Method of recovering metals from metallurgical waste

Abstract

A method for the recovery of metals from metallurgical waste, especially zinc and iron from dusts is characterized in that the zinc-bearing dusts are granulated with the addition of a reducing agent (coal dust, carboniser, fly ash) in an amount of 7 - 25 % by weight of the metal-bearing charge and material thus prepared is directed to an electric arc-resistive furnace where it is melted at a temperature of 1350 - 1600ºC, where the processes of reduction of zinc and iron oxides take place simultaneously, after which the zinc in gaseous form is stripped and oxidised in the air stream, followed by a two-stage dedusting of the gaseous phase, where a coarser dust fractions are collected in the pre-dusting chamber and returned to the process, further in the dedusting system, the gases are cooled to below 200°C and directed to bag filters for the capture and recovery of zinc oxide in the form of fine dust, while an iron alloy and a waste slag are drained from it, after which the ferrous alloy is directed to an induction or arc furnace, where its chemical composition is modified by removing excess of carbon, sulphur and phosphorus and by adding a steel scrap or an alloying additives to a commercial alloys.

Description

Method of recovering metals from metallurgical waste

[0001] The object of the invention is a method of recovering metals from metallurgical waste, especially zinc and iron from a mixture of dust from iron and steel metallurgy.

[0002] A method of recovering zinc from zinc-containing materials is known from Polish patent application No. P.288418. The invention relates to a method of recovering zinc from zinc-containing materials, particularly from dusts recovered from waste gases from metallurgical smelting processes. Zinc-containing material particles are subjected to agglomeration with a carbon-based reducing material and, advantageously, also a flux, and are delivered to a gas-tight electro thermal furnace having a bath maintained in a temperature range between 1200 and 1700°C, in which the agglomerates are melted, subjected to selective reduction and separation of zinc and other readily volatile metals. The inert slag phase and, advantageously, the liquid metal phase are drained from the furnace, while zinc and other readily volatile metals are recovered from the waste gas by condensation. To prevent secondary oxidation of metallic zinc, zinc-containing dusts are agglomerated together with a carbonaceous binder, which is cracked at temperatures below 700°C to form soot, and the temperature of the furnace gas atmosphere is maintained above 1000°C to keep the volume ratio of CO2and CO in the furnace gas atmosphere below 0.3.

[0003] A method of obtaining zinc oxide and an installation for obtaining zinc oxide are known from Polish patent No. Pat.174046. The method for obtaining zinc oxide is based on introducing into a short rotary furnace, in countercurrent portions, zinc-bearing metallic waste of fine-grained, medium-grained and coarse-grained in weight proportions of 0:2 to 1:3 to 5:9, respectively.

[0004] The batch melts. Once the temperature reaches 930-1120 K, the autogenous oxidation process is initiated and sustained. The method of obtaining zinc oxide from zinc-bearing oxide materials is based on the fact that these materials are introduced in a co-current, continuous manner into an overflow furnace in which three temperature zones are maintained. In a first zone, at temperature of 520 - 770 K, pre-oxidation is carried out. In a second zone, at temperature of 570 - 1270 K, primary oxidation is carried out. In a third zone, at 870 - 470 K, final oxidation is carried out. Also described is a method for obtaining zinc oxide, where the batch is a mixture of zinc-bearing oxide materials and zinc-bearing metallic dust materials. The installation has a short rotary kiln and a swing kiln, connected to a post-combustion chamber and is equipped with a dedusting system consisting of a settling chamber, an inert chamber and a filter. The installation has combined air-water cooling systems.

[0005] A method for the recovery of metals, particularly zinc, lead and cadmium from zinc and lead smelting slags is known from Polish patent application No. P.314353. The method for the recovery of zinc, lead and cadmium from the slags of zinc and lead metallurgy consists in high temperature reduction and elimination to volatile dust of Zn, Pb and Cd. Slags with a composition of 3-15% zinc by weight, 1-6% lead by weight, 0.003-0.12% cadmium by weight are introduced into an arc furnace, followed by high-temperature reduction using the coke in the slags, maintaining the temperature above 1400°C. The process thus carried out results in obtaining a slabs containing 50-60% zinc, 8-15% lead, 0.1-0.35% cadmium, a metallic-sulphide phase and waste slags in glassy form.

[0006] From the Polish Patent No. Pat.225632 there is known a method for recovery and separation of zinc oxide in a pyrometallurgical process of agglomeration of fine-grained wastes, especially metallurgical wastes, consisting in forming under pressure raw briquettes with a moisture content of less than 5.5%, with a volume of 5 to 10 cm3, from a mixture containing as iron-bearing waste dry and oily fine-grained post- rolling mill scale with a grain size of less than 5 mm and arc furnace dusts, the mixture being manufactured in two stages by mixing the dry components with water for 5 to 10 minutes using a slow-speed mixer and then with a high-moisture binder using a high-speed mixer. The briquettes are agglomerated in a shaft furnace for at least 20 min, at a temperature of 1100 - 1200°C, maintaining a reducing atmosphere in the furnace chamber and a vacuum of 10 to 30 Pa, ensuring that the air-fume mixture is blown with an oxygen content of at least 10% by volume.

