RU2477820C1 - Treatment method of waste lining from electrolytic molten aluminium - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: treatment method of waste lining from electrolytic molten aluminium involves heating of waste lining in a rotating calcining furnace with addition of solid oxidising and gasifying compounds; at that, as an oxidising compound, calcium carbonate is used, accumulation of gases leaving the rotating furnace, removal of fluorides from them, cooling of heated products and their grinding. Prior to heating, waste lining is crushed to the particle size of 15-20 mm, and then, finally crushed to the particle size of 0.35-1.5 mm by adding an oxidising compound, and namely calcium oxide, and active additives, particularly CaSO₄, FeO in the quantity of 0.23-0.35 wt % of the waste lining weight. Oxygen is used as a gasifying compound. Heating is performed with oxygen blasting to the furnace volume, at the furnace rotation speed of 1.5-2.5 m/min in three stages; at that, at the first stage, heating up to 400°C is performed during 40-60 minutes; at the second stage, the furnace temperature is increased up to 400 to 800°C and exposed during 60-80 minutes; at the third stage, the furnace temperature is increased up to 800 to 1000°C and exposed during 60-80 minutes; at that, when changing over from one stage to another one, reversal of the rotating calcining furnace is changed.

EFFECT: increasing removal degree of fluorides and cyanides.

3 cl, 1 dwg, 2 tbl

Description

The invention relates to the disposal of aluminum production wastes and to environmental protection.

A known method of processing aluminum waste (US Pat. RU No. 2083699, publ. 10.07.1997). SUBSTANCE: flux is placed on the surface of a metal liquid bath with an equal layer, and then protruding pieces are also covered with flux after loading the waste. Heating to 720-800 ° C is carried out in such a way that the degree of overheating of the liquid bath in comparison with its melting temperature is at least 30 ° C. As a flux, a mixture of 20% cryolite (or spent electrolyte of aluminum production) and 80% of spent magnesium electrolyte with composition, wt.%: KCl 70-80; NaCl 10-15; MgCl ₂ 5.8; MgO, CaCl ₂ , H ₂ O - the rest. The processing of aluminum slag should be carried out at a temperature of 750-780 ° C, and scrap - at 720-750 ° C. The ratio of waste and flux should be (0.5-5): 1.

The disadvantage of this method is the insufficient completeness of the extraction of fluorides and cyanides and the formation of gaseous chlorine during the process.

A known method of processing fluorine-containing wastes of aluminum production by electrolysis (application for the invention. RU No. 95113577, publ. 10.10.1997). Usage: in the processing of aluminum production wastes in order to extract partially aluminum, alkali metals and fluorine from them almost completely, as well as to obtain raw materials containing alumina and energy. The method is suitable for disposal of both the spent lining of electrolysis baths and various sludges. SUBSTANCE: fluorine-containing wastes of aluminum production are suspended in a solution of aluminum sulfate at a concentration of 40-165 g / l, heated to 50-100 ° C and stirred for 0.5-4.0 hours until the alkali metal and fluorine compounds pass into the liquid phase almost completely. Then the alumina-carbon fraction (solid residue) is separated by one of the known methods, for example by filtration, and washed with hot water. Alumina carbon residue and fluoride solution are sent for processing.

The disadvantage of this method is the inefficient extraction of fluorides and cyanides associated with high labor costs.

A known method of disposal of spent cyanide-containing coal lining of the electrolyzer (application for the invention. RU No. 97114476, publ. 08.27.1997). The method includes grinding and subsequent processing with the introduction of an iron salt. As the reagent, a ferrous salt is used, which is introduced into the grinding process in the ratio: Fe ^{+ 2} = 0.05-0.25 CN _lib. where Fe ⁺² is the amount of introduced reagent (mol.); CN _fre - the amount contained in the lining of cyanide (mol.).

The disadvantage of this method is the incomplete extraction of fluorides and cyanides.

