GB2334969A

GB2334969A - Removing coatings from scrap metal

Info

Publication number: GB2334969A
Application number: GB9804537A
Authority: GB
Inventors: Martin Cosgrove; Robert Wilton Weaver
Original assignee: British Steel PLC
Current assignee: British Steel PLC
Priority date: 1998-03-05
Filing date: 1998-03-05
Publication date: 1999-09-08
Anticipated expiration: 2018-03-05
Also published as: GB9804537D0; GB2334969B

Abstract

The process comprises treating coated metal such as steel with an acidic de-coating solution comprising ferrous ions (Fe<SP>2+</SP>) and an amine based pickling inhibitor. The coating may be metallic (e.g. tin, zinc, aluminium, lead or chromium) or organic (e.g. a polyester). The de-coating solution ideally comprises waste pickle liquor from a steel manufacturing process and contains 1-4 % (w/v) hydrochloric acid, 10-25 % (w/v) iron II chloride and less than 0.05 % (w/v) amine based pickling inhibitor. Apparatus suitable for performing the process comprises a vessel for containing the solution in which scrap is immersed. To assist in removal of the coating, the scrap lies between two opposing planar mesh plates, one of which is rotatable. The plates engage the scrap and rotation of the rotatable plate causes the scrap to self abrade.

Description

COATED METAL PRODUCTS This invention relates to coated metal products and to a process and apparatus for removing coatings from such products. More especially, the invention relates to apparatus for and a process of recycling coated steel strip.

The demand for coated steel has risen dramatically over the past two decades in response to the enhanced corrosion protection afforded by organic and/or metallic composite products. Accordingly, the volume of coated steel strip has risen substantially and it is estimated that 150,000 tonnes of organic coated steels will be available annually within the UK alone by the year 2000.

Uncoated steel scrap is readily recycled; unfortunately there are inherent problems with directly recycling organic or metallic coated steels via the conventional basic oxygen steel making (BOS)/electric arc furnace (EAF) routes.

The melting and decomposition of organic coatings during steelmaking can result in the production of volatile organic compounds. Of particular concern is the potential for producing dioxins. Dioxins are a byproduct of incomplete combustion and pose a health risk, inter alia, as a potential carcinogen.

The synthesis of dioxins during combustion is facilitated by the presence of catalytic compounds and sources of chlorine. Scandinavian studies have demonstrated that the addition of organic compounds (present for example in a steel composite comprising a polyvinylchloride-rich coating) in EAF charges, will lead to the production of significant quantities of toxic dioxins. Whilst studies have shown that the use of chemical treatments, for example, ammoniacal scrubbing, can reduce the total levels of dioxins produced during such processes, capital costs can in some cases be substantial and prohibitive.

The potential for generating toxic waste gases when recycling coated steel composites may be circumvented by using a process in which the coating, or a substantial part thereof, is removed prior to adding the recycled steel as scrap to a BOS or EAF furnace. However, little work has been expended in this area.

In contrast, metallic coatings are very often removed prior to steel making. In the past, development in this field has focused on processes for the de-coating of galvanised and tinmill products. The removal of such coatings is important to the steel maker; the build up of zinc in process dusts can present disposal and recycling problems, while tin remains entrained in the steel and can affect a range of metallurgical and mechanical properties. Tin and zinc are amphoteric and readily dissolve in alkaline solutions. Hence, caustic solutions are often used to remove tin and zinc.

US-A-3905882 discloses a process for counterflow sulphuric acid leaching of galvanised steel scrap. The process is continuous and involves a series of linked leach tanks and zinc recovery cells. However, acid treatments severely corrode iron based materials and require higher cost polymeric based materials for the de-coating equipment.

Therefore, the high zinc solubility and the passive nature of steel in caustic solutions has meant that caustic de-coating systems are used most.

US-A-4056450 discloses a recycling process which recovers tin from tinplate scrap on a continuous basis by advancing shredded scrap through an electrically anodic current in a caustic solution. The disadvantage of using electric current to anodically remove a coating metal from scrap is that it can be very inefficient. Alternatively, US-A-5106467 discloses a chemical oxidation mechanism for zinc removal.

Other esoteric proposals include high temperature, low pressure volatilisation of steel coatings, or a process of "baking" followed by subsequent shot blast removal of zinc.

In contrast, very little attention has been focused on finding an efficient and cost-effective process of removing composite organic, as well as, or instead of, metallic coating systems prior to steel-making.

EP-A-0727499 discloses a caustic removal system which discloses the benefits of abrading stones and metal powder in effecting the removal of coatings in an agitated vessel.

Therefore, it is an object of the present invention to provide recycled metal, in particular, steel strip, using an improved, efficient and cost effective process and apparatus to remove organic and/or metallic coatings, thereby avoiding the production of toxic by-products generated on combustion of metal in the presence of organic compounds.