[0007] In a single waste gas flow path, zinc oxide is separated in at least two consecutive stages: on the first stage at 800 to 300°C in a recuperation and dedusting column, a first, coarse fraction (B) of the combustion products deposited in the cyclone funnel of this column is precipitated, and on the second stage at 300 to 100°C in a filter node, a second, final fraction (A) containing essentially pure zinc oxide is separated.Technical Problem

[0008] At present, the problem of the generation and management of metallurgical dusts is an important issue from both environmental and economic aspects. Technologies for the recovery of zinc in oxidized form from metallurgical dusts are known, however, due to the economics of the process, these technologies focus primarily on waste with a high percentage of zinc. At the same time, it turns out that metallurgical dusts with a low zinc content and with a high iron content also contain a small amount of copper, the excess of which deteriorates the functional parameters of some steel products. The aim of the invention was to develop a technology allowing the recovery of zinc and iron alloys from low-zinc-content metallurgical dusts as low as 2 % and at the same time to obtain a steel melting charge with reduced copper content and environmentally safe slag waste with energy efficiency.Solution to Problem

[0009] A method for the recovery of metals from metallurgical wastes according to the invention, and in particular zinc and iron from dusts, consists in the fact that a mixture of steelmaking dusts and other dusts from the iron and steel industry, with a zinc content in the range of 2-40% and an iron content in the range of 10-65%, undergoes a granulation process with the addition of a reductant in the amount of 7-25% by weight of the dust being granulated, depending on the amount of zinc and iron oxides in the dusts, until a homogeneous material is obtained. The material prepared in this way is introduced into an electric arc-resistive furnace in an integrated metallurgical unit, and melted in a continuous or semi-continuous process at temperatures between 1350°C and 1600°C, bringing about the reduction of zinc and iron oxides with the precipitated zinc being driven off in gaseous form, followed by its oxidation in the air stream. Then, the gas phase is dedusted in two stages. In the first stage, the gases are introduced into the pre-dusting chamber, from where the coarser dust fractions are collected and directed back to the arc-resistive furnace. In the second stage, the gases downstream of the pre-dusting chamber are cooled to below 200°C and directed to bag filters from where the dusty zinc oxide is collected. The resulting iron alloy and waste slag are drained from the furnace. The iron alloy containing elevated carbon content is introduced in an integrated metallurgical unit, into an induction or arc furnace, where it is modified, with the removal of excess carbon, sulphur and phosphorus, and the addition of steel scrap or alloying additives.

[0010] Advantageously, the pelletisation process is carried out in a disc granulator or intensive mixer with the addition of a reductant which is coal dust or carboniser or fly ash.Advantageous Effects of Invention

[0011] The use of a mixture of metallurgical dusts of different origins, with different zinc contents, allows the efficient recovery of zinc from waste even with a low zinc content, as low as 2 %. Combining the process of metal recovery from the mixture of metallurgical dusts, in an integrated unit containing an electric furnace and an induction or arc furnace, allows an energy- and cost-efficient production of a steel product with low copper, sulphur and phosphorus content. Zinc oxide, as one of the products of this process, is a commercial product due to its high zinc content (60 - 75%), as is the iron alloy produced in the furnace, which, after modification to commercial alloys (cast iron / steel / steel), is cast in a form depending on the requirements of customers.

[0012] Examples of the implementation of the invention are presented below, setting out a detailed breakdown of the input to the aggregate and the products obtained.Examples

[0013] Example 1.

[0014] A mixture of 1,000 g of steelmaking dust with a content of (33.2 % Zn; 25.4 % Fe; 0.20 % Cu), was subjected to a granulation process with the addition of a reductant in the form of 150 g of coal dust, until a homogeneous material was obtained. The material thus prepared is directed to an electric arc-resistive furnace in an integrated metallurgical unit, where the prepared pellets are melted at 1480 ºC in a continuous process lasting 90 minutes, during which the reduction of zinc and iron oxides takes place and the precipitated zinc in gaseous form is driven off and oxidized in the air stream. The dedusting of the gaseous phase is a two-stage process. First, the gases pass through a pre-dusting chamber, where the coarser dust fractions are collected and are returned to the electric furnace. Further on in the dedusting system, the furnace gases are cooled to below 200°C and directed to bag filters for the capture and recovery of zinc oxide in the form of fine dust. The iron alloy and waste slag created in the electric furnace are drained from the furnace. The iron alloy containing increased carbon content is directed in an integrated system to an induction or arc furnace, where it is modified by removal of excess carbon, sulphur, phosphorus and the addition of steel scrap or alloying additives As a result of the process carried out in this way, 449 g of zinc dust containing 71.2 % Zn, 266 g of iron alloy with a composition of:96 % Fe; 0.46 % Cu; 0.023 % Zn; 3.50 % C, and 117 g of slag with a composition of: 1.21 % Fe; 0.21 % Zn.

