FIELD OF INVENTION

The present invention relates to a process in accor ^¬ dance with claim 1. Further, the present invention relates to a plant in accordance with claim 19. More specifically, the invention relates to the off-gas handling and treatment in said process and plant.

BACKGROUND OF INVENTION

Known technique in the art is depicted e.g. in the ar ^¬ ticles "Roasting developments - especially oxygenated roasting, Developments in Mineral Processing, Volume 15, 2005, Pages 403-432 K.G. Thomas, A. P. Cole", "Roasting of gold ore in the circulating fluidized-bed technology Developments in Mineral Processing, Volume 15, 2005, Pages 433-453, J. Hammerschmidt , J. Gunt- ner, B. Kerstiens", and patent documents WO2010/003693, US 6,482,373, AU 650783, US 4,919,715.

Copper and gold concentrates containing arsenic are preferably processed by dearsenifying roasting before further treatment by smelting in a copper plant or cyanide leaching.

The dearsenifying roasting is made by controlling the oxygen potential during arsenic volatilization to maintain iron both as magnetite and pyrrhotite. The calcine is thereafter further processed by conven- tional matte smelting in case the raw material con ^¬ sists of a copper containing concentrate. Gold rich calcine are often processed by cyanide leaching but the leaching is only efficient, if the calcine is dead roasted or sulphating roasted. A conventional method to treat arsenic rich gold concentrates is therefore a two stage roasting process whereby both stages consist of fluidized beds. The first fluidized bed is the dearsenifying step, op ^¬ erating at a very low oxygen potential, and the second fluidized bed is the dead roasting or sulphating step, operating with an excess of oxygen. The process gas leaving the dearsenifying roasting will contain sulphur rich gas compounds such as elementary sulphur, hydrogen sulphide and arsenic sulphide while the process gas leaving the second oxidiz ^¬ ing roasting will contain oxygen and oxidized com- pounds like SO ₃ .

The roasting process gas is normally further processed by separation of calcine and process gas in cyclones, post combustion, gas cooling and dust cleaning in electrostatic precipitator and possibly bag filter and finally conversion of SO ₂ to sulphuric acid.

Known problems in the further processing are: - The forming of accretions which could fall down and damage equipment or block gas passage. The accretions are e.g. formed due to local under cooling of the process gas or on cold surfaces in the equipment.

- Condensation of arsenic on cold surfaces which will form said accretions.

- Condensation of acid mist on cold surfaces which will cause corrosion and contribute to formation of accretion .

- If the acid mist quantity is high, the cost for ef ^¬ fluent treatment will also be high.

- Heat recovery of the system has often been limited to the production of saturated steam which is less fa ^¬ vourable for the production of electric energy.

These problems were solved before in the following ways :

The post combustion air can for instance be added at the cyclone exit. However, in some cases the post com ^¬ bustion can cause accretions in gas ducts and this is more likely if the combustion is made with a large amount of air in ambient temperature.

The forming of accretions on cold surfaces in the equipment is normally solved by the use of preheated air, which requires separate heating equipment with increased investment cost and operating costs (mainte ^¬ nance and possibly heating fuel) . Forming of accre ^¬ tions in the equipment is normally avoided by insulat- ing the equipment well so that no cold surfaces exist, although it is also accepted that accretions will form where the insulation is damaged or not properly done. Gas cooling during two-stage roasting can either be made by direct cooling with water injection in a cooling tower or through indirect cooling through cooling coils in the fluidized beds and by conventional steam boiler. Lead and arsenic contents in the concentrate and SO ₃ concentration in process gas influence suit ^¬ able cooling method, since these elements can cause formation of accretions on cooling surfaces.

Example of compounds that forms accretions are elemen- tal lead at cooling coils in the first dearsenifying stage and SO ₃ or arsenic trioxide at the boiler tubes of the steam boiler. It is today generally accepted that high SO ₃ concentrations cause higher cost in the effluent treatment plant.

