GB2046888A

GB2046888A - Metal treatment

Info

Publication number: GB2046888A
Application number: GB8008449A
Authority: GB
Original assignee: TOLLTRECK Ltd
Current assignee: TOLLTRECK Ltd
Priority date: 1979-03-13
Filing date: 1980-03-12
Publication date: 1980-11-19
Also published as: GB2046888B

Abstract

Metal pieces, especially scrap metal such as swarf, chips or foil, which are contaminated with volatilisable and/or pyrolysable foreign matter are treated to remove the foreign matter by heating them, in the absence of a direct flame, by contact with pre-heated particulate material such as sand. A moving bed of sand, as in a rotary kiln or fluidised bed, may be used in which the metal pieces become immersed. The gases evolved during heating can be used to pre-heat the particulate material. <IMAGE>

Description

SPECIFICATION Metal treatment This invention is concerned with a process and apparatus for treating metal pieces, for example scrap metal pieces, to remove foreign matter.

Scrap metal occurs in a variety of forms, from the fine turnings and chips known as swarf to large articles such as automobile parts and the like. Recovery of scrap metal for recycling involves melting the scrap which can be effected in a number of different types of furnace. A common problem in the melting of scrap metal arises from the presence of foreign matter, either on the surfaces of the metal or otherwise mixed therewith. The foreign matter may, for example, be present on the surface of the metal as thin films of paint, varnish, oil or plastics material. For example, swarf is normally contaminated with cutting or lubricating oils. Alternatively, or in addition, the foreign matter may be in the form of pieces of debris such as paper, wood or plastics mixed with the scrap metal pieces.

For example, tin cans commonly have a paper wrapper, and metal trash often contains socalled tramp materials. Additionally, scrap metal may frequently be wet.

The presence of foreign matter in scrap metal can cause a variety of highly undesirable effects when the scrap is heated in a furnace, including explosion, smoke and fume production and high metal loss by oxidation and otherwise.

With a view to avoiding these problems, it has been proposed to heat scrap metal, prior to melting, both to dry it and to remove other foreign matter such as by pyrolysis or volatilisation so as to produce a dry, relatively clean material. In one known process, swarf is heated indirectly in a rotary kiln and whilst this does effectively dry the swarf and pyrolyse contaminant oils, it also can result in substantial metal losses (partly by oxidation) and environmental pollution problems with smoke and fume emission from the kiln.

We have now devised an improved process for treating metal pieces to remove foreign matter, which process is particularly, but not exclusively, useful for pretreating scrap metal.

According to one aspect of the present invention, there is provided a continuous process for treating metal pieces which are contaminated with volatilisable and/or pyrolysable foreign matter to remove said foreign matter, which comprises heating the metal pieces, other than by direct contact with a flame, by intimately contacting them with a particulate material which has been preheated to at least that temperature at which said foreign matter is removed; separating the treated pieces from the particulate material; exhausting the gases evolved during heating and heating the said gases.

In the process of the invention, the metal pieces are heated indirectly, by contact with the pre-heated particulate material. Thus, the pieces are not heated by direct contact with a flame nor are they subject to direct combustion. Heat supplied to the contaminated pieces from the particulate material dries the pieces and also causes volatilisation and/or pyrolysis of organic surface contaminants and other volatilisable or pyrolysable foreign matter. In a highly preferred arrangement, the pre-heated particulate material is formed into a bed to which the contaminated metal pieces are fed.

By movement of the bed, the pieces become immersed in the particulate material bed and excellent heat transfer from the particulate material to the metal pieces can be achieved.

The bed of particulate material acts as a heat sink, maintaining a uniform and controllable temperature environment. Further, the particulate material shields the metal pieces from the ambient atmosphere, so controlling the ingress of oxygen and reducing the degree of oxidation.

A moving bed of particulate material may be provided in, for example, a rotary tubular member, e.g. a rotary kiln without a direct flame heat source. As the kiln rotates, so the particulate material and metal pieces are mixed in intimate contact and pass along the kiln to an exit point. The dwell time in the kiln will be chosen to provide sufficient heating of the contaminated metal pieces for the desired cleaning and drying. An advantageous feature of the use of such a kiln is that the air supply to the kiln can be carefully controlled and the gases evolved during drying conveniently exhausted.

Another way of providing a moving bed of preheated particulate material is to use a vibratory conveyor on which the bed is formed. A fume hood or the like will then usually be required to collect the gases evolved.

During motion of the bed, the particulate material moves relative to the metal pieces embedded therein, to ensure good heat transfer without localised cooling.

In another arrangement, the contaminated metal pieces can be heated by contact with the pre-heated particulate material in a fluidised bed. The use of fluidised beds in this manner can be highly advantageous in view of the well known properties of such beds for heat transfer purposes.

