CN112689683A

CN112689683A - Method for utilizing a side stream containing metal oxides in a ferrochrome smelting process

Info

Publication number: CN112689683A
Application number: CN201980060083.4A
Authority: CN
Inventors: K·瓦洛; P·林贾
Original assignee: Outokumpu Oyj
Current assignee: Outokumpu Oyj
Priority date: 2018-09-26
Filing date: 2019-09-25
Publication date: 2021-04-20
Also published as: TWI820222B; JP2022501497A; FI130393B; TW202024343A; WO2020065134A1; FI20185805A1; ZA202101409B; KR20210065943A; SE545037C2; SE2150308A1; JP7322141B2

Abstract

The present invention relates to the agglomeration of metal oxide dust and fines with cement-based binders. Thereafter, the agglomerates may be fed through the already existing inlet system into the electric arc furnace for ferrochrome production.

Description

Method for utilizing a side stream containing metal oxides in a ferrochrome smelting process

Technical Field

The present invention relates to the reuse of metal oxides by recovery of metals using cement-based agglomerates in a submerged electric arc furnace for ferrochrome production. In this process, the side streams from ferrochrome and fine steel production form agglomerates with the cement, which agglomerates can be fed through a standard inlet system and through a preheating furnace into a submerged arc furnace. In a submerged arc furnace, the metal oxides are reduced to metal, primarily with carbon, and the metal is recovered in the ferrochrome product.

Background

In ferrochrome furnaces, materials with small particle sizes cannot be used because they will not reach the reaction zone, but will stop due to the presence of the gas flow on the feed material layer. The metal oxide dust generated in ferrochrome and stainless steel production is typically very fine and cannot be so fed into a submerged arc furnace. In addition, the fine metal oxides increase the electrical conductivity of the feed layer within the furnace, which reduces throughput. For these reasons, all fines must be agglomerated before being fed into the submerged electric arc furnace for ferrochrome production.

The material to be formed into a briquette is typically mixed with cement and water in a concrete mixer. The mixture is used to form briquettes of the desired size in a briquetting machine and allowed to dry for the desired time to achieve the desired strength. The production method here is the same as the production method used for producing cement-based slate.

Metal oxides from ferrochrome production that may be reduced in a submerged arc furnace under selected conditions may be used to form agglomerates. In fine steel production, suitable fractions are, for example, dust from filtration plants, flakes from casting and rolling mills, slurries from water treatment, shot-blasting dust from cold rolls and metal precipitates formed by acid treatment in the annealing-picking process. In ferrochrome production, a suitable fraction is, for example, fines formed in the granulation and furnace feed. To improve metal yield, a suitable reducing material (such as carbon) may be added to the agglomerates to accelerate the reaction kinetics.

The potential of the present invention in ferrochrome production is to improve chromium yield, reduce waste, better utilize raw materials and avoid feed landfills. By varying the composition of the mass, the composition of the ferrochrome may be varied to suit the customer. In fine steel production, the advantage would be to improve the recycling of the current side stream and to make the recycling cheaper.

Previous solutions to the problems associated with recirculating sidestreams have been based on separate dust smelting machines, briquetting with organic binders and direct reduction processes. Dust smelters and direct reduction processes are challenging due to the large investment and potentially high operating costs required. The use of organic binders, such as molasses, may cause the agglomerates to break apart before reaching the reaction zone in a submerged electric arc furnace for the production of ferrochrome. In fine steel production, the use of these agglomerates in an electric arc furnace reduces the energy efficiency and therefore the production.

US patent 8409320B2 discloses agglomerating a steel production side stream containing oxides with molasses and feeding these to an electric arc furnace of a smelting plant where the metals are reduced and the slag boiled. This patent does not relate to agglomerating oxide material with cement and feeding the agglomerates into a submerged electric arc furnace for ferrochrome production or into an electric arc furnace for steel production.

US patent publications US2014/0352496 and US 2013192422 disclose the preparation and use of cement and molasses based briquettes in electric arc furnaces for fine steel production. The patent focuses on boiling the slag in an electric arc furnace together with the agglomerates. This patent does not relate to the use of agglomerates in a submerged arc furnace for ferrochrome production.

Detailed Description

Nowhere in the prior art publication is there any mention of the use of a submerged arc furnace for use in ferrochrome production. Submerged arc furnaces are not able to create slag boiling conditions and, in addition, are not suitable for reducing materials from boiling slag due to reactions and disturbed gas flow at the reduction zone.

The solution according to the invention is based on feeding material from the side streams of ferrochrome and fine steel production to a ferrochrome furnace, which would be difficult to use with any other technology. In addition, it is possible and sensible to feed other side streams from the metal industry and the mining industry into the chromium iron electric arc furnace, which side streams contain carbon-reducible metal oxides.

The chemical composition of the main components of the feed is shown in table 1.

TABLE 1 typical composition of agglomerates。

Cr₂O₃

Fe₂O₃

NiO

MoO₃

SiO₂

CaO

C (Coke)

0-30％

20-70％

0-10％

0-5％

0-20％

0-15％

0-20％

Using the material according to table 1, a mixture is formed with cement and water. In addition to cement as reinforcing agent, blast furnace slag, for example, may be used as required. The mixture is cast into briquettes, for example 6 tipped briquettes of 60 x 60mm size. Typically, the finished briquette contains 2% to 30% cement, a portion of which (10% to 70%) can be replaced with, for example, blast furnace slag. The size of the agglomerates depends on or is influenced by the feed or inlet system in the submerged arc furnace used. The briquettes were allowed to dry under outdoor conditions for about 4 weeks to reach final strength before being fed into the furnace. Accelerators and heat may also be used to adjust hardness. If desired, 0-25% reducing agent (coke, ferrosilicon, aluminum, silicon carbide) can be added to the agglomerates, whereby the reduction is better because the reducing agent itself is physically closer to the metal oxide.

