GB2516364A

GB2516364A - Method for processing red muds

Info

Publication number: GB2516364A
Application number: GB1409847.9A
Authority: GB
Inventors: Wassim Mounir Freij; Vladimir Alexandrovich Berdnikov
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-06-14
Filing date: 2014-06-03
Publication date: 2015-01-21
Anticipated expiration: 2034-06-03
Also published as: GB2516364B; UA107873C2; GB201409847D0

Abstract

A method of recovering metal and slag from red mud by adding a carbonaceous reducing agent and sand with limestone or dolomite to the red mud to form a burden which comprises 3.5-5.5 weight % carbon and having a weight ratio of silica:calcia lying in the range 1.3-1.5:1. The burden is heated to a temperature in the range 1690-1790 0C where it is molten and forms slag and metal phases. The slag is poured into water with a copper sulphate concentration in the range 0.3-0.5 g/L where it foams, while the metal is poured into moulds. The carbonaceous reducing agent reduces the ferric oxide and titanium oxide to form an iron-silicon-titanium-carbon alloy. The carbonaceous material is preferably anthracite although charcoal, brown coal and coke are also disclosed. The burden is preferably heated at rate of 16-18 0C/min.

Description

METHOD FOR PROCESSING RED MUDS

The invention relates to the field of nonferrous metallurgy, in particular, to processing red muds as waste products formed during aluminium production.

Red mud is formed in the process of bauxite purification (the basic feedstock for aluminium production) in alumina production as a part of so called Bayer process (the process of obtaining pure aluminium oxide). Red mud, as a source of aluminium oxide, is contaminated with alkali, therefore it is hazardous for environment and humans. Up to million tons of such wastes are discharged to sludge dumps per year, despite the fact that they may be promising sources of useful substances. At present the quantity of accumulated (due to the lack of economically feasible methods of processing) wastes amounts to several hundreds of millions tons.

Chemical composition of red muds is not uniform. The content of individual components in muds according to 8 tests is as follows (wt. %): A1203 8.28 -15.28 Si02 10.40-15.56 CaO 8.28-15.28 Fe203 + FeO 45.61 -59.32 Ti02 1.12 -5.31 Na20 0.76 -2.83 Any remainder being unavoidable impurities.

Many techniques of red mud processing are known in the art. They are usually based on the methods of grinding and separation such as magnetic, air, gravity and flotation separation. Hydrometallurgical and thermal methods are used as well.

The research work Hot tests oF schemes of comp!ex use of red muds (Report oF the nstaute of Metaflurgy of the UF of USSR Academy, Sverdovsk, 1961) discloses a method of complex processing red muds, the method including agglomeration of red muds, melting the agglomerate in electric pit-type heating furnace to give processed cast iron characterized by high content of titanium and phosphorus (up to 1%), and helenite-containing slag for subsequent processing. However, such technique is economically impractical because of the use of several expensive pyrometallurgical processes related to high net input and energy consumption.

The method of complex processing red muds, as disclosed in the book PyrometaHurgica processng comp'ex ores LLLeontev, N.A.Vatohn, S.V.Shavdn, N,S.Shumakov. Moscow: Metaflurgy, 1997, comprises reductive melting the mud with limestone and coal at the temperature of 1500-1600°C in furnace units positioned in series. The method provides for obtaining two finished products, i.e., processed cast iron and self-disintegrating alumocalcium slag.

According to the method of reductive processing red muds, as disclosed in US patent No. 3876749, red mud is coupled to reducing agent, the resulting mixture is melted, and the melt is separated into steel phase and slag phase. Calcinated product is added to the slag beyond the furnace, which provides such concentration that the content of slag phase, together with CaO, has the following molar ratios of Si02, Ti02, Fe203 and A1203 (±10%): CaO:Si02=2; CaO:Ti02=1; CaO:Fe203=2 and CaO:A1203=0,1-0,5. The product of mixing is melted at the temperatures of 1000°C to 1600°C in neutral or oxidizing medium, with subsequent formation of sodium aluminate by leaching the melt. However, addition of calcinated product to the slag, followed by thermal processing at the temperatures of 1000°C to 1600°C, will require substantial additional expenses.

