore preheat stage with stream (200) and the ore is roasted in the reactor. A start-up gas stream such as butane/propane has been shown entering the CFB reactor (100) at the bottom thereof through line (8). Additionally, a combined stream of oxygen-containing off-gas and fresh oxygen via preheater (102) is introduced into the CFB reactor (100). The combined stream is identified as (201). Further, a preheated, oxygen-supplemented air stream (208) is introduced in the CFB reactor (100) and is coming from the post-finishing calcine treatment which will be discussed below. A single cyclone (103) has been shown in Fig. 1, but more than one may be operated in parallel or in series to assure greater particulate removal from the off-gas. Cyclone (103) bottoms i.e. underflow collections are partially reintroduced into the CFB reactor (100) via seal pot (104). A slip stream (105) of calcined product is also taken from seal pot (104) and introduced into four preheaters (recuperators) (107) to (110) which are in a heat recovery unit (106). Air augmented with oxygen is brought up to about 450°C in heat recovery unit (106). The unit (106) consists of four preheaters in the form of fluidized beds (107), (108), (109) and (110), respectively. Because the conditions in each of the preheater beds are different, these preheaters have been identified by separate numbers. Typically, the CFB reactor (100) is operated at 550°C. The resulting calcine (of retention time of 10 minutes in reactor (100)) is introduced into the first preheater (107). The calcine is at a temperature of about 525°C and has a residence time of about 15 minutes in preheater (107); in the second preheater (108), the calcine temperature is about 475°C and residence time is about 10 minutes; in the third preheater (109) the calcine temperature is at about 420°C and the residence time is about 8 minutes; in the fourth preheater (110) the calcine temperature is about 350°C and the residence time is about 8 minutes. Air and oxygen enter these preheaters in parallel, fluidize in each the calcine and are cleaned in cyclone (112). After separation of particulates in cyclone (112), air and oxygen is introduced as stream (208) into the CFB reactor (100). A second preheater unit (not shown) of the same type may be operated in parallel to the first preheater unit (106). The seal pot (104) or a second seal pot (not shown) may feed the second preheater unit. In the data shown in Table 1, these are referred to two parallel cyclones such as (112), and two parallel seal pots such as (104).

Heated air and oxygen from all four preheaters is used and is at about 450°C as shown in Table 1. However, in addition ambient air is introduced via pump (113) into heating coils (114) immersed in the fluidized calcine in preheaters (109) and (110). This air is used to preheat in a CFB type vessel (not shown) the ore introduced as stream (200) in the CFB reactor (100). Hot air exits heating coils (114) at 200°C. As contemplated, but subject to change in the mineralogy of the ore, the balance of the energy requirement for roasting is made up by the addition of butane or pulverized coal to the CFB reactor (100). Calcine in stream (209) is quenched in water in tank (115) to a 15 % solids content and further worked-up for subsequent leaching.

Off-gases, i.e. cyclone (103) overflows are introduced through line (202) into a waste heat boiler (116) where the off-gas temperature is reduced to about 375°C, dust from the waste heat boiler (116) is introduced into the preheater unit in an appropriate place, e.g. preheater (108) and combined with calcine. From waste heat boiler (116), the off-gases are introduced via line (203) into an electrostatic precipitator (117), e.g. a five field, hot electrostatic precipitator, to remove substantially all residual dust in the off-gas. The exit temperature of the off-gas from the electrostatic precipitator is at about 350°C and the off-gas comprises about 36 % by volume of oxygen. About half of the exit gases are recycled via line (205) and blower (118) to the CFB reactor (100). This recycle is a significant benefit because the off-gas cleaning system becomes about half the size if the off-gas is recycled. Precipitates from the electrostatic precipitator are also introduced into the calcine preheat unit (106). The SO₂-laden exit gases may be sent directly to an acid plant and further amounts of oxygen introduced (as needed, for conversion of SO₂ to an acid as it is well known in the art). However, the excess oxygen rich gas from such plant may be recycled to the roasting side of the process and introduced such as in the CFB reactor (100) or used for calcine post-finishing, e. g. in fluidized beds (107), (108), (109) and (110) to aid in sulfating i.e. solubilizing the otherwise cyanide consumers.

Table 1 shows process data for a circulating fluid bed roasting plant shown in Fig. 1 with 160 t/h of ore, preheated to 200°C, being fed through line (200) into reactor (100), 4.1 t/h of said ore being water of crystallization in ore components. About 330 kg/h of coal calculated as carbon is added as fuel into reactor (100). The calcine withdrawn through line (209) in an amount of 154 t/h has a temperature of 350°C.