A known method of processing spent lining from electrolytic smelting of aluminum (patent RU No. 2127850, publ. 03.20.1999), adopted as a prototype. The method includes heating the spent lining. Heating is carried out by feeding the spent lining into the molten salt bath containing sodium, aluminum, calcium, silicon and fluoride salts in a rotary kiln at a salt bath temperature in the range from 1100 to 1250 ° C and maintaining the volume of the molten molten bath with the spent lining throughout the full length of the rotary kiln, with the addition of solid oxidizing and gasifying compounds to reduce the carbon content in the bath, the addition of siliceous material to the molten bath and the melt is cooled to The formation of a vitreous sediment suitable for burial. The method further includes adding a solid oxidizing and gasifying compound to the molten bath to burn carbon to form carbon monoxide. An oxidizing compound containing calcium carbonate is used. Heating and feeding the spent lining, siliceous material and oxidizing compound to form a homogeneous melt is carried out in a rotary kiln. The spent lining, siliceous material and oxidizing compound are supplied to control the viscosity of the salt melt in the range from 1 to 70 Pz and the salt bath is maintained in the molten state from the inlet to the discharge end of the rotary kiln until cooling to form a glassy residue. The method includes collecting gases discharged from the rotary kiln and removing fluorides from them. Fluorides are removed from the exhaust gases leaving the auxiliary combustion chamber and recycled to the melt pool or the rotary kiln pool, and the vitreous frit, the heating product, is removed from the ash temper-cooler.

The disadvantage of this method is the insufficient completeness of the extraction of fluorides and cyanides and the formation of a significant amount of carbon monoxide CO during processing.

The technical result is an increase in the degree of extraction of fluorides (up to 85%) and cyanides to ensure the reuse of the spent lining or its disposal in the form of non-toxic waste.

The technical result is achieved by the fact that in the method for processing spent linings from aluminum electrolytic smelting, which includes heating the spent linings in a rotary kiln with the addition of solid oxidizing and gasifying compounds, calcium carbonate is used as an oxidizing compound, collecting gases discharged from the rotary kiln, removing of them fluorides, cooling the heating products and their grinding, before heating, the spent lining is crushed to a particle size of 15-20 mm, and then grinding to a size class of 0.35-1.5 mm, with the addition of calcium oxide as an oxidizing compound, and active additives, which are used as CaSO ₄ , FeO in an amount of 0.23-0.35 wt.% by weight of the spent lining the heating is carried out with oxygen blowing into the furnace volume at a furnace rotation speed of 1.5-2.5 m / min in three stages, in the first stage heating is carried out to 400 ° C for 40-60 minutes, in the second stage the furnace temperature is increased to 400-800 ° C and withstand 60-80 minutes, in the third stage, the temperature of the furnace is increased to 800-1000 ° C and withstand 60-80 minutes m during the transition from one stage to another, the reverse of the rotary kiln is changed.

Fluorides removed from gases are disposed of in distillers.

After cooling and grinding the solid heating products, calcium fluoride is separated with silicon.

Spent lining is not immediately disposed of as it is a toxic material and contains soluble toxic cyanides and fluorides. The content of soluble cyanides is in the range from about 1000 to 2000 parts per million, and the content of soluble fluorides is about 3000-8000 mg / l. The total fluoride content in the spent lining is usually in the range from about 25-35 wt.%.

Crushing the spent lining to a particle size of 15-20 mm and grinding the spent lining with the addition of calcium oxide as an oxidizing compound, as well as active additives, which use CaSO ₄ , FeO in the amount of 0.23-0.35 wt.% Of the weight of the spent lining, to a particle size class of 0.35-1.5 mm, provide uniform heating of the feedstock and increase the degree of extraction of soluble fluorides and cyanides.

The feedstock is a mixture of waste lining particles, calcium oxide and active additives with a particle size of 0.35-1.5 mm. The input at the grinding stage of calcium oxide and active additives ensures a uniform distribution of the particles of the feedstock in the resulting mixture and increases the degree of extraction of soluble fluorides and cyanides.

The addition of calcium oxide and active additives in an amount of 0.23-0.35 wt.% By weight of the spent lining activates the flow of chemical reactions, which increases the degree of extraction of soluble fluorides and cyanides.

The use of FeO as an active additive creates a bond of soluble cyanides at temperatures up to 400-600 ° C.

The use of CaSO ₄ as an active additive ensures the binding of soluble fluorides of the spent lining to calcium fluoride CaF ₂ and the conversion of alkali metals to sulfates, for example Na ₂ SO ₄ , for their further decomposition to SO ₂ and Na ₂ O at temperatures up to 800-1000 ° С .

