WO2010068167A1

WO2010068167A1 - Method for manufacturing a mangenese alloy

Info

Publication number: WO2010068167A1
Application number: PCT/SE2009/051387
Authority: WO
Inventors: Jörgen LARSSON; Magnus Larsson
Original assignee: Gosta Larssons Mekaniska Verkstad AB
Current assignee: Gosta Larssons Mekaniska Verkstad AB
Priority date: 2008-12-12
Filing date: 2009-12-08
Publication date: 2010-06-17
Anticipated expiration: 2011-06-12
Also published as: NO20085206L; NO330140B1

Abstract

The present invention relates to a method for manufacturing a manganese alloy, said method comprising the steps of: heating at least iron oxide and manganese oxide together with carbon in a furnace; extracting a manganese alloy from said furnace; cleaning furnace gas generated in the furnace in a wet scrubber to form clean furnace gas and a manganese containing composition; adding acid to said manganese containing composition to form a mixture with a p H of 2-6; adding a precipitating agent to said mixture for, at least partly, precipitating zinc from said mixture; separating non-dissolved particles, including at least a portion of the precipitated zinc, from said mixture to form a liquid solution containing dissolved manganese; forwarding at least a portion of the manganese content of said liquid solution as a source of manganese to a furnace for being heated together with at least iron oxide and carbon for forming a manganese alloy.

Description

METHOD FOR MANUFACTURING A MANGANESE ALLOY

Technical field

The present invention relates to a method for manufacturing a manganese alloy.

Technical background

Manganese, in the form of manganese alloys, is often used in iron and steel production e.g. due to its deoxidizing and alloying properties. Such manganese alloys are produced mainly from ore and coke under high temperatures. In the production of manganese alloys process gases containing metals are generated. Before being discharged these gases are normally purified by bringing them into contact with water in a wet scrubber. In the cleaning of the process gases a sludge containing metal particles is generated. Today, such sludge is placed on waste dumps at high costs. The handling and storage of the sludge may create working and environmental problems.

GB 1 371 113 discloses a method of utilizing manganese present in sludge generated by means of the cleaning of process gases discharged from a smelting furnace. However, this method is considered inefficient as regards economically and environmentally considerations.

Summary of the invention

It is an object of the present invention to overcome the above described drawbacks, and to provide an improved method for manufacturing manganese alloys. This and other objects that will be apparent from the following summary and description are achieved by a method for manufacturing a manganese alloy, said method being characterized in comprising the steps of: heating at least iron oxide and manganese oxide together with carbon in a furnace; extracting a manganese alloy from said furnace; cleaning furnace gas generated in the furnace in a wet scrubber to form clean furnace gas and a manganese containing composition; adding acid to said manganese containing composition to form a mixture with a pH of 2-6; adding a precipitating agent to said mixture for, at least partly, precipitating zinc from said mixture; separating non-dissolved particles, including at least a portion of the precipitated zinc, from said mixture to form a liquid solution containing dissolved manganese; forwarding at least a portion of the manganese content of said liquid solution as a source of manganese to a furnace for being heated together with at least iron oxide and carbon for forming a manganese alloy.

A method for manufacturing a manganese alloy that enables reuse of valuable metals in the furnace gas is thus provided. By treating furnace gas from the production of a manganese alloy in this way it is possible to recover a substantial quantity of the manganese content in the furnace gas, still avoiding to accumulate zinc in the system. The recovered manganese is used as raw material in the production of a manganese alloy, i.e. the amount of required externally supplied raw material, e.g. manganese ore, may be reduced. In known methods the presence of high levels of zinc prohibits the reuse of sludge as a source of manganese in the production of manganese alloys. The removal of zinc thus enables reuse of the valuable manganese in the furnace gas. Thus, a cost-effective method for manufacturing a manganese alloy is provided. Furthermore, the method is advantageous from a environmental point of view since natural resources are utilized in a more efficient way. Consequently, less waste is generated compared to known methods for manufacturing of manganese alloys, where sludge generated from gas cleaning is removed and placed on waste dumps where it in many places is considered as being an enviromental problem. In one embodiment the method further comprises, after the step of adding acid to said manganese containing composition and before the step of adding a precipitating agent for precipitating zinc, a step of separating non- dissolved particles from said mixture having a pH of 2-6. The separated material, e.g. in the form of a filter cake, may be forwarded to a furnace as a source of silicon in the production of silicomanganese. An advantage of this embodiment is that the level of reuse of metals contained in the furnace gas is even further improved. Also, the amount of waste generated in the process is further reduced. In one embodiment the method further comprises, after said step of separating non-dissolved particles, including at least a portion of the precipitated zinc, from said mixture to form a liquid solution containing dissolved manganese, a step of adding a manganese precipitating agent for precipitating, at least partly, manganese from said liquid solution, and a step of separating said precipitated manganese from said liquid solution, said step of forwarding at least a portion of the manganese content of said liquid solution as a source of manganese to a furnace for being heated together with at least iron oxide and carbon comprising utilizing said precipitated manganese as said source of manganese. More preferably said manganese precipitating agent comprises a carbonate and/or a hydroxide containing or hydroxide generating compound. An advantage of this embodiment is that less water is forwarded to the furnace, which increases the energy efficiency thereof. Preferably, in said step of adding acid to said manganese containing composition to form a mixture with a pH of 2-6, said added acid comprising sulphuric acid. It has been found that by choosing sulphuric acid the manganese containing mixture is dissolved in a cost efficient manner.

