CZ2013605A3 - Separation process of zinc from zinc-plated iron scrap - Google Patents

Abstract

A method of separating zinc from galvanized scrap by distillation into a hot gas stream in which solid zinc oxide particles are formed due to the presence of oxygen. In this way, a high zinc oxide feedstock is obtained.

Description

Method of zinc separation from galvanized scrap iron

Field of technology

The invention relates to a process for separating zinc from galvanized scrap by distillation into a hot gas stream.

State of the art

Steel production is currently largely based on the reprocessing of recycled scrap iron, which contains a significant proportion of galvanized parts. During the steelmaking process, scrap is melted and oxygen is used to remove carbon and other elements that pass into the slag (eg silicon, phosphorus, sulfur). Zinc and other less common metals (cadmium, lead, etc.) are also oxidized to form particles which, together with iron oxides, are carried out of the feed gas as steel dust. About 10/20 kg of dust is produced per tonne of steel produced, which at the current rate of recycling of galvanized scrap contains, in addition to the predominant proportion of iron oxides, about 10% of zinc. However, zinc in dust from common steelmaking equipment (converters, electric arc furnaces) is largely in the form of franclinite (ZnFe2O ₄ ), which is so chemically stable that its decomposition by hydrometallurgical or pyrometallurgical means is not economically interesting. The possibility of obtaining volatile metals (including zinc) in this way is also mentioned in the patent CS 481789. The process described in CS 48-789 is intended for Waelz furnaces and uses as a reagent the addition of reducing agents by which it decomposes these stable compounds.

A certain proportion of zincite (ZnO) was found in process dust in plants where the steel process takes place in two stages: (1) preheating of the scrap, (2) oxygen process in the molten metal. This is a technology operated in facilities called tandem furnaces. However, even here, it is not yet worthwhile to process dust particles, so they are costly landfilled as hazardous waste.

The process according to the invention solves this problem regardless of the equipment used and without the need for the addition of further reagents.

The essence of the invention

In contrast to existing technologies of zinc enrichment from raw material containing zinc and iron, or recycling of zinc from steel waste, the proposed procedure is based on heating scrap in an oxidizing environment at temperatures of 800 ^ 1200 ^ 0, at which zinc distills while iron scrap remains in solid state. The heating is provided by hot gas from the steelmaking process, which takes place in a separate ladle. The implementation of such a procedure takes place, for example, in tandem steel furnaces. Analysis of the dust particles from the tandem furnace revealed that the composition of the dust particles changes during the batch processing cycle and that a high purity fraction containing over 40% zinc in the form of ZnO can be removed at a certain stage. This already corresponds to a high-quality raw material for zinc metallurgy.

A theoretical analysis of the ongoing thermal and chemical processes has shown that a key condition for the formation of a zinc-rich fraction is to reach a temperature at which zinc from the scrap already evaporates while the iron remains unmelted. This corresponds to a defined stage of the batch processing cycle, during which it is expedient to selectively remove the dust and process it separately. The vapor pressure is galvanized at 70 ° C, and at 80 ° C it is a significant 31 kPa, which is sufficient for the quantitative separation of the zinc layer from the scrap surface.

The invention therefore relates to the separation of zinc from galvanized scrap by distillation into a hot gas stream in which solid zinc oxide particles are formed due to the presence of oxygen. Although the gas leaving the oxygen smelting process of the steel also carries hot particles of iron oxides, zinc ferrite is not massively formed during zinc condensation and a significant proportion of zinc remains in the form of zinc oxide.

The key advantage is the acquisition of a raw material with a high zinc oxide content. This is a process that does not require additional energy sources or auxiliary agents, and the only investment is a parallel branch for the separate processing of the respective part of the dust particles. If trapping (dusting into water) is used, it is a relatively inexpensive apparatus for sedimentation and dewatering of the resulting sludge. '

Examples of embodiments of the invention

Example 1

In the steel plant, unsorted scrap is processed in such a way that it is practically instantaneously converted to the melting temperature of the steel either by contact with molten pig iron or by an electric arc. By slowing down the scrap heating process, the desired boiling point of zinc (90 ° C) is gradually reached first, at which point zinc selectively vapors and escapes from the mixture. If hot gas is used for heating, zinc passes into the gas at a much lower temperature. The zinc vapor produces solid zinc oxide in the oxidizing environment, which is subsequently separated by filtration or sedimentation.

Example 2

A gas with a temperature well above the melting point of iron (150 ° C) leaves the steel ladle in the oxygen smelting process of the steel. The thermal energy of this gas countercurrently preheats the scrap. By capturing all dust solids, a mixture having a zinc concentration of 5/15% is formed. Selective sampling at a time interval in which the temperature of the scrap charge is just around 50 DEG C. yields a mixture of more than 40% zinc, predominantly in the form of zinc oxide, which can be used for other metallurgical or chemical processes.

Example 3

Countercurrent preheating of the tandem furnace scrap according to Figure 1. The furnace consists of two identical compartments 1 and 2. Iron scrap (stream A) is fed into compartment 1. In section 2 there is molten iron, generated from scrap from past smelting and pig iron (stream B). Oxygen (stream C) is introduced into section 2, which causes the steel to be tempered. The hot steel gas (stream D) containing dust particles is passed through section 1 through scrap, which is thus preheated. Next, the gas proceeds (stream E) to the washing machine 3, where dust particles are separated by a stream of water (stream F). The purified gas is further used as stream G. The dedusted water is fed to the settling process as stream H. At a suitable time, a suspension with a high zinc content is removed as stream J, and treated separately. At the end of the smelting, slag (stream K) and steel melt (stream L) are discharged from section 2, the material inlets and outlets are switched, and the cycle begins again with the exchanged functions of sections 1 and 2.

During the specific smelting, the dust from the tandem furnace was collected in a Venturi washing machine in water. Coarse, more easily sedimenting particles contained mainly iron oxides and less than 2% Zn. The fine sludge fraction contained on average about 10% Zn. In the time stage 20 / -30 minutes after the start of oxygen blowing and preheating of the scrap, only fine sludge with a concentration of over 35% Zn with a maximum of 48% Zn was obtained by sampling.

Industrial applicability

The process of the invention is useful for the production of metallic zinc or zinc compounds. The product of the process of the invention, zinc oxide, is also useful as a basic neutralizing agent.

Claims (3)

PATENT CLAIMS A process for the separation of zinc from galvanized iron scrap, characterized in that the scrap is preheated by a stream of hot oxygen-containing gas to a temperature above 800 DEG C. and particles of the formed zinc oxide are separated from the effluent gas. Process according to claim 1, characterized in that the hot gas is the product of an oxygen steel tempering process. Method according to claim 2, characterized in that the galvanized iron scrap is preheated in a tandem furnace.