Stream No.	201	202	203	204	205	206	207	208
Medium	Gas	Gas	Gas	Gas	Gas	Air	Air	Air
Solids, dry t/h	- -	38.5	35
Water t/h		7.8	4.1	3.7
Gas, wet m³n/h	36,100	47,500	47,500	25,000	2,250	1,000	1,360	700
SO₂ vol%	5.7	9.15	9.15	9.15	9.15
SO₃ vol%	0.3	0.45	0.45	0.45	0.45
CO₂ vol%	6.7	10.8	10.8	10.8	10.8
O₂ vol%	56.3	36	36	36	36
N₂ vol%	18.2	23.2	23.2	23.2	23.2
H₂O vol%	12.8	20.4	20.4	20.4	20.4
Temp. °C	325	550	375	350	350	25	325	450
m³n/h = standard cubic meter per hour

The following examples illustrate the process of the present invention in the context of the recovery of gold.

Example 1:

The ore came from a random sampling of arsenic-, sulfidic-, organic carbon-containing, gold-bearing ores from the region around Carlin, Nevada. This ore, for the series of runs showed an average gold content of about 4.5 g/t of gold, an average content of 0.08 percent arsenic, 2.49 percent sulfide sulfur (2.81 percent total sulfur) and 0.79 percent organic carbon (0.84 percent total carbon). The ore had the following mineralogical and chemical analysis:

Quartz	68 Percent
Kaolinite
	10 Percent
Sericite or Illites	8 Percent
Pyrite
	5 Percent
Jarosite	4 Percent
Alunite	3 Percent
Fe_xO_y	1 Percent
Barite	1 Percent
Carbonates
	0 Percent

A chemical analysis of the ore shows an average composition as follows:

Arsenic	824 partas per million
Carbon (Total)	0.84 Percent
Sulfur (Total)	2.81 Percent
Gold	4.5 g/t
Iron	4.0 Percent
Zinc	400 parts per million
Strontium	0.02 Percent

The ore was ground in a small ball mill and it had a moisture content of about 1 percent by weight. The ground ore was placed in a simple rotating tube reactor and roasted in a batch operation to evaluate various reaction conditions using a residence time of two hours for the sake of consistency. The roasted ore, or calcine, was leached by a carbon in leach cyanidation using a dosage of 2.5 kg of sodium cyanide per ton of roasted ore and 30 grams per liter of activated carbon. The leaching was conducted in a continuously rolling bottle under the following conditions:
200 grams of calcine per leach test, 40 % solids and 24 hours leaching time.

A first series of runs was made roasting the ore with 40 % oxygen (by volume) initially in the feed gas, or gaseous atmosphere, at the following temperatures and with the following results:

Roasting Temperature (°C)	450	475	500	525	550	600
Gold Extraction (%)	84	92	86.5	82	80	76.8

When the roasted ore is treated with sodium hypochlorite at a rate of 11 kg per ton of ore and using the same leaching technique, the results were as follows:

A second run was undertaken in which the roasting temperature was held at 475°C and the retention time at 2 hours, but the percent oxygen (by volume) in the feed gas, i.e., the total initial oxygen content of the gaseous atmosphere, was varied as follows and the following percentages of gold extraction were observed:

Total Oxygen (% by vol.)	10	20	30	40
Gold extraction (%)	80	85.5	87.5	92

Example 2:

A series of air roasting tests was run in a rotating tube furnace with off-gas oxygen content. (This resulted in approximately 4 % to 6 % oxygen by volume in the off-gas). These tests used specimens of the same composition as the sample used in Example 1. The ore for this series of test runs showed an average gold content of about 4.5 kg gold per ton, 2.49 percent sulfide sulfur and 0.79 percent organic carbon. The ore was classified as sulfidic-carbonaceous ore. Sample preparation and test procedures used were the same as in Example 1. These tests demonstrate that low gold recoveries are achieved when roasting is conducted with air as the oxidizing atmosphere. These tests also demonstrate that the process of the present invention using oxygen-enriched air (such as 40 % oxygen by volume) allows better process control - at lower temperatures - for maximum gold recoveries.

It is noteworthy, particularly by comparing air roasting, such as those in Example 1, that the present invention effectively lowers the temperature at which optimum gold recovery occurs. For air roasting the maximum gold recovery is at 600°C while with roasting with oxygen-enriched air the maximum gold recovery is at 475°C. The importance of this is that the process of the present invention is more energy-economical. The percent gold extraction generally increases as the total oxygen content in the feed gas increases.

In Fig. 2 -A- is the regime for arsenate formation, and -B- is the regime for stable arsenate. In Fig. 3 -C- is the regime of arsenate volatilization in Fe₂O₃ range and -D- is the regime of stable arsenates.

