PROCESS FOR THE PREPARATION OF MANGANESE SULPHIDE

Abstract: The present invention concerns the preparation of a manganese sulphide by heating of particulate or agglomerate manganese sulphate to a temperature of at least 700 DEG C in the presence of at least one reduction agent.

Description: PROCESS FOR THE PREPARATION OF MANGANESE SULPHIDE The present invention concerns a process for the preparation of a manganese sulphide. More specifically, the present invention concerns a reduction process for the preparation of manganese sulphide from manganese sulphate at an elevated temperature. In the field of powder metallurgy manganese sul¬ phide (MnS) has found increasing utility as an additive for improving the machinability of sintered parts. As the market for PM products is expanding, the need for industrially applicable, cost effective and environmen¬ tally acceptable processes is obvious. The US patent 4 676 970 discloses a process for the preparation of MnS by melting manganese and sulphur. More specifically this patent discloses a process, according to which an aqueous manganese sulphate solu¬ tion is subjected to electrolysis for the preparation of manganese metal, which in particle form is mixed with particulate sulphur. In the obtained mixture, a chemical reaction is initiated by adding eg aluminium and barium peroxide. A very pure MnS product can be obtained if highly pure starting materials are used. Furthermore the patent teaches that it would not be possible to produce MnS on a large scale for commercial purposes by thermal reduction of manganese sulphate due to sulphur pollution and low purity. The JP application 62-288116 discloses a process for the preparation of MnS, whereby an Mn compound such as e.g. MnS0 4 is heated to a temperature between 350 and 700°C in H 2 S atmosphere. Another process for the produc- tion of MnS disclosed in this application concerns the burning of manganese oxide or metallic manganese in car¬ bon disulphide atmosphere. The Japanese patent applica¬ tion also discloses a method wherein an excess amount of aqueous ammonia is added in the boiling state to an aqueous manganese (II) solution containing potassium oxalate and hydrogen sulphide gas. In accordance with the present invention, it has unexpectedly, and in contrast to the teaching in the above-mentioned patent, been found that an MnS product having the required machinability-improving properties can be prepared by thermal reducion of manganese sul¬ phate in a process suitable for large-scale commercial production. From an industrial point of view, this process offers important advantages including the possi¬ bility to use inexpensive starting materials and avail¬ able equipment. Furthermore, the electrolysis step, which is necessary according to the known process, is eliminated. The purity of the obtained product is suffi- cient for industrial purposes and a remarkably pure product can be obtained even without highly pure start¬ ing materials as required in the known process. A thorough and extensive study has shown that the thermal reduction can be carried out both as a dry and as a wet process and that satisfactory results can be obtained in many different ways. Thus, the reduction agent can be selected among a wide variety of known re¬ duction agents. Preferably, the reduction agent is se¬ lected from the group consisting of pit coal, charcoal, lignite, gaseous hydrogen, carbon monoxide, since the environmentally accepted products, carbon dioxide or wa¬ ter, are then obtained as by-products. When a solid car¬ bonaceous reduction agent is used the reduction is pre¬ ferably carried out in a reducing atmosphere. By opti- mizing the reaction parameters, a small and environ-men¬ tally acceptable amount of polluting sulphur containing by-products is obtained. The process according to the invention is defined in the appended claims. The dry method includes heating manganese sulphate in the presence of a gaseous reduction agent, such as hydrogen gas or carbon monoxide, at a temperature of at least 700°C, preferably at least 750°C and more spe¬ cifically at least 800°C. This method is preferably car¬ ried out in a fluidized bed, wherein the manganese sul- phate is in the form of agglomerates having a particle size below 10 mm. Commercially this process is prefer¬ ably carried out in a shaft furnace and sulphur dioxide can be added in order to increase the yield of manganese sulphide. An alternative dry method is mixing particu- late manganese sulphate with particulate carbon in the form of e g pit coal or charcoal at a temperature of at least 700°C, preferably at least 750°C and more spe¬ cifically at least 800°C, whereby the particle size of manganese sulphate should preferably be less than 1 mm and the particle size of the carbon should preferably be less than about 5 mm. The wet method can be carried out by mixing par¬ ticulate manganese sulphate, particulate coal having a mean particle size of less than about 5 mm, and water, heating the obtained slurry to a temperature of at least 700°C during a period sufficient for allowing essen¬ tially complete reduction of manganese sulphate to man¬ ganese sulphide. Preferably, this method is also carried out in a reducing atmosphere such as a carbon monoxide containing atmosphere. The process according to the invention provides an industrially attractive alternative as the starting ma¬ terials are inexpensive and readily available. Manganese sulphate is widely used as a fertiliser and the reduc- tion agents are also commonly used and inexpensive, which contributes to the low cost of the process. An additional advantage is that the process can be carried out in conventional equipment, e.g. furnaces, such as belt furnaces and tunnel kilns optionally simultaneously with the other processes conventionally carried out in such equipment. The MnS product obtained according to the present process may include up to 25% by weight of MnO. Quite unexpectedly, it has been found, and this is well docu¬ mented, that this relatively high percentage of MnO does not negatively influence the machinability-improving properties. It has actually been found that in certain cases the content of MnO can improve the machinability as compared with a purer MnS product, i.e. a product containing 1% by weight or less of impurities/by- products. In addition to MnO the product according to the invention can include up to 2 % by weight of C and preferably between 0.5 and 1.5 % by weight of C. By using a stoichiometrical excess of the reduction agent, the amount of MnO can be reduced, and as can be seen from the following examples a highly pure (98 %) MnS product can be obtained even without highly purified starting materials. The invention is further illustrated by the follow¬ ing examples: Example 1 Manganese sulphate ( nS0 IH2O) obtained from SVERA AB, Sweden, was mixed with 15% particulate charcoal (particle size about 1 mm) and packed in cylindrical SiC-capsules with coke in the center and surrounding the mixture. Each capsule was capable of containing 26 kg of mixture and 9 capsules were simultaneously introduced into the tunnel kiln. The capsules were heated for 36 minutes at a maximum temperature of 1150 - 1180°C. The resulting sponge cakes were crushed, ground and sieved to 45 μm (325 mesh), leaving a material consisting of MnS having a purity of 98 %. Further refining through grinding and drying is possible. Example 2 The following experiments were performed as dry processes at reduction temperatures between 700 and 900°C. a. MnS0 4 + 2S → MnS + 2S0 b. nS0 + 2C → MnS + 2C02 c. MnS04 + 4H2→ MnS + 4H20 Carbon was added in different forms, such as charcoal, coke or black coal. The resulting MnS/MnO ratio depended strongly upon the type of carbon selected. Experiments carried out at 850°C showed, that charcoal gives the highest MnS content. Reducing agent MnS/MnO ratio charcoal 90/10 black coal 72/28 coke 32/68 The experiments were carried out without any excess of reducing agent. All experiments were carried out with 5 minutes of pre¬ heating, a period of 15 minutes of heating to the reduc¬ tion temperature and a period of 60 minutes at the re- duction temperature. Exemple 3 A slurry was formed by mixing 340 kg of particulate manganese sulphate, 80 kg of particulate carbon in the form of lignite and 80 kg of water. The slurry was pumped into capsules as in example 1 and the capsules were introduced into a tunnel kiln having an atmosphere essentially consisting of carbon monoxide and carbon di¬ oxide. The temperature of the kiln was 1150 - 1180°C. All the products prepared according to the above examples had a machinability index of the same magnitude as the MnS product prepared according to the US patent 4 676 970.