Patent RU 2428490 discloses a method for processing red muds in aluminium industry, the method comprising obtaining a burden containing red mud, carbonaceous reducing agent and bauxite, whereby molar ratio of a content of CaO to Si02 in said burden is adjusted to the value of at most 1.2 to 1.4. The resulting burden is melted at the temperature of 1500 -1600°C to form a metal portion of the melt and a slag portion of the melt. The metal portion of the melt constitutes an associate metal subject to be cast in moulds. Limestone and soda are added to the slag portion of the melt beyond the furnace with cooling, then adjusted to the concentration calculated for formation of calcium orthosilicate, calcium orthophosphate, calcium titanate and calcium aluminate in the resulting slag, and then processed with water steam.

The drawback of such method is that it provides for an additional out-of-furnace processing the obtained slags using a leaching method related to substantial material expenses. After recovery of calcium orthosilicate, calcium orthophosphate, calcium titanate and calcium aluminate from slags, the remaining slags are directed to slag dumps.

The invention is based on the object of development of waste-free, environmentally friendly method for processing red muds.

According to an aspect of the present invention, there is provided a method for processing red muds of aluminium industry, the method including obtaining a burden which contains red mud, carbonaceous reducing agent and material for regulating weight ratio of Si02/CaO in the burden, melting the burden to form a metal portion of the melt and a slag portion of the melt, recovery of associate metal and subsequent processing slag portion of the melt, characterized in that carbon content in the burden is brought to 3.5 -5.5 wt. % by introducing carbonaceous reducing agent, sand with limestone or dolomite are introduced as materials to regulate weight ratio of Si02.ICaO in the burden, whereby the ratio of Si02/CaO in the burden is brought to 1.3 -1.5, melting the burden is carried out at a temperature of 1690-1790°C, the slag portion of the melt is poured into water for its foaming and formation of porous vitreous material to be used in construction industry, and the metal portion of the melt is cast in moulds.

Preferably, wherein anthracite is used as carbonaceous reducing agent.

Further preferably, wherein heating the burden to achieve a temperature of 1690 -1790°C is carried out with a rate of 16 -1 8°C/mm.

Advantageously, wherein the slag portion of the melt is poured into water with a copper sulphate concentration (CuSO4 nH2O) of 0.3-0.5 gIL.

In the method for processing red muds, including obtainment of a burden which contains red mud, carbonaceous reducing agent and a material for regulation of weight ratio of SiO2ICaO in said burden, melting the burden to form a metal portion of the melt and a slag portion of the melt, recovery of associate metal and subsequent processing the slag portion of the melt according to the invention, said object is achieved by that the introduction of carbonaceous reducing agent brings carbon content in the burden to 3.5 -5.5 wt. % as a material to regulate weight ratio of Si02ICaO in the burden, sand with limestone or dolomite is used, whereby the ratio of Si02ICaO in the burden is brought to 1.3 -1.5, the burden is melted at the temperature of 1690-1790°C, the slag portion of the melt is poured into water for its foaming and formation of porous vitreous material to be used in construction industry, and the metal portion of the melt is cast in moulds.

Carbon content in the burden of less than 3.5 wt. % does not provide the complete reduction of ferric oxide and titanium oxide to metallic state. Melting a burden with a carbon content above 5.5 wt. % impairs thermodynamic characteristics: it results in increase in concentration of carbon in the melt of iron and ferrous alloys, increase in concentration of carbides of high-melting metals such as titanium, that remain in the slag phase and have a substantial effect on formation of silicon and calcium carbides, resulting in substantial decrease in concentration of silicon and calcium carbides in the melt of slags and, as a consequence, in negative effect on formation of a porous vitreous material.

If the weight ratio of Si02/CaO is less than 1.3 the melt of the burden will be more viscous, and the reduced metal will be hard to sink into molten metal, and it will remain within the slag phase, such that formation of the porous vitreous material will fail. With a weight ratio of Si02!CaO above 1.5 the melt will pass to lower temperature phase, reduction of iron will be substantially decreased, thermodynamic characteristics of the melt will be changed: formation of reguline metal and electric power consumption will be increased, and electric conductivity of the melt will be decreased, whereby recovery of processing products from a melting unit will be complicated.

Anthracite is a more preferred carbonaceous reducing agent; however, other agents such as charcoal or brown coal, as well as coke, may also be used for this purpose.