Corrosion of the equipment is normally avoided by in ^¬ sulating the equipment well so that no or little SO ₃ condensation occurs, although it is also accepted that corrosion will occur over time and for example where the insulation is damaged or not properly done. It would be best to be able to avoid high SO ₃ concentra ^¬ tions in the process gas. This is today, to some ex ^¬ tent, done by controlling the process with modern control systems. Further reductions would be an advan- tage. Heat recovery in form of steam is today done by steam generation coils in the fluidised bed itself often without any superheating. A normal steam boiler is sometimes used in the process gas stream, but with similar risks of accretions forming and corrosion as described .

OBJECT OF INVENTION The object of the invention is to eliminate the above mentioned drawbacks.

A particular object of the invention is to provide a process and plant wherein the risk of corrosion and forming of accretions are reduced during post combus ^¬ tion and down-stream in the gas cleaning system. Further, an object of the invention is to provide a proc ^¬ ess and plant wherein SO ₃ concentration in the process gas can be reduced and the risk of corrosion damages by SO ₃ is reduced. Further, an object of the invention is to provide a process and plant wherein the effluent handling costs are reduced. Further, an object of the invention is to reduce the total process gas volume which saves both investment cost and operating costs

SUMMARY OF INVENTION

The process according to the invention is characterized by what is set forth in claim 1. Further, the plant according to the invention is characterized by what is set forth in claim 19. The invention concerns a process for treating ore concentrate particles containing valuable metal and hav ^¬ ing at least arsenic and sulfur containing components. The process comprises roasting the concentrate parti- cles in a first roasting step operating with a low oxygen potential for dearsenifying the concentrate. The off-gas leaving the first roasting step is treated to separate calcine and a sulphide rich first process gas component. Further, the process comprises roasting the calcine led from the first roasting step in a sec ^¬ ond roasting step operating with an excess of oxygen. The off-gas leaving the second roasting step is treated to separate calcine and a second process gas component. Further the process comprises post combus- tion of the process gas component and treating the process gas in subsequent gas cooling and dust removal steps .

In accordance with the invention the process further comprises forming of a gas mixture of the first proc ^¬ ess gas component and the second process gas component which is oxygen containing warm oxidizer gas, and post combustion of said gas mixture in a post combustion chamber, said post combustion operating with said re- ducing and sulphide rich first process gas component and said second gas component as oxidizer, in order to decompose SO ₃ in the gas mixture to reduce the SO ₃ con ^¬ tent in the exit gas exiting post combustion chamber and to reduce the risk of accretion formation and cor- rosion in the post combustion chamber and in subsequent steps. Finally the process comprises exposing the exit gas to subsequent gas cooling and dust re ^¬ moval steps.

In an embodiment of the process additional post com- bustion air is inserted into the post combustion chamber after main post combustion reaction zone to avoid SO ₃ formation.

In an embodiment of the process residence time of the gas in the post combustion chamber is arranged to be long enough to ensure complete combustion of all eas ^¬ ily oxidized compounds, such as hydrogen sulphide, elemental sulphide, arsenic sulphide, elemental arse ^¬ nic, present in the gas mixture.

In an embodiment of the process the long enough resi ^¬ dence time is provided by arranging a sufficient vol ^¬ ume of the post combustion chamber. In an embodiment of the process the process comprises a step of controlling the temperature in the post com ^¬ bustion chamber.

In an embodiment of the process the step of control- ling the temperature in the post combustion chamber includes direct injection of cooling water into the post combustion chamber.

In an embodiment of the process the step of control- ling the temperature in the post combustion chamber includes indirect cooling of the walls of the post combustion chamber with steam to keep the temperature of the walls of the post combustion chamber above the condensation temperature of AS ₂ O ₃ or SO ₃ .

In an embodiment of the process said indirect cooling is implemented by a radiation cooler formed by a dou ^¬ ble shell structure of the walls of the post combus ^¬ tion chamber the steam flowing within the double shell structure of the wall whereby hot gas within the cham ^¬ ber by radiation converts saturated steam to super- heated steam.

In an embodiment of the process heat is recovered from the superheated steam for internal or external use of energy .

In an embodiment of the process the first roasting step is implemented in a first fluidized bed reactor and the second roasting step is implemented in a sec ^¬ ond fluidized bed reactor.

In an embodiment of the process the process includes extracting heat from the fluidized bed of the first fluidized bed reactor. In an embodiment of the process the process includes extracting heat from the fluidized bed of the second fluidized bed reactor.