The use of calcium carbonate as an oxidizing compound, for example, in limestone, and calcium oxide provides the transition of silicon oxide and fluorine to dicalcium silicate and calcium fluoride or cuspidine - CaF ₂ · 3CaO · 2SiO ₂ at temperatures up to 800-1000 ° С.

The heating of the feedstock is carried out with oxygen blowing into the furnace volume, activating the oxidation processes in the furnace due to exothermic reactions. Oxygen in the composition of the oxygen-air mixture is used as a gasifying compound. Carbon and organic materials are oxidized to carbon dioxide and water vapor. The carbon component of the feedstock burns in the furnace, ensuring the occurrence of reduction reactions, which also contributes to its more complete interaction of the charge components. At the same time, additional heat is released during the combustion of the carbon component, which allows to reduce the specific fuel consumption for sintering. In the temperature range of 600-800 ° C for 60 min, the degree of decarburization of the feedstock increases from 14 to 95%. Inorganic compounds of the feedstock, such as soluble cyanides and fluorides, are oxidized to insoluble non-toxic compounds, which make it possible to reuse the spent lining or to bury it in the form of non-toxic waste.

The rotational speed of the furnace 1.5-2.5 m / min provides the most complete heating of the feedstock and increases the degree of extraction of soluble fluorides and cyanides from the spent lining by increasing the contact area of the solid particles of the charge.

The heating of the furnace to 400 ° C for 40-60 minutes provides a gradual heating of the feedstock and the removal of moisture from the feedstock, which subsequently allows the rotation of the feedstock to effectively remove fluoride particles dispersed on the surface of the carbon or refractory material. In addition, the removal of moisture from the mixture prevents the formation of hydrates with a subsequent increase in temperature after 400 ° C.

The heating temperature in the range of 400-800 ° C provides the decomposition of soluble fluorides and cyanides from the spent lining and their oxidation:

NaCN = Na ⁺ + CN ^-

Na ₄ [Fe (CN) ₆ ] = 4Na ⁺ + [Fe (CN) ₆ ] ^4-

AlF ₃ = Al ³⁺ + 3F ^-

Oxidation of soluble fluorides and cyanides from spent lining:

2NaCN + 1,5O ₂ = СО ₂ + N ₂ + Na ₂ O

2Na ₄ [Fe (CN) ₆ ] + 14.5O ₂ = Fe ₂ O ₃ + 12CO ₂ + 6N ₂ + 4Na ₂ O

4NaF + O ₂ = 2Na ₂ O + 2F ₂

2AlF ₃ + 1,5O ₂ = Al ₂ O ₃ + 3F ₂

The content of soluble fluorides and cyanides in the solid residue at the outlet of the furnace was analyzed using ICS measurements. Existing forms of fluoride at the outlet of the furnace were determined by x-ray diffraction. Studies have shown that the temperature of the furnace must be maintained in the range of 400-800 ° C for 60-80 minutes to ensure complete decomposition and oxidation of cyanides.

The heating temperature of 800-1000 ° C creates an active and complete interaction of soluble fluorides with oxidizing compounds, which use calcium carbonate, such as limestone, and calcium oxide, as well as active additives, which use CaSO ₄ .

The interaction of the spent lining with calcium oxide and CaCO ₃ provides additional extraction of soluble fluorides:

2NaF + CaO = CaF ₂ + Na ₂ O

2AlF ₃ + 3CaO = 3CaF ₂ + Al ₂ O ₃

2NaF + CaCO ₃ = CaF ₂ + Na ₂ CO ₃

2AlF ₃ + 3CaCO ₃ = 3CaF ₂ + Al ₂ (CO ₃ ) ₃

Al ₂ (CO ₃ ) ₃ = Al ₂ O ₃ + 3CO ₂

Na ₂ CO ₃ = Na ₂ O + CO ₂

When reacting with an oxidizing compound, calcium carbonate in the form of limestone at a temperature of 800-1000 ° C, the formation of more complex compounds - NaAlSiO ₄ and Ca ₄ Si ₂ O ₇ F ₂

2NaF + CaCO ₃ + Al ₂ O ₃ · 2SiO ₂ = CaF ₂ + 2NaAlSiO ₄ + CO ₂

When sodium and calcium are converted to complex compounds, their degree of extraction increases.

