FI58349C - FOERFARANDE FOER BEHANDLING AV METALLHALTIGA STOFT OCH AVFALL FRAON METALLURGISK INDUSTRI - Google Patents

Description

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Patent and Registration Office AmMcm utk | d odi utUkrMtwi pubUcund 30.09.80 (32) (33) (31) P) nrde * ty * tuolk * ui — Begird prlorlttt (jl) Outokumpu Oy, Outokumpu, FI; Töölönkatu U, 00100 Helsinki 10, Finland-Finland (FI) (J2) Frans Heikki Tuovinen, Ulvila, Matti Olavi Vattulainen, Kiviranta,

Heikki Pellervo Saari, Pori, Arimo Rainer Kortehisto, Pori, Finland-Finland (Fl) (jU) Berggren Oy Ab (5U) Method for the treatment of metallic dust and waste from the metallurgical industry - Förfarande för behandling av metallhaltiga stoft och avfall fr In metallurgisk Industri the invention relates to a process for the treatment of metallic dust and waste generated in the metallurgical industry and in particular in the stainless steel industry.

The stainless steel industry generates large amounts of various wastes containing significant amounts of iron, chromium, nickel, molybdenum, zinc and lead as metals, oxides and hydroxides. The waste consists of fine dust, moist fines or oily abrasive wool.

The neutralized waste containing dissolved heavy metals from the acid pickling of the stainless steel industry is a filter pressed precipitate containing chromium, nickel, iron and molybdenum hydroxides, calcium fluoride, various sulphates and about 50% by weight of water. However, the storage of such waste poses a potential environmental hazard, as metal hydroxides, especially chromium and nickel hydroxides, become soluble if the pH of the waste drops to the acidic side. Dried neutralization waste has also, due to the calcium fluoride it contains, been used experimentally as a flux in an arc furnace or AOD converter.

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From the gases obtained from the arc furnace of the stainless steel smelter and the AOD converter, dry dust is separated by filters, in which chromium is present as hexavalent, forming chromates. Dust storage is difficult as it contains environmentally hazardous hexavalent chromium, lead and zinc. Dust has been experimentally fed for remelting.

Oily abrasive wool is produced in connection with the surface repair grinding of stainless steel strips. Normally, the cooling oil content of abrasive wool is approx.

10-15%. The metal part of the wool almost completely corresponds to the original abrasive material.

Abrasive wool is either stored as such in a landfill or incinerated. Attempts have also been made to add wool directly to the arc furnace. In this case, however, the problem has been the sharp rise in the amount and temperature of the flue gases due to the combustion of the oil. This and the cracked oil products carried with the gases have caused a lot of malfunctions in the flue gas cleaning equipment. Wool has also been briquetted both separately and together with other dry metal-containing waste from the steel mill. The briquettes can be further added to the arc furnace or AOD converter. Prior to remelting, the oil contained in the grinding wool can also be extracted and the wool added as such to the defrosting.

The environmental damage caused by abrasive wool is mainly due to the oil released from the wool during storage.

In addition, the stainless steel industry generates various oxide wastes, such as ball blast dust, which are inert and do not normally cause any environmental damage.

There are several methods for recovering waste in which the waste is reduced in an electric or drum kiln, usually by dusting and grinding, either as such or pelletized and the wet precipitates dried. The disadvantages of the methods are the occupational hygiene and environmental problems caused by dusting, in which hexavalent chromium plays a large part, the high investment and operating costs, and the fact that the methods cannot treat all the different wastes at the same time.

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In the method according to the invention, all waste is slurried in water or pickling acid slurry, if necessary raising the pH of the slurry above 7 to precipitate heavy metals in solution, then dewatering the slurry, e.g. in a filter press, so as to result in lumpy waste that can be safely landfilled or stored for further storage. to recover existing precious metals. With this method, all waste can be treated together. It also does not cause occupational hygiene or environmental problems, as there are no dusting disadvantages and hexavalent chromium is reduced to trivalent in the process. The advantages also include low investment and operating costs and a fairly high recovery of metals. Zinc and lead can also be recovered and recycled separately. In addition, slurrying the waste into water or pickling acid slurry and filtering off the water results in a homogeneous cake, and it is easy to add and evenly mix various additives into the slurry as needed. Various effluents from the plant can advantageously be used for sludge.

The invention will be described in more detail below with reference to the accompanying drawing, which shows a process diagram for applying the method according to the invention.