The particulate material used to transfer heat to the contaminated metal pieces in the process of the invention must be inert under the conditions of its use. To ensure good heat transfer, it must also be of a particle size which is small, at least in relation to the metal pieces and foreign matter being heated. We prefer to use sand, but other inert particulate materials may be used, alone or in admixture with sand. Preferably, the mean particle size of the particulate material will be in the range 0.05 to 3.0 mm. Normally, the temperature of the particulate material as it first comes into contact with the metal pieces will be from 300 to 600"C, although temperatures outside this range may be used.

The gases evolved during heating of the contaminated metal pieces by the particulate material are removed to a heating zone in which their temperature is raised. This heating step is preferably effected in an after-burner.

According to a preferred feature of the inven tion, the hot gases so formed may then be used to heat the particulate material (which is subsequently to be contacted with contami nated metal pieces). Alternatively, hot gases from another source, e.g. exhaust gases from combustion of a fuel, may be used to heat the particulate material. Various forms of direct or indirect heat exchange between the gases and the particulate material may be used. We prefer, however, to use a direct heat exchange process in which a fluidised bed of the particulate material is formed using the hot gas as the fluidising medium. Very efficient heat exchange can be achieved in this manner. Preferably, the fluidised bed (or other heat exchanger) will be downstream of the afterburner, although it is possible in certain cases to form a fluidised bed within the after-burner.

It will be appreciated that the use of a fluidised bed is only possible under appropriate conditions, e.g. where the particulate material is of an acceptable size and density for fluidisation by the hot gases. One reason for preferring sand as the particulate material is that it is normally fluidisable under the conditions of the process of the invention.

After leaving the fluidised bed, or other heat exchanger, the gases which may now be at about 550"C, are generally vented to the atmosphere. Before venting, it is preferred to pass them through a filter or other device such as a hot cyclone, to remove any entrained solids material.

The particulate material which has been heated in the heat exchanger is then used to contact contaminated metal pieces to be heated. Except at start-up of the process, the particulate material is continuously recycled in the process, i.e. from heat exchanger to kiln (for example) back to heat exchanger. The temperature of the particulate material as it leaves the kiln (for example) will vary but will normally be well above ambient temperature.

The process of the invention is particularly, but not exclusively, useful for the treatment of contaminated scrap metal prior to the melting thereof. In particular, the invention is useful for treating, for example, swarf and chips, scrap and recycled metal components and semi-fabrications, and baled foil and laminates particularly of aluminium. So far as the effectiveness of the process in removing water and other foreign matter is concerned, the size of the metal pieces being treated is not critical, provided that all the contaminant foreign matter can be heated to the necessary extent. For ease of handling, however, it is preferred that the scrap metal be of relatively small piece size, e.g. in lumps of about first size or less e.g. up to about 200 mm, (swarf will of course be very much smaller).Larger pieces of scrap such as automobile gear boxes (and any incidental foreign matter) will preferably be reduced to the appropriately sized pieces before being subjected to the process of the invention (although such reduction in size is not essential). Bales of metal foil will preferably be shredded or otherwise reduced to pieces or lumps before processing.

In another aspect, the invention includes apparatus for a continuous process of treating metal pieces which are contaminated with volatilisable and/or pyrolysable foreign matter to remove said foreign matter, which comprises means for forming a moving bed of preheated particulate material; means for feeding said contaminated metal pieces to said bed for intimate heating contact therein with said particulate material; means for separating particulate material from said metal pieces in said bed; means for exhausing gases evolved (in use) from said bed; means for heating the said exhausted gas and means for pre-heating said separated particulate material.

In order that the invention may be more fully understood, reference is made to the accompanying drawing which shows schematically and by way of illustration only, one arrangement of apparatus of the invention by which the process of the invention may be effected.

Referring to the drawing, there is shown a rotary kiln into one end of which is fed the fresh contaminated scrap metal pieces to be heated, and the pre-heated sand from the fluidised bed. The sand forms a bed in the kiln into which the scrap pieces settle and become immersed. They are thereby heated with evaporation of any water and other volatiles and pyrolysis of other foreign matter. As the kiln rotates, the sand and metal pieces are constantly mixed and move towards the other end of the kiln. At, or shortly before the other (exit) end of the kiln, a screen or sieve is provided (for example built into the wall of the kiln) through which sand falls and is thus separated from the metal pieces. The metal pieces are collected.

The separated sand is then returned to the fluidised bed where it is reheated by hot gases from the after-burner.

The gases evolved in the kiln mix with a controlled amount of air admitted to the kiln.

Whilst the sand and metal pieces are not heated in the kiln by flame or other heater, there will normally be one or more pilot flames in the kiln simply to ignite any combustible gases which may be evolved from the sand/metal pieces mixture. Any heat generated by the combustion may serve in part to heat the metal pieces and sand, but this amount of heat is usually small and incidental to the process. In any event, the metal pieces will normally be buried in the sand and hence not directly exposed to any small amount of combustion which may occur in this manner.

