NICKEL POWDER DESULFURISATION

Abstract: A process and apparatus for desulfurisation of nickel powder is disclosed in which the nickel powder is heated in a hydrogen-containing atmosphere to promote the formation of hydrogen sulfide gas. The hydrogen sulfide gas is then removed using a desulfurisation agent. Hydrogen gas is used as a transfer agent to transport the sulfur from the nickel powder to a suitable desulfurisation agent. In this way the sulfur content of the nickel powder can be substantially reduced. The apparatus (30) includes a rotary kiln (32) in which the off-gases are recycled back into the kiln via an adsorbent H>2<S filter (34). The nickel powder is fed to the rotary kiln (32) via a nickel powder feed hopper (35) which pre-heats the powder prior to entry into the kiln. The kiln is indirectly heated using natural gas. A gas blower (36) maintains a flow of hydrogen containing gas through the rotary kiln in a counter-current direction relative to the direction of transport of the nickel powder through the kiln. Filter (34) incorporates a filter membrane made from a suitable non-volatile desulfurising agent such as calcium, magnesium metal or calcium hydroxide which can be periodically replaced as required. The desulfurised nickel powder exits from the rotary kiln (32) via a discharge hopper (42) where some cooling of the nickel powder occurs. From there the desulfurised nickel powder is fed to a hot briquetting machine (44) which presses the powder into small briquettes. A significant improvement in the production of on specification nickel briquettes can be achieved.

Description: NICKEL POWDER DESULFURISATION FIELD OF THE INVENTION The present invention relates to a process and apparatus for desulfurisation of nickel powder and relates particularly to the use of a desulfurisation agent for the removal of hydrogen sulfide gas produced on heating the nickel powder in a hydrogen containing atmosphere. BACKGROUND TO THE INVENTION At WMC Resources Kwinana Nickel Refinery (KNR) nickel powder is produced in accordance with the Sherritt Gordon Process and pressed into briquettes for sale. The dried nickel powder currently discharges from the wet metals plant at KNR into storage tanks before being fed into briquette making machines. Current refinery operation averages 0.029% sulfur in washed nickel powder. Traces of ammonium sulfate, ammonium sulfamate, nickel sulfate and other sulfur compounds which remain after washing the powder contribute to residual levels of sulfur in the briquettes. This residual sulfur is removed by reduction with hydrogen at high temperature. The briquetted nickel powder is processed through sinter furnaces to produce a final product of <0.008% sulfur. Hydrogen is used to reduce sulfur compounds in the briquettes to hydrogen sulfide (H 2 S) without loss of nickel metal or product purity. The stoichiometric requirement of hydrogen for removal of sulfur from the briquettes constitutes less than 1% of total furnace hydrogen usage. Excess hydrogen is incinerated. The efficiency of hydrogen utilisation is thus poor. Furthermore, up to 25% of briquettes are off-specification (high S content) and must therefore be recycled through the furnace. It is believed that H 2 S diffusion is inhibited through the compacted briquettes thus further diminishing the effectiveness of the process. SUMMARY OF THE INVENTION The present invention was developed with a view to providing a process and apparatus for desulfurising the nickel powder prior to briquetting which is more efficient than the above-noted prior art technique. Throughout this specification the term "comprising" is used inclusively, in the sense that there may be other features and/or steps included in the invention not expressly defined or comprehended in the features or steps subsequently defined or described. What such other features and/or steps may include will be apparent from the specification read as a whole. According to one aspect of the present invention there is provided a process for desulfurising nickel powder, the process comprising the steps of: heating the nickel powder in a hydrogen containing atmosphere to promote the formation of hydrogen sulfide gas; and, removing the hydrogen sulfide gas using a desulfurising agent whereby, in use, the sulfur content of the nickel powder can be substantially reduced. Preferably during said heating step the nickel powder is transported substantially continuously from a nickel powder feedpoint to a desulfurised nickel powder discharge point. Preferably during said heating step the nickel powder is exposed to a hydrogen containing gas flowing in a counter-current direction relative to the direction of transport of the nickel powder. Advantageously hydrogen gas is recirculated in the process following the removal of the hydrogen sulfide thereby significantly improving the hydrogen utilisation. Preferably the nickel powder is heated up to temperatures within the range of 400° to 950°C, more preferably between 650° and 800°. Preferably a non-volatile