Aluminum fluoride has low chemical resistance and goes into a gaseous state during heating in the presence of oxygen.

When the temperature rises above 800 ° С during contact of contact with air during heating, aluminum fluoride partially evaporates, and it is trapped with exhaust gases, and then it is separated and sent to the feed hopper (or furnace).

At a temperature of 800-1000 ° C, the bulk of aluminum fluoride reacts with an oxidizing compound - calcium carbonate in the form of limestone to produce aluminum carbonate, which decomposes to aluminum oxide.

Increasing the treatment time to 60-80 min increases the degree of extraction of soluble sodium and aluminum fluorides with the formation of stable and insoluble compounds (experimental data).

Soluble fluorides bound in the insoluble form during heating are removed from the process as a solid residue. Gaseous fluorides removed from the gases are sent to produce AlF ₃ .

Changing the reverse of the kiln during the transition from one stage to another provides a smooth thermal transition and a reduction in the length of the furnace.

The method is illustrated in Fig. 1, where a schematic diagram of the method is shown, here 1 is a jaw crusher, 2 is a ball mill, 3 is a feed hopper for feedstock, 4 is a rotary kiln, 5 is a heat exchanger, 6 is a refrigerator, 7 is a feeder, 8 - gas vent system.

Table 1 shows the process parameters of the process, table 2 shows the data on the weight loss of aluminum fluoride during heating of the spent lining, wt.%.

The method is as follows. Before heating, the spent lining is crushed to a fineness class of 15-20 mm in a jaw crusher 1 and grinding in a ball mill 2 to a fineness class of 0.35-1.5 mm. At the grinding stage, oxidizing compounds are introduced, which use calcium carbonate (in the form of limestone) and calcium oxide, as well as active additives, which use CaSO ₄ and FeO, in an amount of 0.23-0.35 wt.% linings, which are also crushed to a size class of 0.35-1.5 mm. The feedstock that is accumulated in the feed hopper 3 is a mixture of solid particles from the spent lining, limestone, calcium oxide and active additives. Heating of the feedstock is carried out in a rotary kiln 4 at a rotational speed of the rotary kiln 4 of 1.5-2.5 m / min with periodic exposure to a mixture of air and oxygen with varying degrees of oxygen enrichment. The degree of enrichment with oxygen is regulated depending on the heating temperature. The heating of the feedstock is carried out in three stages. In the first stage, heating is carried out to 400 ° C for 40-60 minutes. There is no oxygen supply. In the second stage, the temperature is increased from 400 to 800 ° C and held for 60-80 minutes. The degree of enrichment with oxygen is chosen 20%. In the third stage, the temperature is increased from 800 to 1000 ° C and held for 60-80 minutes. The degree of enrichment with oxygen is chosen 35%. When moving from one stage to another, the reverse of the kiln 4 is changed. The gases discharged from the rotary kiln 4 are collected using a gas exhaust system 8, and then gaseous fluorides are removed from them. Fluorides removed from gases are disposed of in distillers. The solid heating product is a solid residue suitable for burial and processing, cooled using a refrigerator 6, milled and separated by specific gravity of calcium fluoride and compounds with silicon from the rest of the solid residue.

Examples of the method are given in table 1. After loading the spent lining with a fraction of 0.4 mm into the furnace, heating was carried out to a temperature of 380 ° C; the degree of moisture removal was 86.5% with a holding time of 40 min. During the second regime, at a temperature of 490 ° С and a degree of oxygen enrichment, 20% of the charge material interacted with active additives — CaSO ₄ , FeO — in the amount of 0.3 wt% of the weight of the spent lining and with oxygen, which ensured the complete decomposition of soluble cyanides. The temperature of the third regime was 845 ° С with an oxygen enrichment degree of 35%, and the exposure time was 65 min. The rotational speed of the furnace in all modes was 1.72 rpm. As a result of the process, a solid residue was obtained, which is cooled and ground, and then sent to the separation of silicon and calcium fluoride. The soluble fluoride recovery was 85.7%.

Thus, the method allows to obtain the degree of extraction of soluble fluorides 85% and cyanides. The expected average concentration of soluble fluorides and cyanides in the solid residue is 40.1 and 0.055 mg / l, respectively.

