Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2001-05-04
2002-05-07
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S483000, C075S658000, C075S010330
Reexamination Certificate
active
06383252
ABSTRACT:
The invention relates to a process for thermal treatment of residual materials containing heavy metals, e.g. sludges from surface treatment, pickling and cleaning, metallisation and tin-coating processes on metal parts.
These processes produce large quantities of residual materials containing heavy metals in the form of sludge. These residual materials occur, for example, during filtration of the immersion baths and contain various metals such as zinc, nickel, molybdenum, cobalt, copper and iron in the form of hydroxides, chlorides and sulphates in addition to large quantities of water. The disposal of these residual materials is expensive and their final storage problematical. The sludges are generally stored in special dumps.
Typical compositions of such sludges containing heavy metals are shown in the following table.
TABLE 1
Ni, Cu,
Inert
Fe, Mo,
Cl
CaO
substances
H
2
O
Ko (%)
Zn (%)
S (%)
(%)
(%)
(%)
(%)
Sludges
15-50
>30
1-15
>5
10-30
1-5
10-75
Consequently, the task of the invention is to propose a process for thermal treatment of such residual materials containing heavy metals.
According to the invention this problem is solved by a process for thermal treatment of residual materials containing heavy metals in a multiple-hearth furnace, which is divided into three zones, each zone having several hearths one above the other, and the process comprising the following steps:
continuous introduction of the residual materials containing heavy metals to the top hearth of the first zone of the multiple-hearth furnace, the residual materials being transferred gradually into the second zone and dried;
continuous introduction of reducing agents and desulphurising agents to the top hearth of the second zone, the reducing agents and desulphurising agents being mixed with the dried residual materials, the mixture being heated to about 800° C. and calcined and gradually transferred into the third zone;
heating of the mixture to about 1000° C. in the third zone, the metals being reduced and the waste gases produced in this third zone being exhausted and treated separately;
discharge of the mixture from the multiple-hearth furnace.
An important advantage of the invention is that the metallic oxides, sulphates chlorides, etc. present as a mixture can be reduced and separated (in particular iron and zinc), so that the separated fractions constitute feed materials for other processes. By-products can thus be manufactured from important constituents of the residual materials. After passing through the process, the iron proportion can be returned to production in a steel works. Heavy metal oxides are concentrated to such an extent that they can be used as raw material for recovery of heavy metals. Ashes consisting essentially of inert substances such as SiO
2
, Al
2
O
3
, MgO, . . . and an excess of reducing agents remain.
In the first zone the residual materials containing heavy metals are heated indirectly by heating resistors or directly by burners to approximately 200° C., so that the water is fully evaporated, and subsequently transferred into the second zone. In this zone reducing agents and desulphurising agents are mixed with the residual materials containing heavy metals, and this mixture is heated to about 800° C., the mixture being calcined. The carbonates and sulphates present in the mixture are decomposed and converted to metallic oxides, carbon dioxide and sulphur dioxide being released. The sulphur dioxide reacts with the desulphurising agents, so that the sulphur content in the gases inside the multiple-hearth furnace remains low. After the mixture has reached a temperature of about 800° C. the calcination is completed and the mixture is fed into the third zone and heated further. As soon as it reaches a certain temperature (above about 900° C.) the metallic oxides begin to react with the reducing agents, the heavy metals evaporating and being discharged with the waste gases from the multiple-hearth furnace.
The heavy metals are exhausted from the hearths in the third zone, where they are formed, and treated separately from the other waste gases. These waste gases are subsequently oxidised, e.g. in an after-combustion chamber, the heavy metals being converted to heavy metal oxides, which can then be separated from the waste gases in filter equipment.
At the same time or later the iron oxides remaining in the multiple-hearth furnace are reduced to metallic iron. The metallic iron produced in this way is discharged from the furnace together with the residues of the material introduced, the ashes of the reducing agents and any excess reducing agents.
In this process sludge-type residual materials containing heavy metals can be charged, agglomeration of the particles being prevented by selective process control and continuous circulation. Regardless of the consistency of the feed material the process supplies a fine-grained end product. This is particularly advantageous if ash-forming reducing agents are used. As the solid end product is fine-grained, the ash can easily be separated from the iron. This separation can take place, for example, in the hot condition by screening.
After cooling below 700° C. it is possible on the other hand to separate the reduced iron via magnetic separators from the ash and excess reducing agent.
The quality of the directly reduced iron obtained in this way is virtually independent of the quantity of reducing agent residues. The iron obtained can subsequently be processed into briquettes or introduced directly into a melting furnace (electric furnace etc.) and further processed.
The residues produced can be utilised with any unused reducing agents present therein in a separate gasification reactor, the ash-forming constituents advantageously being separated as liquid slag and the crude gas formed used as combustion or reduction gas in the multiple-hearth furnace.
Accordingly it is possible to use an inexpensive reducing agent which has a relatively high ash content, and/or work with a relatively high excess of reducing agents, which prevents agglomeration of the residual materials.
When working with excess reducing agents it is possible to prepare the residues to separate the unused reducing agents and reuse them. This can be done, for example, by screening the residues, if the unused reducing agents are present in an adequately coarse form. The unused reducing agents can be returned directly to the multiple-hearth furnace.
However, the charging of reducing agents can also be distributed over several stages.
It is thus possible to feed coarse-grained reducing agents (1-3 mm) at a higher point in the second zone of the multiple-hearth furnace and fine-grained reducing agents (<1 mm) at a lower point. Consequently discharge of dust with the waste gases is largely avoided and the course of the reaction is accelerated by the fine reducing agent particles fed in further below.
The charging of coarser particles reduces the consumption of reducing agents, because in the upper hearths, on which an oxidising atmosphere prevails, the small particles are quickly consumed by reaction with H
2
O and CO
2
from the waste gas.
The process space is subdivided into various zones, the solids move continuously from the top downwards and the gases are conducted from below upwards through the furnace. The process conditions in the various zones or even on each hearth can be measured and selectively controlled by the subdivision of the process space into various zones.
The residual materials containing heavy metals can, however, also be mixed with at least part of the required reducing agents or desulphurising agents before they are introduced into the multiple-hearth furnace. This applies in particular in the case of treatment of sludges with a high water content, which are mixed with at least part of the required reducing agents or desulphurising agents before they are fed into the furnace. The sludges generally have a tacky consistency and can be introduced more easily into the furnace if they are mixed with reducing agents or desulphurising agents. The mixi
Frieden Romain
Hansmann Thomas
Roth Jean-Luc
Solvi Marc
Andrews Melvyn
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Paul Wurth S.A.
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