According to Fig. 4 the first circulating fluidized bed system consists of the fluidized bed reactor (401), the recycling cyclone (402), and the recycling line (403). The fluidized bed reactor (401) was 0.2 m in diameter and had a height of 6 m. By a metering screw feeder, gold ore concentrate at a rate of 15 kg/h was charged through line (404) into the reactor. The concentrate contained 2.1 % arsenic, 15 % sulfide sulfur and 45 g gold per 1000 kg. The particle size was below 0.2 mm with a medium size (d₅₀) of 70 µm. Air at a rate of 11 m³n/h was fed through line (405) into the heat exchanger (406) and was preheated therein to 600°C and then fed through line (407) into the reactor (401) as a fluidizing gas. The reactor (401) was fed through line (408) with secondary air at a rate of 9 m³n/h and through line (409) at a rate of 3 m³n/h with tertiary air, which served to combust the residual sulfur in the reactor (401). By the distribution of the air supply, the oxygen potential was adjusted to be in the range in which arsenate is volatilized in the Fe₂O₃ range, see -C- in Fig. 3, above the range in which iron arsenate is formed.

The temperature in the reactor was between 700°C and 750°C. The calcine withdrawn through line (410) contained 0.02 % arsenic and 0.1 % sulfur. The leaching of the calcine resulted in a recovery of gold with a yield of 96 %. The solubility of arsenic during the leaching of gold was very low and amounted only to less than 2 mg/l.

A gas-solid suspension was fed from the reactor (401) through line (411) into the recycling cyclone (402). The solids collected there were recycled through the recycling line (403) into the reactor (401). The exhaust gas conducted in line (412) was dedusted in two cyclones (not shown) and in a candle filter (413) at about 600°C. The collected dusts were returned to the reactor (401) through line (414). The dust-free exhaust gas contained SO₂ and As₂O₃ and was fed through line (415) to the fluidized bed reactor (416) of a second circulating fluidized bed system.

The reactor (416) was 0.16 m in diameter and had a height of 4 m. It was heated by indirect electric heating. Hematitic iron ore having a particle size below 0.5 mm, with a medium size of 30 µm, was charged through line (417) at a rate of 0.3 kg/h. Fluidizing air at a rate of 15 m³n/h was fed into the reactor (416).

The suspension leaving through line (419) was adjusted to contain 6 % oxygen and 4 % water vapor so that the conditions for a formation of stable arsenates (Fig. 2) were established. To adjust a water vapor content of 4 %, the moisture content of the iron ore charged through (417) was controlled in dependence or the water vapor content of the gas entering through line (415) and of the fluidizing air entering through line (418).

The solids collected in the recycling cyclone (420) were returned through the recycling line (421) into the reactor (416). The arsenic-free roaster gas contained 9.1 % SO₂ and was fed through line 22 to a gas purifier and subsequently to a plant for producing sulfuric acid. The solid material which was discharged through line (423) from the reactor (416) contained 17.3 % arsenic. Leaching tests with water showed that the solubility of arsenic was less then 1 mg/l.

According to a preferred feature of the embodiment shown in Fig. 4, the dust-containing gases which contain arsenic vapor and arsenic compound vapor are produced by roasting e.g. of sulfide materials which contain iron and arsenic. Such materials are roasted in the Fe₂O₃ range at temperatures of 500°C to 1100°C in a first stage, which is supplied with oxygen-containing gases. In these materials, arsenic is volatilized mainly as arsenic oxides and part of the sulfur content is volatilized as elementary sulfur. Solids are removed from the exhaust gas at temperatures above the condensation temperature of the volatilized components, and the solids are discharged as calcine.

Claims

A process for treating ore particles containing gold and having arsenic-, carbon- and sulfur-containing components which comprises:
roasting said ore particles in a roasting reactor in presence of at least one substance selected from the group consisting of a free oxide, carbonate, sulfate, hydroxide or chloride of calcium, magnesium, iron and barium, or pyrite, at temperatures in the reactor of 475 to 900°C in presence of water vapor up to 10 % by weight of ore, without formation of a molten phase on the surface of said ore particles, said substance being present in said reactor in a hyperstoichiometric amount to react with arsenic in said ore particles to form stable arsenates, feeding gas into said reactor with an initial oxygen content of 25 % by volume to 65 % by volume and maintaining an oxygen-containing atmosphere of at least 1 % by volume oxygen, referenced to a basis amount of oxygen in air, in said reactor, recovering thus-roasted ore particles as arsenate-containing calcine, said calcine being amenable to recovery of gold by leaching, and withdrawing from the roasting reactor an exhaust gas containing stable arsenates and at least 1 % by volume of oxygen.
A process of claim 1, wherein said ore particles are treated in said reactor in a fluidized bed.
A process of claim 2, wherein said fluidized bed is a circulating fluidized bed.
A process of claim 1, wherein temperatures in said reactor are from 475°C to 600°C.
A process of claim 1, wherein in said reactor a ratio of iron to arsenic is maintained which is sufficient to form a ferricarsenate, but not less than 3.5 moles of iron to one mole of arsenic.
A process of claim 5, wherein the ferricarsenate formed is scorodite or scorodite-like compounds.