The process according to the invention consists of the following unit processes: mixing of dry dusts 1 with neutralization slurry 2, pressure or vacuum filtration 3, drying preheating 4 utilizing melting gases and reducing melting 5. The process diagram is shown in the figure.

All the stored dry waste 1 is mixed with the neutralization slurry in a mixer 2 so that the pH is above 7 and preferably n.

9-10 and filtered through a pressure or vacuum filter 3. The solution 9 is returned to the slurry 2. During the slurry, lime and Fe sulphate are added to adjust the filterability and strength of the filtered cake together with the precipitate of the neutralization plant.

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features. At the same time, Cr is reduced to Cr. Also, part or all of the reducing agent, coke or coal, can be added at this stage.

Filtration 3 gives a cake with a moisture content of 15-40% and a compressive strength when wet of the order of 20-30 kp / cm, which is sufficient to withstand the mechanical stresses of drying. During drying, the compressive strength increases. When dropped from the filter, the cake is crushed into pieces suitable for melting.

The pieces of filter cake are fed 10 to a belt or tumble dryer 4, where drying and preheating is achieved by burning the reduction gases 11 of the melting furnace. No external energy, i.e. other than the heat of combustion of the reduction gases, is required. The flue gases 12 are cleaned before being introduced into the chimney.

The preheated material 13 and the additional coke and any necessary additives are fed to an electric furnace 5, where reduction, melting and formation of CO gas take place. An alloy 6 containing precious metals and slag 7 are obtained, which are periodically taken out, as well as dusty flue gases 14. The yields of iron, chromium, nickel and molybdenum in the alloy 6 are 85-95%. 94-98% of zinc and lead are transferred to dust.

The dusty flue gases 14 are cleaned and the CO-containing gas 11 is blown onto the dryer 4 for combustion. The separated dust 8, which has a high content of zinc and lead, goes to further processing.

The waste treatment process described above can combine the treatment of all wastes containing precious metals related to the operation of a stainless steel plant and, in addition, achieve a fairly high metal recovery. In addition, pilot-scale test runs have shown that there are no environmental impacts from the treatment, as the treatment reduces hexavalent chromium to trivalent and binds the oils to waste. The advantage of the treatment is that the process produces unpolluted waste suitable for landfill or an intermediate that is easy to store and contains all the waste, a filter precipitate from which, when remelted, a Fe-Cr-Ni-Mo alloy is available as a raw material, a zinc- and lead-rich recyclable dust as well as dumped wreckage.

The process is cheap in terms of both operating costs and investment. The process can avoid occupational hygiene and environmental problems because dust is slurried and hexavalent chromium is reduced to trivalent. Waste can already be pre-treated at each steel plant or collected at one central treatment plant.

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The invention for treating dusts in the steel industry has been described above. The invention is equally suitable for the treatment of other types of metallurgical dusts.

Example 1470 kg of a waste mixture containing pickling acid sludge, ballwood dust, AOD converter gas cleaning dust and abrasive wool, 50 kg of lime, 20 kg of ferrous sulphate and 80 kg of coke dust were slurried in 2 water to a pH of about 10 and filtered. to give 1640 kg of a cake with a moisture content of 30%, which was dried and preheated in a tumble oven 4. The temperature of the dried product 3 was 350 ° C. CO gas consumption was 180 m and combustion air 430 m.

The dried product and 80 kg of coke were melted in an arc furnace 5.

An alloy of 340 kg, slag 340 kg and dust 170 kg were obtained, with the following analyzes:

Alloy Slag Dust

Fe 72.3 3.5 0.9

Cr 13.5 2.3 0.4

Ni 10.1 0.6 0.1

Zn 0.03 0.03 6.2

Pb 0.06 0.06 3.5

Mo 1.2 0.06

SiO 2 10.0 6.0

CaF2 41.8 35.8

CaO 32.2 21.2

The temperature of the alloy was 1550 ° C, the slag 1600 ° C and the exhaust gases 1300 ° C. 900 kWh of electricity was consumed. The metals were divided into alloy, slag and dust as follows:

Fe Cr Ni Zn Pb Mo

Alloy% 95.0 85.0 94.2 0.9 3.1 95.5

Slag% 4.5 13.7 4.9 0.9 3.1 45.5 Dust% 0.5 1.3 0.8 98.2 93.8 4.

It is clear that a filtered cake can be dumped if further processing is not profitable. The waste does not pollute the environment and can therefore be safely disposed of in landfills. If desired, such waste can be further treated in any known manner to recover the precious metals therein.