Claims: CLAIMS 1. Process for the preparation of an manganese sulphide, c h a r a c t e r i s e d by heating particu¬ late or agglomerate manganese sulphate to a temperature of at least 700°C, preferably at least 750°C, in the presence of a at least one reduction agent. 2. Process according to claim 1, c h a r a c t e ¬ r i s e d in that the reduction is carried out in the presence of a reduction agent selected from the group consisting of pit coal, charcoal, lignite, gaseous hydrogen and carbon monoxide. 3. Process according to claim l or 2, c h a r a c ¬ t e r i s e d in that the reduction is carried out as a dry process. 4. Process according to claim 3, c h a r a c t e - r i s e d in that the reduction agent is gaseous hydro¬ gen and/or carbon monoxide and that the manganese sul¬ phate is present in the form of agglomerates. 5. Process according to claim 4, c h a r a c t e ¬ r i s e d in that the reduction is carried out in a shaft furnace. 6. Process according to claim l or 2 c h a r a c - t e r i s e d in that the reduction is carried out as a wet process. 7. Process according to claim 6, c h a r a c - t e r i s e d in that the reduction agent is a solid, particulate, carbonaceous reducing agent, which is mixed with particulate manganese sulphate and water, that the obtained slurry is heated to a temperature of at least 700°C, preferably at least 750°C, for a period sufficient for allowing an essentially complete reduc¬ tion of the manganese sulphate. 8. Process according to claim 7, c h a r a c t e ¬ r i s e d in that the reduction is carried out at a tem¬ perature of at least 1100°C in a reducing atmosphere. 9. Process according to claim 8, c h a r a c t e ¬ r i s e d in that the reduction is carried out at a tem¬ perature of 1100 to 1180°C in an atmosphere essentially consisting of carbon monoxide and carbon dioxide. 10. Process according to any of the claims 6 to 9 c h a r a c t e r i s e d in that the process is car¬ ried out in a tunnel kiln.

Inventors: ARVIDSSON JOHAN, ENGSTROEM ULF, ARVIDSSON, JOHAN, ENGSTROEM, ULF
Applicants: ARVIDSSON JOHAN, ENGSTROEM ULF, HOEGANAES AB, ARVIDSSON, JOHAN, ENGSTROEM, ULF
Application year: 1996
Main classification: C01G45
IPC codes: C01G1, C01G45