desulfurising agent is employed. Preferably the desulfurising agent is a solid material which reacts with hydrogen sulfide. Suitable adsorbing desulfurising agents include calcium and magnesium hydroxide, carbonate and oxide. Advantageously the desulfurising agent can be regenerated by desorbing the hydrogen sulfide at lower temperatures. Advantageously the process includes the further step of forming the desulfurised nickel powder into briquettes. Preferably the nickel powder is formed into briquettes upon exiting from the desulfurised nickel powder discharge point, while the powder is still hot. According to another aspect of the present invention there is provided an apparatus for desulfurising nickel powder, the apparatus comprising: a reactor for heating the nickel powder in a hydrogen containing atmosphere to promote the formation of hydrogen sulfide gas; and, a means for removing the hydrogen sulfide gas using a desulfurising agent whereby, in use, the sulfur content of the nickel powder can be substantially reduced. Preferably said reactor is a kiln through which the nickel powder is transported substantially continuously from a nickel powder feed point to a desulfurised nickel powder discharge point. Preferably said reactor is a rotary kiln. Preferably the apparatus further comprises a device for blowing a hydrogen containing gas through the kiln in a counter-current direction relative to the direction of transport of the nickel powder. Preferably said means for removing the hydrogen sulfide gas comprises a filter having said desulfurising agent provided in connection therewith. Preferably said filter is provided in a gas by-pass stream through which the hydrogen containing gas is recycled after is passes through the kiln. Alternatively said means for removing the hydrogen sulfide gas comprises said desulfurisation agent located within the kiln adjacent said nickel powder. Preferably a non-volatile desulfurisation agent is employed. Preferably the desulfurisation agent is a solid material which reacts hydrogen sulfide. Suitable adsorbing desulfurisation agents include alkaline earth metals, metal oxides, carbonates and hydroxides such as Ca, Mg, CaO, MgO, CaCO 3 , Ca(OH) 2 , MgCO 3 and Mg(OH) 2 . BRIEF DESCRIPTION OF THE DRAWINGS In order the facilitate a more comprehensive understanding of the process and apparatus according to the invention a preferred embodiment of the process and apparatus for desulfurising nickel powder will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a scanning electron micrograph (SEM) image of particles of nickel powder; Figure 2 is a schematic diagram of a laboratory scale rotating tube furnace employed to obtain experimental results; and, Figure 3 is a schematic diagram of a possible embodiment of the process and apparatus for desulfurising nickel powder in accordance with the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Figure 1 illustrates a typical sample of washed and dried nickel powder viewed under SEM examination. Individual grains of the nickel powder resemble small cauliflowers, with some of the grains having fissures leading into the interior of the particles. The bulk of the powder (86%) is in the size range between 212 and 106μm, with 99% of the particles passing through a screen aperture of 300μm. A sulfur analysis of two size fractions between 212-150μm and 150-106μm yielded 0.017% and 0.029% sulfur content respectively. The present invention is based on the discovery that hydrogen gas can be used as a transfer agent to transport the sulfur from the nickel powder to a suitable desulfurising agent. Desulfurisation of the nickel powder occurs through the formation of hydrogen sulfide when the powder is contacted with a hydrogen containing gas. Laboratory test work indicated that the extent of the desulfurisation reaction is determined more by gas equilibrium than kinetic considerations. Any suitable method of contacting the nickel powder with a hydrogen containing gas may be employed. In experimental test work, a drop tube furnace, a horizontal tube furnace, a fluidised bed and a rotating tube furnace were all employed with varying degrees of success. However, in view of the observation that better hydrogen utilisation can be achieved in counter-current flow than in co-current flow, the use of an indirectly heated rotary kiln is favoured over that of a fluidised bed, as it is difficult to achieve counter- current flow with a fluidised bed. Furthermore, in a rotary kiln the capital and operating costs of a blower to maintain the flow of gas would be less than for a fluid bed, in which the blower has to create sufficient pressure drop to levitate the bed. In addition, it is believed that should agglomeration of the nickel powder occur, it can be more easily dealt with in a rotary kiln than in a fluid bed. Laboratory test results were produced by using an experimental rotating tube furnace as illustrated schematically in Figure 2. The rotating tube furnace assembly 10 as illustrated in Figure 2 consists of an electrically