Accordingly, in one aspect, the invention provides a process for removing metallic and/or organic coatings from coated metal, the process comprising treating coated metal with an acidic de-coating solution which comprises Fe2+ and an amine-based pickling inhibitor.

The term "pickling inhibitor" is used herein to denote a group of conventional commercially available inhibitor compounds which are used to retard metal dissolution in the industrial process of removing scale (oxide) layers from hot rolled steel strip.

In another aspect, the invention provides a process for removing metallic and/or organic coatings from coated steel, the process comprising the treatment of coated steel with a de-coating solution which comprises Fe2+ and an amine-based pickling inhibitor.

The rate of removal of metallic and/or organic coatings is markedly enhanced in the presence of iron II chloride. Without wishing to be bound by any theory, it is believed that the enhanced ionic content of the solutions may be an important factor in accelerating the de-coating process.

Therefore, it is preferred that the de-coating solution comprises ferrous chloride.

Preferably, the de-coating solution further comprises an amine-based inhibitor. Amine-based inhibitors are amine-based organic compounds which protect the base metal from acid attack after coating or scale layers have been removed. The inhibitors are physically or chemically adsorbed onto the surface of the metal and act as a "barrier" to the acid in order to retard metal dissolution. A number of proprietary chemicals are commercially available for this purpose. Therefore, the yield of recycled metal is maximized.

Preferably, the de-coating solution comprises hydrochloric acid, iron II chloride and an amine based pickling inhibitor.

Typically, the de-coating solution may comprise between 1 and 4% (w/v) hydrochloric acid, and/or between 10 and 25% (w/v) iron 11 chloride, and/or less than 0.05% (w/v) amine based inhibitor. For example, the decoating solution may comprise 2% (w/v) hydrochloric acid, 20% (w/v) iron II chloride, and 0.04% (w/v) amine based inhibitor.

In particular it is preferred that the de-coating solution comprises waste pickle liquors. Pickling is carried out in the steel manufacture process, between hot and cold rolling. It is an important stage, during which the metal surface is directly exposed for the first time after pickling eliminates the oxides formed during the process of hot rolling. Effective pickling is essential for the success, not only of cold rolling but also for subsequent annealing and coating operations. For mild steels, chemical pickling is performed in baths of hot concentrated acid. Although sulphuric acid used to be the standard pickling agent, this has gradually been replaced by hydrochloric acid which gives greater productivity and more efficient pickling.

There are several advantages in using waste pickle liquors as the decoating solution. Firstly, the process utilises a waste product from steel plant cold mills and therefore saves on costs and waste. Secondly, the residual acid levels (about 2%) in pickle liquor can initiate the de-coating process. In addition, pickle liquor comprises ferrous chloride which accelerates the rate of removal of the coating on steel scrap. Lastly, the waste pickle liquor contains residual amine based pickling inhibitors which significantly reduce the loss of iron substrate.

In one embodiment of the invention the process further comprises the step of subjecting the coated steel to abrasive forces when immersed in the de-coating solution. Preferably, the coated steel is self-abrading. This obviates the need to use abrading stones, shredding or galvanic coupling mechanisms to assist the removal of the steel coating; instead, the invention utilises the self-abrasive properties of steel scrap.

In a preferred embodiment, use of a tumbling reaction vessel allows rapid movement of coated steel scrap segments and promotes multiple collisions between these segments within the de-coating solution.

The process of the present invention may be used to remove metallic coatings such as tin, zinc, aluminum, lead and chromium and/or organic coatings for example, polyesters.

In another aspect of the invention, there is provided apparatus for removing metallic and/or organic coatings from coated metal, said apparatus comprising a vessel for containing a de-coating solution comprising Fe2+ and a amine-based pickling inhibitor and means for imparting movement to coated metal segments which, in use of the apparatus, are immersed in the de-coating solution.

Typically, the coated metal is coated steel.

The apparatus may comprise a reaction vessel comprising a means for abrading metal scrap, wherein said means comprises two opposing plane surfaces, a lower plane surface and an upper plane surface, said upper plane surface being rotatable about a central axis perpendicular to said surface, wherein said plane surfaces engage with said metal scrap.

In one embodiment the plane surfaces are roughened to grip the metal scrap segments. Preferably, the plane surfaces comprise mesh sheets.

In a preferred embodiment, coated metal scrap is trapped between opposing mesh sheets. The metal scrap itself has the potential to act as an abrasive cutting face. Accordingly, rotation of an upper mesh sheet forces the metal scrap to self abrade. Preferably, the upper mesh sheet is positioned so as to exert a compressive force on the metal scrap.