Heating the burden to the temperature of 1690 -1790°C was preferably carried out at a rate of 16-18°C/mm. Such heating rate for burden materials is determined by the preferred rate of phase changes for ferric oxides (from higher to lower oxides) and high-melting metals according to the reaction of solid phase reduction (MeO) + C = [Me] + CO and melting thereof, with a subsequent formation of solid iron based solution capable of dissolution of drop (molten) and reguline (spongy) metal iron that is formed, as well as heavy and high-melting metals. Within the temperature range of 1000 to 1100°C a shift in thermodynamic equilibrium in gaseous phase occurs towards formation of carbon monoxide according to reaction 2C02 = 2C0 + 02, which provides the more complete reduction of metals from their oxides by creation of fluidized bed within the system. During this melting period reduction occurs according to reaction (MeO) + CO = [Me] + CO2. In the temperature range of 1530-1650°C appropriate separation of metallic and silicate portions of the melt takes place. Further raise of temperature to 1700-1790°C provides the formation of necessary amount of silicon, calcium and aluminium carbides that serve dual purpose in this process: they promote foaming the silicate portion of the melt in the melting unit and interaction of metal oxides within the slag melt with carbon monoxide as a gaseous reducing agent and provide foaming by contact of the slag melt with water. Formation of high temperature foam and excessive concentration of CO in gaseous phase results in deeper reduction of metals within the system according to the reaction (MeO) + CO = [Me] + C02, more complete precipitation of reduced metals from the silicate melt, and promotes the removal of dissolved gases from metal phase.

The process of reductive melting also includes reduction of sulphur (S) contained in natural burden materials such as sand and limestone and forming, upon contact with water, hydrogen sulphide (H2S) having unpleasant odor. Therefore, it is desirable to pour a silicate portion of the melt off into water with a copper sulphate concentration (CuSO4 nH2O) of 0.3-0.5 gIL for binding thereof into insoluble chemical compounds and removal thereby such unpleasant odor from the porous vitreous material.

Calcium and silicon carbides formed in the process of melting upon contact with water form a large amount of gases that create porosity of vitreous material, whereby it is provided with a thermal conductivity coefficient of 0.03-0.06 WImK and bulk density of 50-150 kg/m3.

Examples of embodiment of the invention are presented below. In all examples burden components of 0-50 mm fractions were used. Particle size distribution of the burden is not critical for carrying out this method, and its preparation by uniformity is not necessary.

Example 1.

Carbon content in the burden containing red mud, which had the following composition (wt. %): A1203 12.51 Si02 14.56 CaO 13.98 Fe2O3 + FeO 52.52 1102 3.63 Na2O 2.80, was brought to 3,5 wt. % by addition of the appropriate amount of anthracite, and the ratio of SiO2ICaO + MgO was adjusted to 1.3 by addition of sand and limestone. The burden was heated with a rate of 16°C/mm to the temperature of 1750°C. Upon achievement of the indicated temperature the melt was held for 30 minutes at such temperature, wherein, due to formation of silicon and calcium carbides in the melt, the latter is intensively saturated with carbon monoxide (00), i.e., foamed, which promotes active stirring of slags and deeper reduction of metals, as well as their appropriate precipitation on the bottom of a melting unit. Upon expiry of the indicated time, the slag portion of the melt was poured off into water with a copper sulphate concentration of 0,3 gIL. Herewith instantaneous foaming of the mass took place. Metal portion of the melt was poured into the mould. The obtained porous vitreous material had an averaged thermal conductivity coefficient of 0.036 WImK and bulk density of 90 kglm3, in accordance with a specification TU U2.7-14.32553104-001-2004 Artificial porous rubble and sand of expanded silicate", with the following chemical composition, wt. %: Ti02 -0.02, Fe203 -0.91, Si02 -52.00, CaO -39.60, A1203 -6.70. Chemical composition of a metal phase was as follows, wt. %: Fe -91.962, Ti -1.988, Si - 5.150, C -upto 0.900. Yield of metal made 98.2% of its initial content in the slag.

Example 2.