wound horizontal tube furnace 12 capable of producing temperatures of up to 1200°C. Within the furnace 12 a long pythagoras tube 14 (38mm ID x 46mm OD x 500mm) was provided, capable of being rotated to agitate the contents. Approximately 115mm of the tube 14 extended outside the furnace 12 and its open end was fitted with a plug and rotary gas inlet/exit assembly 16. The other closed end of the tube 14 was located in the central hot zone of the furnace 12. A 100mm diameter aluminium pulley wheel 18 was attached to the part of the tube extending from the furnace 12, and was coupled to a 30mm diameter pulley on a variable speed electric drive 20 by means of a suitable pulley. The speed of electric drive 20 was selected to ensure that the speed of rotation of the tube 14 was approximately 20rpm. The furnace assembly 10 was tilted to an incline of 30mm in 580mm so that feed solids would move towards and remain in the closed end of the tube within the hot zone of the furnace 12. The cool open end of the tube 14 was sealed with a rubber bung which could be removed to add feed solids and remove products. Gas entered the tube 14 through a brass rotating seal which penetrated the centre of the bung. High purity hydrogen gas (supplied by BOC Gases), reduced to 100 kpa prior to a needle valve, was metered through a mass flow (0-0.3 Nm 3 h "1 ) meter 22, and blended with a flow of nitrogen gas controlled by a mass flow (0-0.6 Nm 3 h '1 ) controller 24. Both gas flows were calculated to give a total flow of approximately 0.17 Nm 3 h "1 at the required gas composition. The gas mixture was transferred directly to the hot end of the pythagoras tube 14 via a quarter inch stainless steel tube 26 supported concentrically within the pythagoras tube 14 by a freely rotating gas inlet. The stainless steel tube 26 also passed concentrically through a half inch stainless steel Tee and the rotary gas inlet/exit assembly 16. The exhaust gases flow back to the cool end of the pythagoras tube 14, through an annular space in the rotary gas inlet/exit assembly 16 and the half inch stainless steel Tee, then out via a side branch before passing through a gas bubbler (not shown) containing lead acetate solution to remove hydrogen sulfide and then vented to a forced draft exhaust. The pythagoras tube 14 was purged with a flow of 2 L/min of nitrogen. Approximately 30g of nickel powder was pre- weighed in a glass boat and transferred into a stainless steel tube for insertion into the pythagoras tube 14 which had been pre-heated to the experimental temperature. The rubber bung was removed to allow the stainless steel tube to be inserted into the pythagoras tube 14 to transfer the feed into the tube 14. Purging with nitrogen continued during feeding. The bung was replaced, rotation commenced and the timer started for six minutes of nitrogen purging while the powder made its way to the hot end of the tube 14. The hydrogen control valve was kept closed while the cylinder was opened and the mass flow indicator warmed up. At the end of the purge cycle, the timer was restarted and hydrogen flow adjusted to give a total flow of 2.8 L/min and hydrogen in nitrogen concentrations of 14.8%, 50% and 100% hydrogen. At the end of the reduction period the furnace was allowed to cool, the hydrogen flow stopped and nitrogen flow increased to 2 L/min. Rotation of the tube 14 continued while the furnace cooled, and when it was sufficiently cool (less than 300°C) rotation was stopped and the inner tube containing the sample removed. Sulfur analysis of the desulfurised nickel powder from the rotating furnace tube experiments are listed in Table 1 below. Table 1 Average values of the sulfur content and pH 2 S/pH 2 ratios have been calculated from the results in Table 1 for the rotating tube experiment at 800°C, with 30g of nickel powder and 2.8 L/min of hydrogen containing gas (15% H 2 ) and are shown in Table 2. Table 2 Tests were also carried out to determine the feasibility of using desulfurising agents in a hydrogen containing atmosphere to adsorb the H 2 S produced by the desulfurisation of nickel powder and thus regenerate the hydrogen in situ in the kiln. Two desulfurising agents, namely slaked lime and Mg ribbon, were separately trialled using the rotating tube furnace assembly 10. The nickel powder and the desulfurising agent were placed in separate alumina boats in close proximity to each other. Two temperatures (500°C and 600°C) were tested, and the results are shown in Table 3 below. Table 3 * There may have been oxide on the surface of the Mg in this experiment. The materials were brought up to temperature under argon (0.5 L/m). When the furnace reached the desired temperature, the flow of argon was cut off, and a flow of 1 L/m of hydrogen was maintained for two minutes to purge the argon. The flow of hydrogen was then cut back to just maintain a positive pressure within the pythagoras tube 14 (less than 0.2 L/min). At the end of the time the hydrogen was purged and the materials allowed to cool under a flow of argon. The flow