In one embodiment of the invention, the reaction vessel further comprises an ultrasonic plate to aid the de-coating process.

In yet another aspect of the invention, there is provided recycled metal prepared according to the process and by the apparatus herein described.

The invention will now be described by way of example only with reference to the following diagrammatic drawings, in which: Figure 1 illustrates the effect over time of various solutions on the de-zincing of galvanised steel strip; and, Figure 2 illustrates the effect over time of various solutions on the de-coating of PVC coated steels.

The efficacy of various solutions on the de-coating of G295 galvanised (galvanneal) or PVC-coated (PVC Plastisol) steel strip was evaluated in the laboratory. Samples of either galvanneal or PVC Plastisol were treated for various times with a solution of: (a) 2% HCI; (b) 2% HCI + inhibitor; (c) 2% HCI + 20% FeCI2; or, (d) Synthetic pickle liquor, i.e. 2% HCI, 20% FeCI2 and 0.04% inhibitor.

The results of the study are illustrated in Figures 1 and 2 (galvanneal and PVC Plastisol respectively). In both cases, the synthetic pickle liquor provides the fastest removal of 100% of the coating.

The de-coating mechanism is similar for both organic and galvanised steels. Organic coated steels will, in most cases, have a zinc layer between the base metal and the coating. De-coating is dependent on the access to and the dissolution of this zinc layer. As the amount of zinc available for reaction is much greater with a galvanised only steel, rates will subsequently be much greater.

It will be appreciated that the foregoing is merely exemplary of the process and apparatus for de-coating coated steel scrap in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention.

Claims

Claims 1. A process for removing metallic and/or organic coatings from coated metal, the process comprising treating coated metal with an acidic de coating solution which comprises Fe2+ and an amine-based pickling inhibitor.
2. A process as claimed in claim 1 wherein the coated metal is coated steel.
3. A process as claimed in claim 1 or claim 2 wherein the de-coating solution comprises iron ll chloride.
4. A process as claimed in any one of the preceding claims wherein the de-coating solution comprises hydrochloric acid, iron II chloride and an amine based pickling inhibitor.
5. A process as claimed in claim 4 wherein the de-coating solution comprises between 1 and 4% (w/v) hydrochloric acid, and/or between 10 and 25% (w/v) iron II chloride, and/or less than 0.05% (w/v) amine based pickling inhibitor.
6. A process as claimed in claim 5 wherein the de-coating solution comprises 2% (w/v) hydrochloric acid, 20% (w/v) iron II chloride, and 0.04% (w/v) amine based pickling inhibitor.
7. A process as claimed in any of the preceding claims wherein the de coating solution comprises waste pickle liquors.
8. A process as claimed in any one of the preceding claims further comprising the step of subjecting the coated metal to abrasive forces when immersed in the de-coating solution.
9. A process as claimed in claim 8 wherein the coated metal is self abrading.
1 0. A process as claimed in claim 8 or claim 9 comprising the use of a tumbling reaction vessel.
11. A process substantially as hereinbefore described and with reference to the accompanying drawings.
1 2. Apparatus for removing metallic and/or organic coatings from coated metal, the apparatus comprising a vessel for containing a de-coating solution comprising Fe2+ and a amine-based pickling inhibitor and means for imparting movement to coated metal segments which, in use of the apparatus, are immersed in the de-coating solution.
1 3. Apparatus as claimed in claim 1 2 wherein the coated metal is coated steel.
14. Apparatus as claimed in claim 12 or claim 13 comprising a reaction vessel comprising a means for abrading metal scrap, wherein said means comprises two opposing plane surfaces, a lower plane surface and an upper plane surface, said upper plane surface being rotatable about a central axis perpendicular to said surface, wherein said plane surfaces engage with said metal scrap.
15. Apparatus as claimed in claim 14 wherein the plane surfaces are roughened to grip the metal scrap.
16. Apparatus as claimed in claim 14 or claim 15 wherein the plane surfaces comprise mesh sheets.
1 7. Apparatus as claimed in claim 1 6 wherein coated metal scrap is trapped between opposing mesh sheets.
18. Apparatus as claimed in claim 17 wherein the rotation of an upper mesh sheet forces the metal scrap to self abrade.
1 9. Apparatus according to any one of claims 1 6 to 1 8 wherein the upper mesh sheet is positioned so as to exert a compressive force on the metal scrap.
20. Apparatus as claimed in any one of claims 1 2 to 1 9 further comprising an ultrasonic plate to aid the de-coating process.
21. Apparatus substantially as hereinbefore described and with reference to the accompanying drawings.
22. Recycled metal prepared according to the process as claimed in any one of claims 1 to 11 and/or by the apparatus as claimed in any one of claims 12 to 21.