In said step of adding acid to said manganese containing composition to form a mixture with a pH of 2-6, the added acid may in one embodiment comprise hydrochloric acid.

In said step of adding acid to said manganese containing composition acid is preferably added in such a quantity that a mixture with a pH of 3.0-5.0, more preferably with a pH of 3.5-4.5, is formed in order to dissolve metal particles contained in the sludge acid in an even more efficient way. Most preferably, acid is added in such a quantity that a mixture with a pH of 3.8-4.2 is formed.

In said step of adding a precipitating agent to said mixture for, at least partly, precipitating zinc from said mixture, said precipitating agent preferably comprises a sulphide containing compound. An advantage of adding a sulphide containg compound is that sulphides forms complexes with many metals including zinc. In said step of adding a precipitating agent to said mixture for, at least partly, precipitating zinc from said mixture, said precipitating agent may be an alkaline substance.

Further advantages of the invention will be apparent from the following description and the appended claims.

Brief description of the drawings

The present invention will now be described in more detail with the reference to the accompanying schematic drawings which shows an embodiment of the invention and in which:

Fig. 1 shows a method for manufacturing a manganese alloy according to an embodiment of the present invention.

Fig. 2 shows selected steps of the method shown in Fig. 1.

Technical description

As used in the present application, "ferroalloys" refers to various per se known alloys of iron with a high proportion of one or more other elements, e.g. manganese or silicon. For instance, ferromanganese is a ferroalloy with a high content of manganese, while silicomanganese is a ferroalloy with a high content of manganese and silicon. Ferromanganese and silicomanganese are also referred to as manganese alloys. Due to their deoxidizing and/or alloying properties manganese alloys are, e.g., used in the production of steel.

Ferromanganese is made by heating a mixture of manganese oxide,

MnO₂, and iron oxide, Fe₂O₃, with carbon, such as coke, in a furnace, e.g., a blast furnace or an electric arc furnace.

Normally, iron oxide in the form of iron ore, manganese oxide in the form of manganese ore, and carbon in the form of coke is used.

The metallurgical process of manganese alloy production in a furnace is known per se, see for instance GB 2 255 350. In order to purify furnace gas generated in the production of a mananganese alloy before being discharged, gas cleaning in a wet scrubber is normally carried out, which process also is known per se.

In the following, a method for manufacturing a manganese alloy, in this case ferromanganese, according to an embodiment of the present invention will be described referring to Fig. 1 , illustrating the method in a schematical manner, and Fig. 2, illustrating the specific steps S1 to S8 of the method. As indicated above the steps of heating raw materials, extracting a manganese alloy and cleaning process gas are known per se and they will therefore be described only briefly.

In step S1 , a mixture comprising iron oxide, Fe2θ3, and manganese oxide, MnO2, is heated together with carbon, C, in a furnace 1 , such as a blast furnace or an electric arc furnace. For this purpose raw materials, e.g. ores containing manganese and iron, and coke, are supplied to the furnace 1 , as indicated by arrow A, through an opening 3 arranged in the upper portion of the furnace 1. The main source of manganese is often manganese containing ore, also referred to as manganese ore, such ore often containing also iron. Additionally, manganese in the form of a manganese containing liquid solution and/or in the form of a solid manganese salt, such as manganese carbonate, MnCO3, is supplied, as will be described later.