of hydrogen was too small to measure conveniently, however estimates of the flow rate through a bubbler gave values less than 7.5 cc/min. On the basis of this estimate and the weight of nickel powder used in the test (lOgm), hydrogen utilisation was calculated to be in excess of 124 tNi/tH 2 . Better hydrogen utilisation figures could have been produced if shorter residence times had been employed. From Table 3 above it can be seen that on specification nickel powder was produced at low hydrogen flow rates, as well as at relatively low temperatures using slaked lime. From the test results, it is evident that H 2 S is absorbed onto desulfurising materials like slaked lime or magnesium, according to the following equation: Ca(OH) 2 + H 2 S → CaS + 2H 2 O ( 1 ) From equation 1 it can be seen that some hydrogen is consumed as well as water vapour produced. For 54,000 tpa Ni production containing 0.025% S the complete desulfurisation would theoretically consume 0.8 tpa H 2 ie, 67,500 tNi:tH 2 . Using magnesium or calcium metal for the desulfurising agent, the H 2 S can be adsorbed according to the following equation: H 2 S + Ca/Mg → Ca MgS + H 2 (2) In a properly engineered system (ie. sealed with no purge requirement) the hydrogen would be completely regenerated and there would be zero consumption of hydrogen. This test work clearly demonstrates that hydrogen can transport the sulfur to a non-volatile desulfurising agent. Figure 3 illustrates a possible commercial scale embodiment of an apparatus 30 for desulfurising nickel powder using a rotary kiln 32 in which the off-gases are recycled back into the kiln via an adsorbent H 2 S filter 34. The nickel powder is fed to the rotary kiln 32 via a nickel powder feed hopper 35 which pre-heats the powder prior to entry into the rotary kiln 32 at a feed rate of 6 tonne per hour. The rotary kiln is kept at a temperature of approximately 750°C and is indirectly heated using natural gas. In the illustrated embodiment, the rotary kiln has a length of 12,000mm and an internal diameter of 1200mm and is inclined with a slope of 50mm per 1000mm. The rotary kiln 32 would typically be rotated at approximately 20 rpm. A gas blower 36 produces a pressure drop within the rotary kiln 32 to maintain a flow of hydrogen containing gas of 1200m7min through the rotary kiln in a counter-current direction relative to the direction of transport of the nickel powder through the kiln. Off- gas from the rotary kiln 32 exits from rotary outlet 38 and is made up with fresh hydrogen gas before passing through the adsorbent filter 34. Preferably the filter 34 incorporates a filter membrane made from a suitable non-volatile desulfurising agent such as calcium or magnesium metal or calcium hydroxide which can be periodically replaced as required. Gas passing through the filter 34 is recycled back into the rotary kiln 32 via a gas preheater 40 which brings the gas up to temperature before entry into the rotary kiln 32. The off gas composition is approximately 9.9% H 2 , 0.1% H 2 S and 90% N 2 , whereas the recycled gas entering via the preheater 40, is approximately 10% H 2 and 90% N 2 . Test work indicated that a range of hydrogen concentrations can be used in the kiln. The desulfurised nickel powder exits from the rotary kiln 32 via a discharge hopper 42 where some cooling of the nickel powder occurs. From there the desulfurised nickel powder is fed to a hot briquetting machine 44 which presses the powder into small briquettes at a temperature of approximately 400°C. The nickel briquette product which is discharged from the hot briquetting machine 44 is ready for commercial sale. From the above description of experimental test results and a possible plant scale embodiment it will be seen that the process and apparatus for desulfurising nickel powder has significant advantages over the prior art, including the following: (a) a significant improvement in the production of on specification nickel briquettes can be achieved. (b) the reaction equilibrium of the desulfurisation reaction using hydrogen can be significantly improved by reacting the hydrogen sulfide onto an adsorbent material. (c) most if not all of the hydrogen can be regenerated resulting in greatly improved hydrogen utilisation. (d) reduced operating temperatures and low flammability gases can be employed, thus improving plant safety. (e) the process lends itself to hot briquetting of the desulfurised nickel powder, thus eliminating the need for sinter furnaces and conventional briquette making machines. It will be apparent to persons skilled in the mineral processing and metallurgical arts that numerous variations and modifications may be made to the described process and apparatus for desulfurising of nickel powder, in addition to those already described, without departing from the basic inventive concepts. Thus, for example, any suitable desulfurising agent can be employed, and the process is not limited to the particular adsorbent materials described which are exemplary only. All such variations and modifications are to be considered with in the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A process for desulfurising nickel powder, the process comprising the steps of: heating the nickel powder in a hydrogen containing atmosphere to promote the formation of hydrogen sulfide gas; and, removing the hydrogen sulfide gas using a desulfurising agent whereby, in use, the sulfur content of the nickel powder can be substantially reduced. 