GB2398069A

GB2398069A - Reducing the oil content of steel mill scale

Info

Publication number: GB2398069A
Application number: GB0302549A
Authority: GB
Inventors: Roy Szulakowski
Original assignee: Hanson Support Services Ltd
Current assignee: Hanson Support Services Ltd
Priority date: 2003-02-05
Filing date: 2003-02-05
Publication date: 2004-08-11
Anticipated expiration: 2023-02-05
Also published as: GB0302549D0; GB2398069B

Abstract

A method of reducing the oil content in mill scale produced in the operation of a steel rolling mill, the rolling mill having a recirculating cooling and cleaning water system. The method comprising maintaining in the recirculating water an active concentration of hydrocarbon-consuming micro-organisms which when mixed with the waste material break down the oil within the waste.

Description

TREATMENT OF FERROUS WASTE

Field of the Invention

This invention relates to a method of treating particulate ferrous waste from steel rolling mills and the like to reduce the oil content.

Background to the Invention

In steel rolling mills, a significant quantity of coarse and fine mill scale is produced, entering the cooling water and being carried to the cleaning station, where it is separated from the water by screening and settling. The scale is contaminated with the oil used in the rolling process, and this leads to problems with the disposal of the scale. Regulations to control pollution limit the oil content for materials disposed of in landfill sites, and these limits become ever lower.

The use of hydrocarbon-degrading bacteria in the treatment of waste materials is well-known. For example, the oceans have some 20 to 25 families of microbes that feed on long and short hydrocarbon chains, breaking them down ultimately to carbon dioxide and water. Some families act to break down complex hydrocarbons into less complex molecules, some act on these less complex molecules to yield even simpler hydrocarbons, while other families complete the breakdown. In general, these processes take time. For example, complete natural degradation of crude oil in water or solid sewage on land can take 4 to 6 months.

It has been proposed to treat mill scale using bacterial treatment in the settlement lagoons where the scale separated from the process water is passed in the form of a slurry. This involves the construction of aeration and agitation means for the lagoons to ensure that the bacteria act on as much of the waste as possible in as effective a manner as possible. It has been found, however, that consistently low levels of oil content are difficult to achieve, and that because of the long residence time of the waste in the lagoons, the cost of the process is high.

Summary of the Invention

It has now been found that the conditions occurring in the recirculating cooling water system in a steel rolling mill, involving elevated temperature and agitation/aeration, are conducive to the growth of hydrocarbon-consuming micro-organisms. - 2

Accordingly the invention provides a method of reducing the oil content in mill I scale produced in the operation of a steel rolling mill, the rolling mill having a recirculating cooling and cleaning water system, the method comprising maintaining in the recirculating I water an active concentration of hydrocarbon-consuming micro-organisms.

The recirculating cooling and cleaning water system in a rolling mill will include a settlement area where the solids (mill scale) contained in the water are accumulated.

Where the residence time of the scale in the settlement area is of the order of 3-6 weeks, it has been found that the oil content can be reduced to below 0.5% by weight, I thus meeting requirements for recycling of the material. Where the system does not have the capacity to permit residence times of this order, it may be necessary to remove the solids, as a sludge or slurry, to a secondary treatment tank through which water from the cooling system can be circulated, and to which further treatments of micro-organisms can be introduced.

The micro-organisms introduced are those well-known for aqueous treatment of hydrocarbon contamination. The micro-organisms are dosed into the system in an aqueous medium, preferably containing growth nutrients such as BACTNITE nitrogenous feeding compound containing rare earth elements, supplied by DOVETAIL UK LIMITED in the UK. For secondary treatment systems, the liquid containing the micro-organisms is suitably sprayed on to the solid material spread on to a concrete pad. The material is periodically removed from the pad, to be replaced with a fresh batch of material for treatment.

The solids are preferably sampled at intervals and the residual oil content determined. Techniques for this are well-known. When the oil content has reduced to the desired level, the material may be removed for landfill or other use, and a fresh batch may then be treated.

For introduction of the micro-organisms into the recirculating water system in the rolling mill, it has been found that a point as near as possible to the rolling process is the most effective; the water is elevated in temperature and is subject to turbulence, ensuring intimate contact between the organisms and the particulate ferrous waste. ! Since the bacteria not only break down the oily waste associated with the scale, but also assist in release of oil from the waste, it will be necessary to skim off the released - 3 oil at regular intervals to ensure that its build-up does not affect the efficiency of the bacteria.