In an embodiment of the process the off-gas leaving the first roasting step is separated by at least one first cyclone separator. In an embodiment of the process the off-gas leaving the second roasting step is separated by at least one second cyclone separator. In an embodiment of the process the second process gas component which is mixed with the first process gas component, is hot, preferably the temperature of the second process gas component is about 650 ... 700°C, to ensure fast reaction with the first process gas compo- nent .

In an embodiment of the process the additional combus ^¬ tion air is preheated to at least 200°C, preferably by the heat extracted from air cooled conveyors or cal- cine coolers.

In an embodiment of the process the calcine is com ^¬ pletely roasted in the second roasting step. In an embodiment of the process the valuable metal is any of platinum-group metals, gold, silver, copper or zinc .

The invention also concerns a plant for treating ore concentrate particles containing valuable metal and having at least arsenic and sulfur containing compo ^¬ nents. The plant comprises a first roasting reactor operating with a low oxygen potential for dearsenify- ing the concentrate and a first separator arranged to receive off-gas from the first roasting reactor and to separate from said off-gas calcine and a sulphide rich first process gas component. Further, the plant com- prises a second roasting reactor arranged to receive calcine from the first roasting reactor and from the first separator, said second roasting reactor operating with an excess of oxygen, and a second separator arranged to receive off-gas from the second roasting reactor and to separate from said off-gas calcine and a second process gas component. Moreover, the plant comprises means for post combustion of the process gas component and gas cooling and dust removal equipment for further treating the process gas.

In accordance with the invention the plant further comprises means for forming a gas mixture of the first process gas component and the second process gas comp- ponent which is oxygen containing warm oxidizer gas. Said means for post combustion includes a post combus ^¬ tion chamber for post combustion of said gas mixture, said post combustion chamber operating with said reducing and sulphide rich first process gas component and said second process gas component as oxidizer, in order to decompose SO ₃ in the gas mixture to reduce the SO ₃ content in the exit gas exiting post combustion chamber and to reduce the risk of accretion formation and corrosion in the post combustion chamber and in subsequent gas cooling and dust removal equipment.

In an embodiment of the plant the plant comprises a first pipeline for guiding the first process gas com ^¬ ponent from the first separator to the post combustion chamber, and that the means for forming the gas mix ^¬ ture comprises a plurality of connections at several positions along the first pipeline for introducing the second process gas component via said connections into the stream of the first process gas component.

In an embodiment of the plant the post combustion chamber comprises a first chamber part forming a reac ^¬ tion chamber to which the gas mixture is fed; a second chamber part comprising means for inserting additional combustion air; and a third chamber part from which the gas exits from the post combustion chamber.

In an embodiment of the plant the post combustion chamber comprises cooling means for controlling the temperature in the chamber. In an embodiment of the plant the cooling means com ^¬ prises a water spraying nozzle for injecting cooling water into the post combustion chamber for direct cooling . In an embodiment of the plant the cooling means com ^¬ prises a radiation cooler formed by a double shell structure of the walls of the post combustion chamber for indirect cooling of the walls of the post combus ^¬ tion chamber with steam streaming between the shells.

In an embodiment of the plant the plant comprises a first heat exchanger for extracting heat from the flu- idized bed of the first fluidized bed reactor. In an embodiment of the plant the plant comprises a second heat exchanger for extracting heat from the fluidized bed of the second fluidized bed reactor. In an embodiment of the plant the plant comprises a third heat exchanger for recovering heat from the superheated steam generated by the radiation cooler for internal or external use of energy.

