FI130393B - Method for using sidestreams containing metal oxides in ferrochrome smelting processes - Google Patents

Abstract

The invention is directed to the briquetting of metal oxide dusts and finely divided materials using cement-based binders. After this, the briquettes can be fed into the electric arc furnaces of FeCr production using the current feeding systems.

Description

METHOD OF SIDE FLOWS CONTAINING METALLIC OXIDES

FOR UTILIZATION IN FERROCHROME Smelting PROCESSES

Technology area

The invention is related to the recycling of metal oxides in the submerged arc furnace of ferrochrome production using a cement-based briquette to recover the metals. In the method, the side streams of ferrochrome and stainless steel production are briquetted with the help of cement into briquettes that can be fed into the immersion arc furnace of ferrochrome production through a normal dosing system and a preheating furnace. In the submerged arc furnace, metal oxides are reduced to metals mainly with the help of carbon, and the metals are recovered in the ferrochrome product.

The level of technology

Materials with a small particle size cannot be processed in the ferrochrome furnace because they cannot reach the reaction zone in the immersion arc furnace due to the gas flow in the feed bed. The metal oxide dusts generated in ferrochrome and stainless steel production are typically fine-grained and thus cannot be fed directly into the submerged arc furnace. In addition, finely divided metal oxides increase the electrical conductivity of the feed mat in the submerged arc furnace, which weakens the production capacity. For the reasons mentioned above, all fines must be agglomerated before being fed into the submerged arc furnace for ferrochrome production.

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IN Briquetting materials are typically mixed in a concrete mixer together with cement 2 and water. The mixture is briquetted with a briquetting machine into briquettes of the desired size and the e 25 briquettes are allowed to harden for the desired time to achieve the desired final strength.

T The production method is the same as for cement-based paving stones. 2

The metal oxides suitable as raw materials for LO Briquette are those that are reduced under the prevailing conditions in the 2 submerged arc furnaces of ferrochrome production. Fractions generated in stainless steel production are, for example, dust from filter plants, scale from casting machines and rolling operations, waste from water treatments, ball-blowing dust from cold rolling mills, and metal deposits from acid treatments from annealing-pickling lines. Fractions produced in ferrochrome production include, for example, fine-grained feed materials formed during pelleting and dosing of submerged arc furnaces. To improve the metal supply, a suitable reducing agent, such as carbon, can be added to the briquette to speed up the reaction kinetics.

The potential of the invention is, for example, in FeCr production, improving chromium supply, reducing the amount of waste, more accurate use of raw materials and avoiding landfill tax. The composition of the ferrochrome product can be modified by changing the composition of the briquette to suit the customer. On the side of steel production, the benefits would be the improvement and cheaper recycling of current side streams.

Previous attempts to solve the problems related to the recycling of side streams have been based on separate dust melts, briquettes made with organic binders and direct reduction processes. The challenges of dust smelters and direct reduction processes are the necessary large investments and possible high operating costs. The problem of using briquettes containing an organic binder, e.g. molasses, in submerged arc furnaces for ferrochrome production is the briquette's — decomposition = before the € reaction zone.

The use of briquettes in stainless steel production — arc furnaces — weakens — the energy efficiency of the furnace and thus reduces production.

The US patent publication US8409320 B2 is known for briquetting materials from the steel industry's oxidic side streams with molasses and feeding them to the electric arc furnace of a steel smelter, where the oxidic metals in the briquette are reduced and slag is obtained

N to effervescence. The patent does not deal with the briquetting of oxidic materials with cement and

N feed into the submerged arc furnace used in the production of ferrochrome or in steel production

S 25 for the arc furnace used. ©

E In the US patent publications US 2014/0352496 and US 2013192422, 2 the production of cement- and molasses-based briquettes containing metal oxides and 3 their use in arc furnaces for stainless steel production are known. The patent focuses on evaporating slag using briquettes in an electric arc furnace. The patent does not deal with the use of briquettes in submerged arc furnaces used for the production of ferrochrome.

From US patent publications US 5654976 A, a process is known which feeds a mixture of scrap metal, chromite, melt material, carbonaceous material and wood shavings into a submerged arc furnace in order to obtain base iron suitable for iron and steel use. The mixture contains about 1-60% chromite, about 1-20% fluxes, about 5-35% carbonaceous material, about 0-10% wood chips, all based on the weight of the mixture scrap.

By keeping the charge in the oven, good energy efficiency is achieved.