[0015] Example 2

[0016] A mixture of 1,000 g of converter dust with a content of (2.82 % Zn; 58.9 % Fe; 0.01 % Cu), is subjected to a granulating process in a granulator with the addition of a reductant in the form of 200 g of coal dust, until a homogeneous material is obtained. The material thus prepared is directed to an electric arc-resistive furnace in an integrated metallurgical unit, where, in a continuous process lasting 40 minutes, the prepared granulate is melted at a temperature of 1510 ºC, at which the reduction of zinc and iron oxides takes place. The precipitated zinc in gaseous form is de-gassed and oxidised in the air stream, with the de-gassing of the gaseous phase in two stages. First, the gases pass through a pre-dusting chamber, where the coarser dust fractions are collected, which are then returned to the electric furnace. In the next part of the dedusting system, the furnace gases are cooled to below 200°C and directed to bag filters for the capture and recovery of zinc oxide in the form of fine dust. The iron alloy and waste slag produced in the electric furnace are drained from the furnace, with the iron alloy containing increased carbon content being directed in an integrated system to an induction or arc furnace, where it is modified by removal of excess carbon, sulphur, phosphorus and the addition of steel scrap and / or alloying additives The process results in 44 g of zinc dust containing 62.7 % Zn, 608 g of iron alloy with a composition of: 93.8 % Fe; 0.014 % Cu; 0.009 % Zn; 3.02 % C; 2.5 % Mn), and 181 g of slag with a composition of: 1.83 % Fe; 0.08 % Zn.

[0017] Example 3

[0018] A mixture of 500 g of steelmaking dust with a content of 33.2 % Zn; 25.4 % Fe; 0.20 % Cu and 500 g of converter dust with a content of 2.82 % Zn; 58.9 % Fe; 0.01 % Cu, is subjected to a granulation process in a granulator with the addition of a reductant in the form of 175 g of coal dust, until a homogeneous material is obtained. The material thus prepared is then directed to an electric arc-resistive furnace in an integrated metallurgical unit, where the prepared granulate is melted in a continuous process at 1490 ºC for a period of 70 minutes, during which the reduction of zinc and iron oxides takes place. The precipitated zinc in gaseous form is driven off and oxidised in the air stream, and the dedusting of the gas phase is in two stages. First, the gases pass through a pre-dusting chamber, where the coarser dust fractions are collected and are returned to the electric furnace. In the next part of the dedusting system, the furnace gases are cooled to below 200°C and directed to bag filters for the capture and recovery of zinc oxide in the form of fine dust. The iron alloy and waste slag created in the electric furnace are drained from the furnace, with the iron alloy containing increased carbon content being directed in an integrated system to an induction or arc furnace, where it is modified by removal of excess carbon, sulphur, phosphorus and the addition of steel scrap and / or alloying additives The process thus carried out yields: 249 g of zinc dust with 68.9 % Zn, 441 g of iron alloy with a composition of: 94.6 % Fe; 0.21 % Cu; 0.012 % Zn; 3.2 % C; 1.5 % Mn and 153 g of slag with composition: 1.56 % Fe; 0.12 % Zn.

Claims

A method for recovery of metals from a metallurgical waste, characterized in that a mixture of steel mill dusts and other dusts from an iron and steel industry, having a zinc content in the range of 2-40% and an iron content in the range of 10-65% is subjected to a granulating process with addition of a reducing agent until a homogeneous material is obtained, wherein the amount of the added reducing agent is 7-25% by weight of the dust being granulated and depends directly on the amount of zinc and iron oxides, the material thus prepared is then introduced into an electric arc-resistive furnace in an integrated metallurgical unit and melted in a continuous or semi-continuous process at a temperature in the range of 1350-1600°C, leading to the reduction of zinc and iron oxides with stripping of the precipitated zinc in a gaseous form followed by its oxidation in a stream of air, afterwards, the gaseous phase is dedusted in two stages - in the first stage, the gases are introduced into the pre-dusting chamber, from where the coarser dust fractions are collected and directed back to the arc-resistive furnace, and then, in the second stage, the gases located behind the pre-dusting chamber in the further part of the dedusting system are cooled to temperature below 200°C and directed to bag filters, from which a dusty zinc oxide is received, while resulting an iron alloy and a waste slag are drained from the furnace, wherein the iron alloy containing elevated carbon content being introduced, in an integrated metallurgical unit, into an induction or arc furnace, where it is modified with the removal of excess carbon, sulphur and phosphorus and the addition of a steel scrap or alloying additives.The method according to claim 1, characterized in that the granulation process is carried out in a disc granulator or intensive mixer with the addition of a reductant selected from the group: coal dust, carboniser or coke or mixtures thereof.