The agglomerates are preferably fed into the submerged arc furnace via a preheating furnace, in which the agglomerates are charged in CO₂Dried in an atmosphere and heated to about 500 ℃. This breaks the silicate bonds and replaces them with carbonate bonds, while the agglomerates retain their strength. The agglomerates flow as plug flows through the inlet pipe into the tank of the submerged arc furnace and at the same time start to heat up due to the furnace gas. When the agglomerates reach the melting zone, the metal oxides inside begin to reduce, first reducing the iron oxide to some extent by the gas in the tank and finally reducing the chromium oxide. In a submerged electric arc furnace for ferrochrome production, the cement contained in the agglomerates increases the pH of the slag, thereby reducing the chromium content of the slag by about 0.5-5%. The reduced metal is melted and dissolved to metal in the furnace and the melt is melted outside the furnace as a castable alloy, the composition of which depends on the metal content of the feed. In practice, for example, all Ni, Mo and Fe fractions in the feed are reduced to metal. The compositions of the metal and slag are shown in table 2.

TABLE 2 composition of metals and slags obtained from submerged arc furnace

Typical analysis of metals
						Cr	Si	Ni	Mo	C
40-55％	3-10％	0-5％	0-2％	5-8％
						Typical analysis of slag
Cr	Fe	SiO₂	Al₂O₃	MgO	CaO
						0.5-10％	0-4％	25-30％	22-28％	20-25％	2-5％

In one embodiment, various auxiliary raw materials, such as catalysts, are used as the raw material for the agglomerates, thereby allowing the metals in the metal oxides to be recycled to the ferrochrome. These raw materials may be metal oxides comprising nickel, molybdenum, titanium, copper, manganese or cobalt.

The invention is explained in more detail below with reference to the figures.

Fig. 1 shows an Ellingham (Ellingham) diagram, which shows the reduction sequence of metal oxides.

Figure 2 shows how the levels of nickel and manganese in the ferrochrome product varied during the feed experiments.

Fig. 3 shows the change in chromium content in the ferrochrome product during the experiment.

Figure 4 shows the carbon and silicon content in the ferrochrome product during the experiment.

The ellingham diagram of fig. 1 defines the reduction sequence of the metal oxides. Different metals that can be reduced by carbon in an electric arc furnace under selected conditions can be seen. The carbon is able to reduce the metal above the line where the carbon reaction occurs. The reduction itself depends on temperature and pressure. In the process, the noble metal is reduced first, so the reduction order is Ni, Mo, Fe, Cr. The figure also shows the reduction reaction formula as a function of the oxidation stage, e.g. the respective formulas for the different oxidation stages of iron.

According to the invention, cement is the only binding material that can hold the agglomerates together at preheat furnace temperatures of 400 to 600 degrees. In addition, it gives the agglomerates sufficient mechanical strength so that they can be fed into the furnace through the inlet system. Under the heating of the preheating oven, the chemical bonds of the cement become carbonate bonds, thereby almost completely maintaining the initial strength of the briquettes. The use of cement-based briquettes also provides lime for submerged arc furnaces, which increases the pH of the slag, resulting in higher degree of reduction and higher chromium production.

The particle size distribution in the agglomerates depends on the raw materials used in the formation of the agglomerates. The particle size distribution should follow the Fuller (Fuller) curve as closely as possible, as this allows to minimize the amount of cement used and provides a raw material saving. The amount of added agglomerates may be up to 20 wt%, preferably 3-10 wt% of the total material feed, according to the analysis of the slag material currently obtained.

The present invention is not limited to the above raw materials. By means of this method, other side streams containing metal oxides can also be used economically. For example, oxides from the nickel industry blend nickel into ferrochrome, and the ferrochrome thus formed would be better suited for the manufacture of austenitic steel grades.

Figures 2 to 4 show the results of an experiment in which cement-based agglomerates containing flakes from fine steel production are fed into a submerged electric arc furnace for ferrochrome production.

Figure 2 shows the variation of nickel and manganese content in the ferrochrome product during the feed experiments, i.e. the reduction of metal oxides to the final product.

Figure 3 shows the change in chromium concentration in the ferrochrome product in the final product during the experiment. As the ratio of the other metals increases, the chromium concentration decreases as expected.

Figure 4 shows that the carbon and silicon concentrations in the final product were maintained at normal levels during the lump experiment.

Claims

1. A method of utilizing a side stream containing industrial metal oxides, characterized in that a material containing metal oxides is agglomerated together with cement and the agglomerates are fed into a submerged electric arc furnace for ferrochrome production.

2. The method according to claim 1, characterized in that the material comprising metal oxide is a flake from a casting machine, a flake from a rolling mill, dust from a filtration operation, a side stream from water treatment or a metal slurry from an annealing-pick-up line.

3. A method according to claim 1 or 2, characterized in that the briquettes are fed through a preheating furnace.

4. A method according to claim 3, characterized in that the temperature in the preheating furnace is between 400 ℃ and 600 ℃.

5. A method according to any of the preceding claims, characterized in that lumps are added which represent at most 20% of the total feed.

6. The method according to any of the preceding claims, characterized in that the metal oxide containing material comprises an oxide of a metal selected from chromium, iron, nickel, titanium, cobalt, manganese and copper.