Carbon content in the burden containing red mud, which had the following composition (wt. %),: A1203 8.43 Si02 14.66 CaO 14.22 Fe203 + FeO 56.62 Ti02 4.81 Na20 1.26, was brought to 4,5 wt. % by addition of the appropriate amount of anthracite, and the ratio of SiOWCaO was adgusted to 1.5 by addition of sand and limestone. The burden was heated with a rate of 1 7°C/mm to the temperature of 1770°C. Upon achievement of the indicated temperature the melt was held for 20 minutes at such temperature, wherein, due to formation of silicon and calcium carbides in the melt, the latter is intensively saturated with carbon monoxide (CO), i.e., foamed, which promotes in active stirring of molten slags and deeper reduction of metals, as well as their appropriate precipitation on the bottom of a melting unit. Upon expiry of the indicated time slag portion of the melt was poured off into water with a copper sulphate concentration of 0.5 gIL. Herewith instantaneous foaming of the mass took place. Metal portion of the melt was poured into the mould. The obtained porous vitreous material had an averaged thermal conductivity coefficient of 0.031 W/mK and bulk density of 100 kglm3, in accordance with a specification TU U2.7-14.32553104-001-2004 "Artificial porous rubb'e and sand at expanded siflcate', with the following chemical composition, wt. %: Fe203 -0.33, Si02 -55.80, CaO -37.90, Ti02 -0,09, A1203 -5,88. Chemical composition of a metal phase was as follows, wt. %: Fe -90.745, Ti -2.703, Si - 5/752, C -up to 0.80. Yield of metal made 99.6 % of its initial content in the slag.

Example 3.

Carbon content in the burden containing red mud, which had the following composition (wt. %),: A1203 15.21 Si02 12.02 CaO 10.65 Fe203+FeO 59.12 1102 1.48 Na20 1.52, was brought to 5,5 wt. % by addition of the appropriate amount of anthracite, and the ratio of Si02/CaO was adgusted to 1.4 by addition of sand and limestone. Burdeny was heated with a rate of 18°C/mm to the temperature of 1790°C. Upon achievement of the indicated temperature the melt was held for 15 minutes at such temperature, wherein, due to formation of silicon and calcium carbides in the melt, the latter is intensively saturated with carbon monoxide (00), i.e., foamed, which promotes active stirring of molten slags and deeper reduction of metals, as well as their appropriate precipitation on the bottom of a melting unit. Upon expiry of the indicated time slag portion of the melt was poured off into water with a copper sulphate concentration of 0.3 gIL. Herewith instantaneous foaming of the mass took place. Metal portion of the melt was poured into the mould. The obtained porous vitreous material had an averaged thermal conductivity coefficient of 0.041 W/mK and bulk density of 110 kg/rn3, in accordance with a specification TU U 2.7-14.32553104-001-2004 Artificial porous rubbe and sand at expanded slicate', with the following chemical composition, wt. %: Fe203 -0.61, A1203 -18.19, Si02 -48.74, CaO -31.83, Ti02 -0.63. Chemical composition of a metal phase was as follows, wt. %: Fe -84.51, Ti -1.86, Si -12.83, C -up to 0.80. Yield of metal made 98,9 % of its initial content in the slag.

The metal obtainable using the above described method may be used as a basis for manufacture of special steels and alloys, and the obtainable porous vitreous material may be used for manufacture of products intended for construction such as beat-insulating material, light concrets, filters, ceramics, glass foam and glass fiber.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS1. Method for processing red muds of aluminium industry, the method including obtaining a burden which contains red mud, carbonaceous reducing agent and material for regulating weight ratio of Si02/CaO in the burden, melting the burden to form a metal portion of the melt and a slag portion of the melt, recovery of associate metal and subsequent processing slag portion of the melt, characterized in that carbon content in the burden is brought to 3.5 -5.5 wt. % by introducing carbonaceous reducing agent, sand with limestone or dolomite are introduced as materials to regulate weight ratio of Si02./CaO in the burden, whereby the ratio of Si02/CaO in the burden is brought to 1.3 -1.5, melting the burden is carried out at a temperature of 1690-1790°C, the slag portion of the melt is poured into water for its foaming and formation of porous vitreous material to be used in construction industry, and the metal portion of the melt is cast in moulds.
2. Method according to claim 1, characterized in that anthracite is used as carbonaceous reducing agent.
3. Method according to claim 1 or claim 2, characterized in that heating the burden to achieve a temperature of 1690 -1 790°C is carried out with a rate of 16 -1 8°C/mm.
4. Method according to any one of claims 1-3, characterized in that the slag portion of the melt is poured into water with a copper sulphate concentration (0uSD4 nH2O) of 0.3-0.5 gIL.
5. Method as hereinbefore described with reference to the examples.
6. Any novel feature or combination of features disclosed herein.