In step S2, ferromanganese, FeMn, in the form of a molten metal, is extracted from the furnace 1 by means of conventional metallurgical processes. It is realized that the mixture inside the furnace 1 is melted together with coke at high temperature, e.g. 1400⁰C. The molten ferromanganese is tapped, as indicated by arrow B, through a tap hole 5 arranged in the lower portion of the furnace 1. As a by-product slag is generated. Molten slag is removed from the furnace 1 , as indicated by arrow C, through another tap hole 7 arranged in the lower portion of the furnace 1. The melting of materials in the furnace 1 generates a hot process gas, also referred to as a furnace gas, containing various metals in the form of metal particles. The furnace gas is forwarded to a wet scrubber 9 via a gas duct 10. The wet scrubber 9 comprises a tower 11 , nozzles 13 for distributing a scrubbing solution 12 in the tower 11 , and a pipe 15 for recirculating scrubbing solution 12. The scrubbing solution 12 is recirculated by means of a pump 17 arranged in the recirculation pipe 15. The wet scrubber 9 also comprises a fresh water supply pipe, and, optionally, a pipe for supplying an absorbent, such as NaOH, both of which are not illustrated in Fig. 1 for reasons of clarity.

In step S3, furnace gas is cleaned in the wet scrubber 9 to form clean furnace gas and a manganese containing composition, also referred to as sludge. The clean furnace gas is forwarded to a stack 19, via a gas duct 21 , and is discharged to the atmosphere. A portion of the scrubbing solution 12 flowing through the recirculating pipe 15 is forwarded to a conventional water treatment unit 23 via a pipe 25 connected on the high-presure side of the recirculating pipe 15. The water treatment unit 23 includes, e.g., means 27, such as a series of tanks with agitators, for pH-adjustment and/or flocculation, and a sedimentation device 29, such as a sedimentation tank or a lamella separator. In the water treatment unit 23 scrubbing solution is thus treated in subsequent steps in order to form purified water, as indicated by arrow D, and a manganese containing sludge. The purified water may be released to the recipient.

Optionally, the water treatment unit 23 may comprise a sludge dewatering device 31 , in which water is removed from the sludge by means of, e.g., filtering using a filter press or a bucher press, to form solid filter cakes. Alternatively, in the sludge dewatering device 31 , water may be removed by means of sedimentation or centhfugation.

In the gas cleaning step, S3, metal particles contained in the furnace gas is thus collected in the circulating scrubbing solution 12 of the wet scrubber 9, and is then, in the water treatment unit 23, precipitated and separated as a manganese containing composition. The separated sludge contains, in general, elements such as Mn, Si, Fe, Al, Ca, Mg, Zn, C, Pb and water. The composition of the sludge generated in the gas cleaning step S3 may vary due to, for instance, variations in raw material and process conditions. At least some of the mentioned elements contained in the sludge are in general present as oxides and/or carbonates.

Hence, the manganese containing sludge leaving the sedimentation device 29 may thus, optionally, be dewatered by means of the sludge dewatering device 31 , the resulting filter cakes leaving the sludge dewatering device 31 for being processed further as indicated by arrow E.

Optionally, the manganese containing sludge is dewatered by means of the sludge dewatering device 31 and is forwarded to a waste dump 33, as indicated by arrow F, for being treated further at a later occasion, as indicated by arrow G.

The obtained sludge and/or the filter cakes is forwarded to a tank 35, as indicated by arrow H. In the case illustrated in Fig. 1 , sludge is forwarded directly from the sedimentation device 29 to the tank 35.

In step S4, acid, in this case sulphuric acid, arrow I, and water, arrow J, are supplied to the sludge 37 received in the tank 35. Consequently, the sludge 37 is diluted, and its content of metal salts, such as oxides and carbonates as indicated hereinbefore, is at least partly dissolved as a consequence of the pH of the sludge 37 being reduced. In order to dissolve desired types and/or a sufficient amount of particles acid need to be added in such a quantity that the pH of the sludge 37 is reduced to a pH of 2-6. The lower the pH the more particles are dissolved. It is preferred to reduce the pH of the solution to a pH of typically about 4. The mixture of sludge, water and acid is preferably stirred in order to accelerate the dissolving of particles.

In step S5, particles not dissolved in step S4 are separated from the liquid to form filter cakes, by means of, e.g., filtration, sedimentation or centrifugation, by means of a suitable separation device 38, such as a filter press or a belt filter, the filter cakes leaving the device 38 as indicated by arrow K. By separating non-dissolved particles, a liquid solution 39 containing dissolved manganese and zinc is obtained and is forwarded to a second tank 41 via a pipe 40. The filter cakes obtained in step S5, and leaving the separation device 38 as indicated by arrow K, mainly consist of silicon dioxide and may optionally be used as a source of silicon in the production of silicomanganese.