In the Japanese patent publications JP 2010202942 A, the calcining = reduction equipment for steel by-products is known, which = includes: briquetting means for briquetting powdered steel by-products into briquettes; a belt conveyor that transports briquettes; first powder removal means arranged in the center of the belt conveyor to remove the generated powder; = transport tank — for receiving briquettes transported from the belt conveyor; drying means for drying the briquettes held in the tank for transfer; a second dedusting means which removes the dust generated when the briquettes are loaded from the belt conveyor to the hopper for transfer; calcining medium for calcining briquettes; and an arc-type electric furnace for melting-reducing calcined briquettes.

In Korean patent publications KR 20120075322 A, an invention is known concerning a pH-neutral binder composition, which is a kind of inorganic binder related to the existing cement industry, and its manufacturing method. The pH-neutral binder composition contains: 40-48% by weight alkali component material; 40-

N 45% by weight neutral component material; 7-20% by weight acid soil

N component materials; where 3 25 slag powder or fly ash is used as a binder in the alkali component material; and the strength-improving stimulant consists of 2 inorganic calcium sulfoaluminate (CSA) cements or alkali activators.

E As an alkali activator, one chosen from Na2SO4 or liquid glass is used; 2 as a neutral component material, one component selected from 3 anhydrous gypsum, dihydrate gypsum and aluminum oxide powder is used. In the acidic component material, silicic acid and organic acid are used to lower the pH concentration of the main binder of the alkaline component, and the organic acid consists of one compound selected from malic acid, citric acid, oxalic acid, isocitric acid and succinic acid.

EP — known from patent publications EP 958385, method for agglomeration of steel mill waste containing oil — includes a step in which a first steel mill waste component is combined with a second steel mill waste component to form a mixture. The first waste component contains essentially dry matter

Oil-free steel mill waste and the second waste component contains steel mill waste containing oil. The first waste component and the second waste component are agglomerated.

In US patent publications US 3276859 A, a method for reducing metals from oxide is known.

From WO patent publications WO 2010103343 A1, a method for improving the production process of high-carbon ferrochrome (HCFeCr) and charge chrome is known, which method — comprises: mixing dried chromium concentrate and finely divided chromium powder in all possible proportions to form raw feed ore; bringing the raw feed ore to a stage where hydrated lime, molasses and bentonite are mixed as binders into said mixture to form a hriketting mixture feed; forming briquettes from said mixture by compaction.

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N Description of the invention 3 25 The published state-of-the-art publications have also presented the utilization of the submerged electric arc furnace used in FeCr production 2. It is not possible to use a submerged electric arc furnace

E to create the conditions for slag effervescence and it is otherwise not suitable for the reduction of effervescent 2 slag-forming materials due to the reactions in the reduction zone and the disruption of 3 gas flows. & 30

The solution according to the present invention is based on the fact that the FeCr furnaces are fed with side streams generated in the production of FeCr and stainless steel, which are materials difficult to utilize with other technologies. Taman | in addition, other by-streams from the metal and mining industry, which contain oxidic metals that can be reduced with carbon, can also be sensibly fed into the FeCr electric arc furnace. 5 The chemical composition of the main components of the feed materials is shown in Table 1.

Table 1. Typical variation limits of briquette composition 0-30% [20-70% 10 -|0-5% |0-20% |0-15% |0-20% [je TT br

The materials according to table 1 above are mixed together with cement and water in a mixer. In addition to cement, e.g. blast furnace slag can be used as reinforcement if desired. The mixture is cast into a mold into a briquette, which is, for example, a 6-cornered 60 x 60 x 60 mm sized briquette. The finished briquette typically contains 2 to 30% cement, of which a part (10 to 70%) can be replaced with e.g. blast furnace slag.

The size is affected by the dosing system of the submerged arc furnace used. The briquettes are allowed to harden, for example, for about 4 weeks in outdoor conditions to achieve final strength before feeding the furnace. The use of various accelerators or heating to increase the hardening is also possible. If desired, a reducing agent 0 — 25% (coke, ferrosilicon, aluminum, silicon carbide) can be added to the briquette, in which case the briquette is reduced

S 20 better because the reducer itself is physically closer to the metal oxide. a

S The briquettes are preferably dosed to the immersion arc furnace of ferrochrome production = through a preheating furnace, where the briquette dries and heats up to a temperature of approximately 500 *C

E in a CO2 atmosphere. In this case, the silicate bonds of the briquette break down and are replaced by 3 25 carbonate bonds, and the briquette itself retains its strength properties. The briquette flows as a plug flow 3 through the supply pipes into the cauldron of the submerged electric arc furnace and at the same time begins to heat up due to the effect of the furnace gases. When the briquette reaches the melting zone, the metal oxides in it begin to be reduced, so that iron oxide is the first to be reduced, partly due to the influence of the furnace gases, and chromium oxide is the last to be reduced. In ferrochrome production, the cement in the briquette increases the pH value of the slag and thus reduces the amount of chromium in the slag by an estimated 0.5 to 5%. The reduced metals melt and dissolve into metal in the furnace, and during melting, a metal alloy is obtained from the furnace to be cast, the composition of which depends on the amount of metal contained in the inputs. In practice, for example, all the Ni, Mo and Fe fractions contained in the feed are reduced to metal. The composition of the obtained metal and slag is shown in Table 2.

