Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2000-10-06
2002-12-31
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S431000, C075S432000, C075S961000
Reexamination Certificate
active
06500229
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to the technical field of the treatment of dust from steelworks, notably electric steelworks.
BRIEF SUMMARY OF THE INVENTION
According to a first general object, the invention discloses a method and an installation for treating said dust allowing elimination of the heavy metals it contains.
According to a second general object, the invention discloses a method and an installation for treating said dust allowing maximum economic recovery of the most abundant fraction of this dust.
The term heavy metal, which has come into use in many legislations, relates to metals which are generally:
non-degradable over time;
toxic to living systems at very low concentrations;
inclined to accumulate in living organisms and to become concentrated in the course of transfers of materials in food chains.
Steelworks dust, for example issuing from remelting in the electric furnace of scrap iron for the production of steel, contains such heavy metals such as zinc, cadmium and lead, for example.
Dust production is between 15 and 22 kg per tonne of liquid steel.
As a guide, steel production, in Europe, was of the order of over 500,000 tonnes per year at the beginning of the 1990s.
DESCRIPTION OF THE PRIOR ART
Consequently, there is a considerable need for effective treatment of this dust, for the sake of protecting the environment notably.
A number of methods have been envisaged for the treatment of this dust, the methods varying depending on the composition of said dust.
Two main classes of dust are distinguished, depending on the nature of the steels produced, carbon steels and stainless alloyed steels.
In the case of carbon steels, zinc (17 to 29%) is present in the dust in two forms: zinc oxide, ZnO, and zinc ferrite, ZnFe
2
O
4
, while the lead (4 to 5%) is in oxide form (PbO) The cadmium contents are lower, of the order of around 800 ppm.
Unlike zinc oxide, zinc ferrites are not easily soluble during hydrometallurgical treatments for releasing the zinc.
For alloyed and stainless steels, the quantity of dust generated in Europe was around 50,000 tonnes per year during the 1990s.
Various procedures and processes, many still at the experimental laboratory stage, have been devised for the treatment of dust from steelworks, notably electric steelworks.
The BUS (BERZELIUS UMWELT) procedure proposes a treatment of dust from steelworks for carbon steels combining two processes: the WAELZ process and the ISP (IMPERIAL SMELTING PROCESS). This procedure uses a conventional pyrometallurgical process and condensation of the zinc vapours by means of liquid lead spray condensers.
The FLAME REACTOR process, used for the treatment of dust from alloyed and stainless steels, consists essentially of vertical flame smelting/reduction cyclone separation. The waste products, rich in zinc and lead, are converted into slag to be resmelted and into oxide to be recovered.
The PLASMADUST process has been used since the 1980s for the treatment of electric steelworks dust rich in zinc and lead. The treatment furnace is of the tank furnace type, whose energy is provided by blown arc plasma torches. The energy of the plasma is used to heat the reagents, and to provide the heat necessary for the endothermic reduction reactions. The plasma-producing gas is introduced into the furnace by water-cooled copper nozzles. The powdery charge composed of dust, flux and coal is injected into the nozzles and mixed with the plasma-producing gas. The fumes charged with metallic vapours leave the furnace at a temperature of around 1150° C.
When the PLASMADUST process is used for dust from alloyed and stainless steels, the zinc content in the fumes is generally too small for condensation of the zinc to be efficient, the zinc in this case being retrieved from the sludge issuing from the washers.
The PLASMINOX process uses a treatment by means of hot cathode plasma torches operating by transferred arc at D.C. and installed vertically on the furnace. In the furnace, the metals present—chromium, nickel, molybdenum and iron—are separated from the dust by melting and the ferroalloy which results therefrom is poured into moulds for subsequent recharging at the dust-producing steelworks. The scorias obtained are, in accordance with the standards, inert. The fumes escaping from the furnace are cooled and have their dust removed. The secondary dust obtained contains volatile metals Z
n
, P
b
which are in the form of oxides and are not recovered.
The modified ZINCEX process produces zinc by hydrometallurgical means, this process having the following successive steps:
atmospheric leaching of the dust by a dilute sulphuric acid solution;
purification of the leaching solution by precipitation of the iron and aluminium;
selective extraction of the zinc by D2EHPA;
stripping of the zinc from the organic phase by the highly acid electrolytic solution;
electrodeposition of the zinc on to aluminium cathodes.
The ZINCEX process is relatively complex in its implementation since it requires organic solvents combined with sulphuric acid. Furthermore, this process requires large quantities of wash water to avoid the formation of SO
2
from the oxides which are recycled in the furnace. This ZINCEX process is not easily exportable, owing to the above-mentioned problems and since it necessitates too heavy investments.
The GLASSIFICATION process comprises a step of mixing the steelworks dust with other steelwork waste products and constituents of glass. The mixture is melted in a submerged arc furnace. The metals vaporized during the melting condense in the upper part of the furnace before being trapped in the melt. The glass obtained can be put in the form of granules for the manufacture of glass tiles or grit.
In total, three main ways of recycling have been envisaged for recycling dust from electric steelworks.
A first way consists of performing reductions at various temperatures in accordance with the processes for vaporizing the heavy metals and re-injecting the ferrous fraction into the steel production furnaces:
the document EP 336 923 proposes treating steelworks dust in an iron smelting furnace after conversion into pellets;
the document EP 441 052 proposes a thermal treatment between 1200 and 1700° C. by the addition of reducing agents;
the document WO 91/12 210 describes a method of treating steelworks dust in an iron reduction furnace, the heavy metals being retrieved by condensation from the hot gases;
the document EP 453 151 describes a method of treating dust in the form of pellets by an agent selectively reducing iron oxide;
the document FR 2 666 592 describes a device for extracting volatile metals acting by oxidation;
the document WO 93/69 619 describes an arc furnace specially designed for the reduction of dust containing oxides;
the document EP 551 992 describes a method of retrieving recoverable metals from steelworks dust by reduction and vaporization of the recoverable metals;
the document FR 2 373 612 describes a method of extracting zinc contained in steelworks dust, by selective oxidation under heat.
A second way consists of directing the dust towards a use where it would be “made inert” by various means with a view to use as construction or filler materials:
the document EP 402 746 proposes recycling by incorporation in a clay for a mine packing material;
the document WO 91/12 210 proposes the use of steelworks dust for the treatment of sewage with the addition of a flocculating agent;
the document FR 2 689 881 describes a method of manufacturing bricks whose properties are improved by the addition of electric steelworks dust first calcined at the vaporization temperature of heavy metals contained in said dust;
the document FR 2 700 161 describes a mixture for a road surfacing comprising 2 to 6% by weight of electric steelworks filter dust.
Pyrometallurgical processes, with or without electricity, allow the recovery of oxides of zinc, of iron or nickel chromium alloy or of liquid zinc most often by condensation.
A third way, hydrometallurgical, has also been envisaged.
Va
Roux Gilles
Tedjar Farouk
King Roy
McGuthry-Banks Tima
Perman & Green LLP
Recupac
LandOfFree
Method for treating steel works dust by wet process does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for treating steel works dust by wet process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for treating steel works dust by wet process will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2977649