The advantage of the invention is that it solves ac ^¬ cretion problems during post combustion and downstream in the gas cleaning system as well as gas cool- ing during post combustion. The risk for corrosion damages caused by SO ₃ is also reduced. The suggested mixing of the process gases reduces the amount of SO ₃ , thus reducing corrosion risks and costs for effluent treatment. The suggested mixing of the process gases gives a cost-free pre-heated post combustion gas (oxy ^¬ gen containing) . Only a minor part needs, possibly, to be heated in another way. The pre-eating gives no forming of accretions neither in the post combustion chamber nor in the following equipment without the need for extra, costly, pre-heating equipment. The suggested post combustion unit makes it possible to insert steam-superheating coils after the post combus ^¬ tion unit without the risk for forming of accretions or corrosion by SO ₃ . The system can superheat steam without the need of external steam superheating (sepa ^¬ rately fired superheater) . The production of steam and superheating is well balanced to the need for process gas cooling, this will avoid a costly and complicated control systems (as needed for an external superhea- ter) . LIST OF DRAWINGS

The accompanying drawing, which is included to provide a further understanding of the invention and consti- tutes a part of this specification, illustrates an em ^¬ bodiment of the invention and together with the de ^¬ scription helps to explain the principles of the in ^¬ vention . Figure is a schematic flow sheet of one embodiment of a process and plant according to the invention.

DETAILED DESCRIPTION OF INVENTION The flow sheet in Figure shows a two stage roasting plant with its off-gas handling system. This process layout is suitable when the raw material is an arsenic contaminated sulfide ore concentrate which valuable particles contain precious metals like gold and sil- ver. Copper and zinc can also be present in large or small quantities. The concentrate is fed at the inlet 29 to the first roasting step 1 which is implemented in a first roasting reactor 16. The first roasting reactor 16 is a first fluidized bed reactor. The first roasting step 1 is a dearsenifying step operating at a very low oxygen potential. A first cyclone separator 18 is arranged to receive process gas with a lot of calcine from the first roasting reactor 16 and to sep ^¬ arate from said process gas the calcine and a sulphide rich first process gas component 2 with less calcine. The calcine contains the valuable metals and has a low content of arsenic. The first process gas component 2 leaving the dearsenifying roasting contains sulphur rich gas compounds such as elementary sulphur, hydrogen sulphide and arsenic sulphide. A second roasting reactor 17 is arranged to receive calcine from the first roasting reactor 16 and from the first separator 18. The second roasting step 3 is made in a second roasting reactor 17 which is the second fluidized bed reactor wherein the calcine is com- pletely roasted, ie. dead roasted or sulfating roasted, and it operates with an excess of oxygen. The meaning of dead roasted or sulfating roasted material is that all sulfide sulfur has been removed and any remaining sulfur consists of sulfates. A second cy- clone separator 19 is arranged to receive process gas from the second roasting reactor 17 and to separate from said process gas calcine and a second process gas component 4. The second process gas component 4 leav ^¬ ing the second oxidizing roasting will contain oxygen and oxidized compounds like SO ₃ .

The calcine from the second roasting reactor 17 and from the second cyclone separator 19 is fed via outlet 30 to further processing of the calcine, cooling, leaching etc. (not shown in Figure) .

The plant and the process further comprises means for forming a gas mixture of the first process gas compo ^¬ nent 2 and the second process gas component 4 which is oxygen containing warm oxidizer process gas. The means for forming a gas mixture may be arranged so that a plurality of connections 21 is arranged at sev ^¬ eral positions along the first pipeline 20 which leads the first process gas component 2 from the first sepa- rator 18 to the post combustion chamber 6. The second process gas component 4 may be introduced via said plurality of connections 21 into the stream of the first process gas component 2. This improves mixing and reduces combustion time.

The second process gas component 4 is hot, typically 650 - 700 °C, which ensures a fast reaction with the roaster gas. When warm additional post combustion air 12 is used, air cooled conveyors (not shown in Figure) or calcine coolers (not shown in Figure) could be used as air preheaters to preheat the air to approximately 200 °C.

Post combustion of the gas mixture is implemented in a post combustion chamber 6. The post combustion chamber 6 operates with said reducing and sulphide rich first process gas component 2 and said second process gaas component 4 and, if needed, with warm air 12. Condi ^¬ tions during post combustion, e.g. an increased gas temperature and presence of reducing gas components, will make it possible to decompose SO ₃ and thus reduce the SO ₃ content in process gas at the post combustion exit 7. This is an important feature since it will re ^¬ duce the risk of acid condensation and the subsequent formation of sticky dust, especially in the conditioning tower 10 and bag filter 11 that operate close to or below the SO ₃ dew point. The post combustion chamber 6 comprises a first cham ^¬ ber part 22 forming a reaction chamber to which the gas mixture is fed. Further the post combustion cham- ber comprises a second chamber part 23 having means for inserting additional combustion air 12. Additional post combustion air 12 is inserted into the post com ^¬ bustion chamber 6 after main post combustion reaction zone Z to avoid SO ₃ formation. The gas exits from the post combustion chamber 6 via a third chamber part 24. The exit gas 7 exiting the post combustion chamber 6 is led to conventional gas cooling and dust removal steps 8 to 11, which may include leading the exit gas 7 via a cooling tower 8 to an electrostatic precipita- tor 9 and then to a bag filter 11.