2. A process for desulfurising nickel powder as defined in claim 1 , wherein during said heating step the nickel powder is transported substantially continuously from a nickel powder feedpoint to a desulfurised nickel powder discharge point. 3. A process for desulfurising nickel powder as defined in claim 2, wherein during said heating step the nickel powder is exposed to a hydrogen containing gas flowing in a counter-current direction relative to the direction of transport of the nickel powder. 4. A process for desulfurising nickel powder as defined in claim 3, wherein hydrogen gas is recirculated in the process following the removal of the hydrogen sulfide thereby significantly improving the hydrogen utilisation. 5. A process for desulfurising nickel powder as defined in claim 1 , wherein the nickel powder is heated up to temperatures within the range of 400° to 950°C. 6. A process for desulfurising nickel powder as defined in claim 5, wherein the nickel powder is heated up to temperatures within the range of between 650° to 800°. 7. A process for desulfurising nickel powder as defined in claim 1, wherein a non- volatile desulfurising agent is employed. 8. A process for desulfurising nickel powder as defined in claim 7, wherein the desulfurising agent is a solid material which reacts with hydrogen sulfide. 9. A process for desulfurising nickel powder as defined in claim 8, wherein said desulfurising agent is selected from the group consisting of alkaline earth metals, metal oxides, carbonates and hydroxides such as Ca, Mg, CaO, MgO, CaCO 3 , Ca(OH) 2 , MgCO 3 and Mg(OH) 2 . 10. A process for desulfurising nickel powder as defined in claim 8, wherein the desulfurising agent can be regenerated by desorbing the hydrogen sulfide at lower temperatures. 11. A process for desulfurising nickel powder as defined in claim 1, wherein the process includes the further step of forming the desulfurised nickel powder into briquettes 12. A process for desulfurising nickel powder as defined in claim 11, wherein the nickel powder is formed into briquettes upon exiting from the desulfurised nickel powder discharge point, while the powder is still hot. 13. An apparatus for desulfurising nickel powder, the apparatus comprising: a reactor for heating the nickel powder in a hydrogen containing atmosphere to promote the formation of hydrogen sulfide gas; and, a means for removing the hydrogen sulfide gas using a desulfurising agent whereby, in use, the sulfur content of the nickel powder can be substantially reduced. 14. An apparatus for desulfurising nickel powder as defined in claim 13, wherein said reactor is a kiln through which the nickel powder is transported substantially continuously from a nickel powder feed point to a desulfurised nickel powder discharge point. 15. An apparatus for desulfurising nickel powder as defined in claim 14, wherein said reactor is a rotary kiln. 16. An apparatus for desulfurising nickel powder as defined in claim 13, wherein the apparatus further comprises a device for blowing a hydrogen containing gas through the kiln in a counter-current direction relative to the direction of transport of the nickel powder. 17. An apparatus for desulfurising nickel powder as defined in claim 13, wherein said means for removing the hydrogen sulfide gas comprises a filter having said desulfurising agent provided in connection therewith 18. An apparatus for desulfurising nickel powder as defined in claim 17, wherein said filter is provided in a gas by-pass stream through which the hydrogen containing gas is recycled after is passes through the kiln. 19. An apparatus for desulfurising nickel powder as defined in claim 13, wherein said means for removing the hydrogen sulfide gas comprises said desulfurisation agent located within the kiln adjacent said nickel powder. 20. An apparatus for desulfurising nickel powder as defined in claim 18 or 19, wherein a non-volatile desulfurisation agent is employed. 21. An apparatus for desulfurising nickel powder as defined in claim 20, wherein said desulfurisation agent is selected from the group consisting of alkaline earth metals, metal oxides, carbonates and hydroxides such as Ca, Mg, CaO, MgO, CaCO 3 , Ca(OH) 2 , MgCO 3 and Mg(OH) 2 .

Inventors: CLARK IAN, FRENCH SEAMUS, LALLENEC SERGE, PICKERING GARETH, CLARK, IAN, FRENCH, SEAMUS, LALLENEC, SERGE, PICKERING, GARETH
Applicants: CLARK IAN, FRENCH SEAMUS, LALLENEC SERGE, PICKERING GARETH, WMC RESOURCES LTD, CLARK, IAN, FRENCH, SEAMUS, LALLENEC, SERGE, PICKERING, GARETH
Application year: 2000
Main classification: C22B23
IPC codes: B22F1, C22B1, C22B5, C22B9, C22B23