While the process can achieve the desired reduction of oil content in normal operation of the mill, if the capacity of the settlement tanks is sufficient to give a long enough residence time, after-treatment of the scale may be necessary if unusually high contamination occurs, for example as a result of spillage of oil, or plant cleaning.

A benefit of the method of the invention is that the pipework in the system is kept freer from contamination, requiring less frequent closure of the mill for cleaning. In addition, the cooling water is cleaner, requiring less frequent replacement. The costs of disposing of waste water are high, and so a reduction in the frequency with which this needs to be done can significantly reduce the operating costs for the mill.

Brief Description of the Drawings

In the drawings, which illustrate exemplary embodiments of the invention: Figure 1 is a diagrammatic illustration of a rolling mill water recirculating system; 1 5 and Figure 2 is a diagrammatic illustration of a secondary treatment system in accordance with a preferred embodiment of the invention.

Detailed Description of the Illustrated Embodiment

Referring first to Figure 1, the water recirculating system of a steel rolling mill comprises a plurality of channels beneath the rolling area to receive cooling water 1 associated with the rollers and associated scale particles produced from the steel during the rolling operation. The channels converge in a conduit 2 which leads the water and suspended solids to a clarifier 3, where the solids settle out into a slurry 4 in the lower conical part of the clarifier, while the separated water is recycled back to the rolling operation in pipes represented at 5 in the diagram. The slurry 4 is removed periodically to a thickener 6, in which further water settles out for recycling via pipeline 7. When the thickener 6 becomes full, the material is removed for disposal or, if necessary, further treatment in a secondary system as described hereinafter with reference to Figure 2. An aqueous suspension of oil- consuming bacteria and growth nutrients is introduced into the conduit at 13, just downstream of the channels, in a continuous manner, so that the bacteria and the scale solids are intimately mixed in the turbulent aerated flow of warm sp - 4 water from the rolling operation. The mixture then runs into the clarifier, where bacterial growth occurs. Bacterial activity is monitored by sampling the water regularly and culturing the samples. Oil released from the scale by the action of the bacteria floats to the surface of the clarifier and is skimmed off. The oil content of the scale in the thickener is monitored by sampling and testing at intervals.

Referring now to Figure 2, the secondary treatment system comprises a treatment tank 11 provided with a concrete pad 12 on which the dried slurry from a steel rolling mill recirculating water system, for example as described with reference to Figure 1, is spread in a layer approximately 300mm thick. This is done in a progressive strip manner referred to as "land farming", each strip being moved along the surface of the pad as the next is applied behind it. In this way, each strip undergoes treatment for substantially the same residence time. Warmed water from the recirculating system is dosed with bacteria and nutrients and is sprayed on to the layer through sprinklers 13 arranged around the edge of the pad 12.

Water draining from the pad passes to a settlement tank 14 before being returned to the mill recirculating system. After a suitable residence time on the pad 12 sufficient to reduce the oil content to the desired level, the solids are removed to a storage area for recycling. ; - 5

Claims

1. A method of reducing the oil content in mill scale produced in the operation of a steel rolling mill, the rolling mill having a recirculating cooling and cleaning water system, the method comprising maintaining in the recirculating water an active concentration of hydrocarbon-consuming micro-organisms.

2. A method according to Claim 1, which comprises separating solids from the water system as a sludge or slurry, transferring the separated solids to a secondary treatment tank, circulating water from the water system through the tank, and introducing further micro-organisms to the secondary treatment tank.

3. A method according to Claim 2, comprising spreading the solid material on to a concrete pad and spraying liquid containing the micro-organisms on to the material on the pad.

4. A method according to any preceding claim, comprising periodically skimming off oil released from the solids.

5. A method according to any preceding claim, wherein the micro-organisms are introduced into the system in an aqueous medium containing growth nutrients.

6. A method of reducing the oil content in mill scale produced in the operation of a steel rolling mill, the rolling mill having a recirculating cooling and cleaning water system, substantially as described with reference to, and/or as shown in, the drawing.