The post combustion chamber 6 must have such a volume that the residence time is long enough to ensure com ^¬ plete combustion of all easily oxidized compounds pre- sent in the gas mixture, e.g. hydrogen sulphide, ele ^¬ mental sulphide, arsenic sulphide and elemental arse ^¬ nic .

The reaction temperature during post combustion is in- creased due to exothermic reactions but must be con ^¬ trolled to avoid over-heating since that should cause formation of partially melted and sticky material in the post combustion chamber 6. The temperature control of post combustion is either made by direct injection of cooling water via a water spraying nozzle 25 into the post combustion chamber 6 or by indirect cooling of walls 15 of the post combus ^¬ tion chamber 6 by means of steam superheating or by a combination of both. Direct injection of cooling water is used when no energy recovery is included in the roaster flow sheet while indirect steam cooling is used when energy re ^¬ covery 28 is included. Direct water injection in combination with indirect cooling steam cooling can be necessary to control the flame temperature during combustion to avoid formation of sticky material. The indirect steam cooling in the post combustion walls 15 will serve three purposes, the walls will be warm enough so that no accretions will form or corrosion occur, and the walls will be cold enough to avoid overheating of both the walls and the post combustion gas and at the same time produce superheated steam.

The indirect cooling means comprises a radiation coo ^¬ ler 14 formed by a double shell structure of the walls 15 of the post combustion chamber 6 for indirect cool ^¬ ing of the walls of the post combustion chamber with steam streaming between the shells. The walls 15 may be made of cooling panels that by radiation from the hot gas inside the chamber 6 converts saturated steam within the walls 15 to superheated steam. By using steam can be ensured that the wall temperature of the post combustion chamber 6 doesn't drop to levels where condensation could form at the walls. It is also possible to have indirect steam cooling, partly or only, by inserting superheating coils into the gas stream in the third part 24 of the post com ^¬ bustion chamber 6 or in the following gas duct. As the SO ₃ concentration is reduced, any metallic lead vapour in the roaster gas has been oxidized to non-condensing lead compounds like PbO and the process gas has a tem ^¬ perature that is controlled at a level giving no or little sticky materials in the post combustion cham- ber. Correct insulation of the post combustion chamber would then be needed.

If lead free concentrates are treated, a first heat exchanger 26 may be provided to extract heat from the fluidized bed of the first fluidized bed reactor 16 (steam circuit shown with dot-and-dash line in Figure) , and a second heat exchanger 27 for extracting heat from the fluidized bed of the second fluidized bed reactor 17 (steam circuit shown with a solid line in Figure) . The heat exchangers 26 and 27 may be steam coils 26, 27 with the same superheating possibilities as described above.

If lead containing concentrates are roasted, only the second fluidized bed reactor 17 can be provided with the steam coil 27, because lead vapour would conden ^¬ sate on the steam coil 26 in the first reactor 16 which would cause accretions and harm the roasting operation. In that case heat is extracted only from the fluidized bed of the second fluidized bed reactor 17. Preferably the superheated steam generated by the ra ^¬ diation cooler 14 is led via a third heat exchanger 28 for recovering heat from the superheated steam for internal use in the process itself, e.g. steam heating of leaching tanks, and/or external use of energy. The external use may include e.g. production of electric energy by a turbine.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. For ex ^¬ ample, although a two-stage roasting process has been described, it should be understood that the process may include more than two, e.g. three or more, roast- ing stages, and, respectively, the plant may include more than two, e.g. three or more, roasting reactors. The invention and its embodiments are thus not limited to the examples described above, instead they may vary within the scope of the claims.