In step S6, a sulphide solution is added, as indicated by arrow L, to the liquid solution 39 in the tank 41 , in order to precipitate zinc. A molar ratio of sulphide to zinc of 1.1-1.2 to 1 is preferred to effectively precipitate zinc present in the liquid solution 39. The mixture of liquid solution and sulphide solution is preferably stirred to accelerate the precipitation of zinc. By adding, e.g., disodium sulphide, Na₂S, dissolved zinc is precipitated from the liquid solution 39 in the form of solid zinc sulphide, ZnS. In step S7, zinc precipitated in step S6 is separated, e.g., by means of filtration, sedimentation or centrifugation, by means of a suitable separation device 42, such as a filter press or a belt filter, to form solid filter cakes leaving the device 42, as indicated by arrow M, to form a liquid solution containing dissolved manganese. In step S7, a manganese rich liquid solution essentially free from zinc is thus obtained.

In step S8, the manganese content of the liquid solution obtained in step S7 is forwarded as a source of manganese to the production of a manganese alloy. The liquid solution is in this case forwarded without further treatment to a furnace 1 , as indicated by the arrow N, for being heated together with manganese ore, iron ore and coke.

Alternatively, the liquid obtained in step S7 may be forwarded to a tank 43 via a pipe 44, in order to precipitate solid manganese carbonate, MnCO3, by adding carbonate, such as a sodium carbonate, Na₂CO₃, solution, as indicated by arrow O. The solid manganese carbonate, MnCO3, is then separated by means of, e.g., a separation device 45, such as a filter press, a belt filter, a centrifuge, or a sieve, and is forwarded to the furnace 1 , as indicated by arrow P. In this case the step S8 thus includes precipitating and separating MnCO₃ from the liquid solution leaving the separation device 42 in S7. An advantage of this embodiment is that less water is forwarded to the furnace 1 , which increases the energy efficiency thereof. As an alternative to adding a carbonate containing compound, such as Na₂COs, it is also possible to add a hydroxide containing compound, such as, NaOH, to precipitate the manganese in the form of manganese hydroxide, Mn(OH)₂.

An embodiment of the present invention will now be described in more detail with reference to the following example.

Example 1 In a first example manganese containing sludge was obtained in accordance with steps S1 , S2 and S3 described above. In the first example 0.5 kg of the obtained sludge, with the chemical composition as displayed in Table 1 , was mixed with 216.5g of water. Sulfuric acid having a concentration of 96 percent by weight was added in step S4 for reducing the pH of the sample. In total 235.4g of sulfuric acid was added. The sample was then kept with intensive stirring for 2 hours at approximately 100⁰C. When the pH of the sample decreased below a pH of 6 the sample started to foam. A defoamer, of the polyethyleneoxide-polypropyleneoxide modified fatty acid type was then added. The pH of the sample was reduced to a pH of 4.1. Table 1. Composition of initial sludge sample used in example 1.

Non-dissolved particles were then removed from the sample by filtration in step S5. Of the total amount of manganese and zinc in the initial sludge 8 % of the manganese and 10.5% of the zinc were removed in this filtration. The total weight of the removed particles was 77.7 g. The filter cake was washed with water to rinse out any remaining dissolved manganese. After the separation of non-dissolved particles the sample contained 974 g of liquid solution including 9.4 % Mn and 2.3 % Zn. To the liquid solution was then added, in step S6, 53.1 g of a Na2S solution having a concentration of 60 percent by weight, corresponding to 1.2 mol S to 1 mol Zn. Zinc sulphide ZnS was formed and was removed, in step S7, by means of a second filtration of the sample. After this second filtration the remaining liquid sample had a weight of 884g and contained 10.4 % Mn and 0.004% Zn. The remaining liquid thus contained as much as 90 % of the amount of manganese of the initial sludge, but very little of the zinc. The total amount of Zn removed from the initial sludge as precipitated ZnS was 21.9g. Hence, as demonstrated, most of the manganese that leaves the furnace with the furnace gas can be brought back to the furnace, after treatment in accordance with one embodiment of the present invention, still avoiding to accumulate zinc in the system.

A method according to an embodiment of the present invention may be considered as a closed-loop method since manganese recovered from the furnace gas is reused in manufacturing a manganese alloy. It will be appreciated that the described embodiment of the invention can be modified and varied by a person skilled in the art without departing from the inventive concept defined in the claims.

It is for instance realized that the manganese content of the liquid solution obtained in S7 do not necessarily, in step S8, need to be forwarded to the furnace of step S1 , i.e., the furnace from which the furnace gas treated in step S3 originates. Recovered manganese, e.g., in the form of a liquid solution containing dissolved manganese or solid manganese in the form of MnCO3 precipitated therefrom, may thus be forwarded to any furnace for being heated together with at least iron oxide and carbon to manufacture a manganese alloy.