The use of this method of processing spent lining from electrolytic smelting of aluminum allows you to:

- increase the completeness of the extraction of fluorides and decomposition of cyanides;

- get refined carbon-graphite and refractory materials, for example refined mullite;

- increase the safety of waste disposal.

Table 1 METHOD FOR PROCESSING WASTE LAYING FROM ELECTROLYTIC ALUMINUM Smelting Experience number The size of the fraction on crushing, mm The size of the fraction on crushing, mm Temperature 1 mode, ° C The degree of moisture removal,% Temperature 2 modes, ° С Temperature 3 modes, ° С Exposure time (3 modes), min Rotation speed rpm Fluoride recovery,% one 13.9 0.15 365 80.5 401 810 61 1.56 82 2 14.5 0.25 365 80.3 422 813 62 1.60 83.7 3 15.0 0.30 360 78.1 430 815 64 1.65 84.8 four 15.2 0.35 370 82.5 455 830 65 1.70 85.2 5 15.2 0.40 380 86.5 490 845 65 1.72 85.7 6 15.5 0.47 385 90.6 495 860 66 1.75 85.8 7 15.5 0.56 385 90.5 510 875 67 1.85 86.3 8 15.7 0.60 387 90.5 515 880 68 1.87 86.9 9 15.9 0.65 388 90.3 530 890 70 1.90 87.3 10 16.0 0.72 383 88.7 540 900 70 1.90 87.6 eleven 16.0 0.80 390 92.1 580 910 71 1.92 88.2 12 16.5 0.86 390 91.6 585 915 72 1.95 88.3 13 16.8 0.95 391 92.3 600 923 73 2.00 88.1 fourteen 17.2 1.00 388 90.2 620 945 74 2.10 87.5 fifteen 17.5 1.15 395 94.4 650 950 74 2.15 87.2 16 17.7 1.2 393 93.5 680 960 75 2.17 86.8 17 18.1 1.25 394 94.1 710 975 75 2.25 86.4 eighteen 18.5 1.32 391 92.0 755 983 77 2.28 86.1 19 18.7 1.35 390 91.4 765 991 78 2.31 85.9 twenty 19.0 1.40 385 90.1 785 994 79 2,35 85.6 21 19.3 1.45 370 81.7 780 1000 79 2.41 85.4 22 19.5 1.50 365 80.1 800 1000 80 2.49 83.7 23 19.9 1.52 365 79.8 800 1005 80 2.48 82.8 24 20.4 1.55 360 77.9 790 1010 80 2.49 81.5

table 2 METHOD FOR PROCESSING WASTE LAYING FROM ELECTROLYTIC ALUMINUM Smelting Arr. Wednesday 750 ° C 800 ° C 850 ° C 900 ° C 950 ° C one Air 38.8 58.9 65.4 69.2 72.6 2 CaO 22.3 21.8 21.7 23.9 25.0 3 Heating product 29.1 28.9 30.1 31.2 30.5

Claims (3)

1. A method of processing spent linings from aluminum electrolytic smelting, including heating the spent linings in a rotary kiln with the addition of solid oxidizing and gasifying compounds, using calcium carbonate as the oxidizing compound, collecting gases discharged from the rotary kiln, removing fluorides from them, cooling heating products and their grinding, characterized in that before heating, the spent lining is crushed to a particle size of 15-20 mm, and then grinding to class fineness of 0.35-1.5 mm with the addition of calcium oxide and active additives as an oxidizing compound, which are used as CaSO ₄ , FeO in an amount of 0.23-0.35% by weight of the spent lining, oxygen is used as a gasifying compound and the heating is carried out with oxygen blast into the furnace volume at a furnace rotation speed of 1.5-2.5 m / min in three stages, while in the first stage they are heated to 400 ° C for 40-60 minutes, in the second stage the temperature the furnace is increased from 400 to 800 ° C and held for 60-80 minutes, in the third stage, the temperature of the furnace yshayut from 800 to 1000 ° C and maintained 60-80 minutes, while at the transition from one stage to the other changing reverse rotary kiln. 2. The method according to claim 1, characterized in that the fluorides removed from the gases are disposed of in distillers. 3. The method according to claim 1, characterized in that after cooling and grinding the solid heating products, calcium fluoride and compounds are separated with silicon.

Images