Table 2. Composition of metal and slag from the submerged arc furnace

A typical analysis of metal e ls [Ww [we | <]

Typical analysis of slag 0.5—10% 25-30% | 22-28% | 20-25%

One application form of the invention is the use of various secondary raw materials, such as

The use of N catalysts as a raw material for briquettes, in which case the metals of metal oxides are obtained

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S for ferrochrome. Metal oxides containing e.g. 2 15 nickel, molybdenum, titanium, copper, manganese or cobalt can be chosen as raw materials.

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= The invention is explained in more detail below with reference to the attached drawings, of which a

E

> 9 Figure 1 shows an Ellingham diagram that describes the reduction order of metal oxides,

S 20 figure 2 shows how the nickel and manganese levels of the = ferrochrome product change during the feed test,

Figure 3 shows the change in the chromium content of the ferrochrome product during the test, and Figure 4 shows the carbon and silicon contents of the ferrochrome product during the test.

The order of reduction of metal oxides is determined by the Ellingham diagram, Figure 1.

Let's see the different metals that can be reduced with carbon in the arc furnace under the prevailing conditions. Carbon can reduce metals that are above the line that describes the reaction of carbon. This reduction reaction itself depends on temperature and pressure.

In practice, the more noble elements are reduced first, i.e. the order of reduction is Ni, Mo,

Fe, Cr. The diagram also shows the reaction equations for reduction, which are different for different oxidation states of metals, e.g. iron has its own equations according to the oxidation state.

According to the invention, the cement used as a briquetting agent is the only binder that holds the briquette together at a temperature of 400 — 600 °C in the front heating furnace. In addition, it provides sufficient mechanical strength = to the briquette so that it can be fed into the furnace through the dosing system. The cement bonds are replaced by carbonate bonds at the temperature of the preheating furnace, so that the strength of the briquette remains almost original.

The use of cement-based briquettes also brings lime to the submerged electric arc furnace, which makes the pH value of the slag more basic, and thus the yield of chromium itself improves as the degree of reduction increases.

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N The particle size distribution of the raw materials contained in the briquette = depends on the material to be briquetted 3 25 and it should be adjusted as close as possible to the Fuller curve, because then the amount of 2 cement can be minimized and this creates savings in raw materials

E in material costs. 2 Briquettes can be added up to 20% by weight, preferably 3 to 10% by weight of the total feed, always based on the waste material analysis obtained at the time.

S 30

The invention is not limited to the raw materials presented above. With the help of the method, other side streams containing industrial metal oxides can also be used economically.

For example, oxides from the nickel industry would mix nickel with ferrochrome, and the resulting nickel-containing ferrochrome would be more suitable for the production of stainless austenitic steel types.

Figures 2-4 show test results from an experiment in which cement-based briquettes containing scales from stainless steel production were fed into the submerged arc furnace used in ferrochrome production.

Figure 2 shows the change in the nickel and manganese levels of the ferrochrome product during the feed test, i.e. the oxide metals are reduced to the final product.

Figure 3 shows the change in the chromium content of the ferrochrome product in the final product during the experiment. As expected, the chromium content decreased as the proportions of other metals increased.

Figure 4 shows that the carbon and silicon content of the metal remains at a normal level during the briquette test in the final product.

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Claims (4)

PATENT CLAIMS 1. A method for the — utilization of industrial side streams containing metal oxides, — known — for = that the material containing metal oxides is briquetted with the help of cement and the briquettes are fed through a front heating furnace to the submerged arc furnace of ferrochrome production, where the material containing metal oxides is scale from casting machines, scale from a rolling operation, = dust from a filter plant, — side stream from water treatment or metal precipitate of the annealing-pickling line. 2. The method according to claim 1, characterized in that the temperature of the front heating oven is between 400 and 600 °C. 3. A method according to one of the — above — patent claims, characterized in that briquettes are added to a maximum of 20% of the total feed. 4. A method according to one of the — above — patent claims, characterized in that the substance containing metal oxides contains oxides of metals that belong to the group chromium, iron, nickel, titanium, cobalt, manganese and copper.