It is further realized that furnace gas from several furnaces may be forwarded to a wet scrubber for being processed further, or that furnace gas from one furnace may be divided between several wet scrubbers. Furthermore, it is realized that it is possible to forward manganese recovered from the manufacturing of a first manganese alloy, e.g. ferromanganese or silicomanganese, to the manufacturing of a second manganese alloy, e.g. ferromanganese or silicomanganese, said second manganese alloy being of the same or of different type as said first manganese alloy. As an example, manganese recovered from manufacturing of ferromanganese may be forwarded to a furnace, for being heated together with iron oxide, manganese oxide, silicon dioxide (SiO2) and carbon for manufacturing silicomanganese (SiMn).

In step S6, a sulphide solution containing disodium sulphide is added in order to precipitate zinc. It is realized that other water soluble sulphide salts, such as dipotassium sulphide, may be used as alternative to disodium sulphide. As an alternative to inorganic sulphide salts it is also possible to use organo-sulphide containing compounds, such as thmercapto-s-thazine trisodium salt, dithiocarbamate, or sodium trithiocarbonate to precipitate zinc. As mentioned above, sludge and/or filter cakes obtained from the gas cleaning step S3 may, for a shorter or longer period of time, be removed and placed on a waste dump 33 before being further treated in step S4. In this case the loop is closed when filter cakes are brought back and further treated in accordance with the steps S4, S6, S7 and S8 of an embodiment of the method. Hence, sludge that has been recovered several years ago in a waste water treatment plant 23, and has then been transported to a waste dump 33 for being stored for many years, such as 2-20 years, may very well, in accordance with one embodiment of the present invention, be utilized for producing manganese, such that the manganese loop is closed.

As mentioned above, the main source of manganese used in the manufacturing of a manganese alloy according to an embodiment of the present invention is manganese containing ore. However, for a shorter time period, it may be possible to use recovered manganese as the main, or even the only, source of manganese.

Claims

1. A method for manufacturing a manganese alloy, said method being characterized in comprising the steps of: heating at least iron oxide and manganese oxide together with carbon in a furnace (1 ); extracting a manganese alloy from said furnace; cleaning furnace gas generated in the furnace (1 ) in a wet scrubber (9) to form clean furnace gas and a manganese containing composition; adding acid to said manganese containing composition to form a mixture with a pH of 2-6; adding a precipitating agent to said mixture for, at least partly, precipitating zinc from said mixture; separating non-dissolved particles, including at least a portion of the precipitated zinc, from said mixture to form a liquid solution containing dissolved manganese; forwarding at least a portion of the manganese content of said liquid solution as a source of manganese to a furnace (1 ) for being heated together with at least iron oxide and carbon for forming a manganese alloy.

2. A method according to claim 1 , further comprising, after the step of adding acid to said manganese containing composition and before the step of adding a precipitating agent for precipitating zinc, a step of separating non-dissolved particles from said mixture having a pH of 2-6.

3. A method according to any one of the preceding claims, further comprising, after said step of separating non-dissolved particles, including at least a portion of the precipitated zinc, from said mixture to form a liquid solution containing dissolved manganese, a step of adding a manganese precipitating agent for precipitating, at least partly, manganese from said liquid solution, and a step of separating said precipitated manganese from said liquid solution, said step of forwarding at least a portion of the manganese content of said liquid solution as a source of manganese to a furnace (1 ) for being heated together with at least iron oxide and carbon comprising utilizing said precipitated manganese as said source of manganese.

4. A method according to claim 3, wherein said manganese precipitating agent comprises a carbonate and/or a hydroxide containing or hydroxide generating compound.

5. A method according to any one of the preceding claims, wherein in said step of adding acid to said manganese containing composition to form a mixture with a pH of 2-6, said added acid comprising sulphuric acid.

6. A method according to any one of the preceding claims, wherein in said step of adding acid to said manganese containing composition to form a mixture with a pH of 2-6, said added acid comprising hydrochloric acid.

7. A method according to any one of the preceding claims, wherein said step of adding acid to said manganese containing composition comprises adding acid in such a quantity that a mixture with a pH of 3.0-5.0 is formed.

8. A method according to any one of the preceding claims, wherein in said step of adding a precipitating agent to said mixture for, at least partly, precipitating zinc from said mixture, said precipitating agent comprises a sulphide containing compound.

9. A method according to any one of the preceding claims, wherein in said step of adding a precipitating agent to said mixture for, at least partly, precipitating zinc from said mixture, said precipitating agent being an alkaline substance.