The gases in the kiln are exhausted to an after-burner in which heat is supplied to raise their temperature. Thereafter, they pass to the fluidised bed and from there through a hot cycline and after quenching, a fan, to the atmosphere.

The arrangement illustrated and described above is merely one example of many possibilities. As stated previously, the fluidised bed may be formed within the after-burner. In another arrangement, two rotary kilns are used. The first kiln is used as described above, and the second kiln is used to pre-heat the sand or other particulate material with the after-burner gases.

The following Example of the invention is given by way of illustration only.

Aluminium scrap as foil, sheet and tube arises in large volumes and is frequently coated or laminated with plastics, varnish or other materials, either for corrosion protection or as a base for printing. The scrap is generally baled at the collection centre or point of arising to form dense, compact bales which may then be economically transported to a melting works.

When the bales are charged directly to a furnace, then the high organics content, which is typically up to 8 wt %, causes excessive smoke generation and overheating.

As a consequence there are severe operational problems and metal losses by oxidation are high. For instance, when baled aluminium scrap is melted in a hot-well furnace, metal yields of less than 60% are typical, the remainder being lost as oxide dross.

In order to avoid these problems we processed the baled aluminium scrap, prior to melting, in accordance with the present invention, as follows: (1) The bales were shredded in a rotary shredder to produce fist-sized pieces and smaller.

(2) The pieces were immersed in a fluidised bed of sand of 0.1 5 mm particle size, at temperatures in the range 400 to 550"C and for times from 6 to 2 min utes.

(3) The products of pyrolysis from the proc essing bed were burnt in a further fluid ised bed acting as an after-burner at around 800"C.

(4) The products of combustion from the after-burner bed were drawn through a fluidised bed heat exchanger where they were cooled to 350"C prior to discharge.

The sand was not circulated from the heat exchanger to the processing bed, but this is practicable and affords further fuel economy.

The treated pieces of baled aluminium foil scrap were then melted in a clean pool of aluminium to simulate the conditions of the hot-well furnace. Yields were typically around 80% as recovered metal. There was no perceptible smoke during melting.

Claims

1. A continuous process for treating metal pieces which are contaminated with volatilisable and/or pyrolysable foreign matter to remove said foreign matter, which comprises heating the metal pieces, other than by direct contact with a flame, by intimately contacting them with a particulate material which has been pre-heated to at least that temperature at which said foreign matter is removed; separating the treated pieces from the particulate material; exhausting the gases evolved during heating and heating the said gases.

2. A process according to claim 1, wherein the particulate material is in the form of a moving bed thereof, and the said contaminated pieces become immersed in the bed.

3. A process according to claim 2, wherein the said bed is formed in a rotary kiln.

4. A process according to claim 2, wherein the said bed is formed on a vibratory conveyor.

5. A process according to claim 2, wherein the said bed is a fluidised bed.

6. A process according to any of claims 1 to 5, wherein the mean particle size of the particulate material is from 0.05 to 3.0 mm.

7. A process according to any of claims 1 to 6, wherein the particulate material is sand.

8. A process according to any of claims 1 to 7, wherein the temperature of the particulate material as it first comes into contact with the contaminated metal pieces is from 300 to 600"C.

9. A process according to any of claims 1 to 8, wherein the said evolved gases are heated in an afterburner.

10. A process according to any of claims 1 to 9, wherein the particulate material is preheated by heat exchange with hot gases.

11. A process according to claims 9 and 10, wherein the gases from the after-burner are used to pre-heat the particulate material.

1 2. A process according to claims 10 or 11, wherein the pre-heating is effected in a fluidised bed of the particulate material.

1 3. A process according to any of claims 1 to 12, wherein the metal pieces are no greater than about 200 mm in size and comprise swarf, chips, foil or metal-containing laminates.

14. A process according to any of claims 1 to 13, wherein the metal pieces comprise aluminium.

1 5. A continuous process for treating metal pieces to remove volatilisable and/or pyrolysable foreign matter therefrom, substantially as herein described with reference to the accompany drawing.

1 6. Metal pieces which have been treated by the process of any preceding claim.

1 7. A method of recovering scrap metal by melting pieces thereof, wherein the pieces are first treated by a process as claimed in any of claims 1 to 1 5.

1 8. Metal recovered by the method of claim 17.

1 9. Apparatus for carrying out the process of claim 2, which comprises means for forming a moving bed of pre-heated particulate material; means for feeding metal pieces, which are contaminated with volatilisable or pyrolysable foreign matter, to said bed for intimate heating contact therein with said particulate material; means for separating the heated pieces from said bed of particulate material; means for exhausting gases evolved (in use) from said bed; means for heating the said exhausted gases and means for preheating said particulate material.

20. Apparatus for carrying out the process of claim 2 substantially as herein described with reference to the accompanying drawing.