GB2529875A - Reuse of by-products from metallurgical processes - Google Patents

Abstract

The present invention provides a method of processing contaminated mill sludge 402. The method comprises: diluting the mill sludge with water to form slurry; subjecting the mill sludge slurry to attrition using a jet pump 414; treating the mill sludge slurry by electro-coagulation 418; and separating the ferrous material from the contaminants. The contaminants in the sludge can include one or more of: oil, grease, carbon, silica and/or salts. The ferrous material may be separated from the contaminants in a flotation tank 420 and/or lamella clarifier 426. The method can also comprise a step of magnetically beneficiating 430 the mill sludge. A chemical agent, such as a surfactant, may be added to the contaminated mill sludge. Contaminants which are separated from the mill sludge may be sent to an emulsion splitter. An apparatus for carrying out the method is also claimed. The present invention reduces the level of contaminants, such as oil, which are present in the mill sludge.

Description

Reuse of by-products from metallurgical processes

Field of the Invention

The present invention concerns the re-use of metallurgical process by-products. More particularly, but not exclusively, this inventicn concerns the reuse of by-products generated from steel production.

Background cf the Invention

The production of hot rolled steel in steel rolling mills may produce large quantities of mill scale as a by-prcduct. Mill scale comprises varidus ircn oxides that are formed on the surface of the steel; after production the mill scale becomes flaky and breaks off or is removed. Mill scale typically comprises high levels of ferrous metals, for example it may be 65% to 75% iron by mass. However it is typically contaminated with oils used to lubricate machinery used in the rolling mills, generally oil contamination levels are between 0.5% and 2.5%. Smaller particles of mill scale are similarly produced as mill sludge and are often further contaminated with between 2% to 8% oil. Sintering can recycle mill scale for use in a blast furnace; however the hydrocarbons which make up the oil impurities can contaminate the air when they burn and the oil can otherwise cause fouling of filters and other machinery.

By-products, such as those mentioned above, have often been regarded as unrecoverable waste and sent to landfill.

However several factors including: increases in the cost of raw materials, increases in the costs associated with sending waste to landfill, environmental concerns, and changing attitudes regarding waste, have led to the need for processes for recovering useable raw materials from waste.

The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved method of treating by-products of metallurgical processes.

Summary of the Invention

The present invention provides a method of processing mill sludge, the method comprising steps of: providing contaminated mill sludge; diluting the contaminated mill sludge with process water; removing contaminants from the mill sludge through electro-coagulation; removing contaminants from the mill sludge using a iameiia clarifier; separating the process water from the mill sludge.

The mill sludge which is provided to the process may be produced as a by-product of one or more processes relating to steelmaking, and predominantly, but not exclusively, from hot rolling of steel (in steel rolling mills) . The mill sludge may be a mixture of mill sludge from a plurality of processes or sources.

The mill sludge may largely comprise iron and/or iron oxide particles. The particle size may vary depending on the process from which the mill sludge originates. The particle size may be 1 micron to 3000 microns (1 micron = l*105m) The particle size may be 5 microns to 950 microns. The mill sludge may comprise small flakes or be substantially granular. The mill sludge may comprise 50% to 90% ferrous metal. The mill sludge may comprise 55% to 75% ferrous metal.

Throughout this specification where proportions of

materials are given, whether as percentages or parts ratios, those proportions are by mass (unless otherwise stated) The mill sludge so provided may be contaminated with contaminants including, but not limited to, oil, fatty oil emulsion, grease, carbon, silica, and/or salts. The mill sludge contaminated with oil may comprise an oil contamination level of 0.1% to 25% and it may be 1% to 25%.

The mill sludge may comprise an oil contamination level of 1% to 15%. The mill sludge may comprise an oil contamination level of 2% to 8%. The contaminated mill sludge may be tested for the level and/or type of contaminants present. It will be understood that mill sludge provided to the process will be considered to be contaminated mill sludge, regardless of the actual levels and types of contaminants present.

Contaminated mill sludge, provided directly from a steel plant or otherwise, may contain unwanted debris and tramp material. The debris and tramp material typically comprises waste such as stones, shredded tyre rubber, pieces of scrap metal, lumps of steel and the like. This debris and tramp material can damage machinery, introduce further contaminants into the mill sludge and/or reduoe the effectiveness of the mill sludge processing. As a result the debris and tramp material may preferably be removed from the mill sludge so provided. Preferably the debris and tramp material is removed from the mill sludge prior to any other processing steps. The debris and tramp material may be removed by screening the mill sludge. The debris and tramp material may be removed by screening the mill sludge through a first screening unit. The first screening unit may comprise at least one mesh screen. The at least one mesh screen may vibrate to help material pass through it. The at least one mesh screen may have openings in the range 60mm to 80mm.

The material may be transported by an auger screw. The auger screw may be used to transport material from one step of the treatment process to a subsequent step of the treatment process. The auger screw may transport the material, from which debris and tramp material has been removed, to a subsequent step of the treatment process.

The mill sludge may be screened to remove oversized lumps of mill sludge. Oversized lumps of mill sludge may be too heavy or too large to be treated. The mill sludge may be screened in a second screening unit. The second screening unit may comprise one or more mesh screens having predetermined mesh sizes. The screens may vibrate to help material pass through them. The second screening unit may comprise consecutive screens having increasingly small openings. Having multiple screens may reduce the tendency of the screens to clog up with material. The second screening unit may alternatively or additionally comprise a rotating trommel. In the second screening unit water sprayers may be provided to spray water over the screens. Water sprayers may help material pass through the screens and prevent the screens from clogging up. Oversized particle which do not pass through the screens may be collected and disposed of or sent for alternative treatment.

The mill sludge may be received by a jet pump. The jet pump may be configured to dilute the mill sludge with a liquid, for example water, preferably in a predefined ratio, thereby forming mill sludge slurry. For example, the mill sludge may be mixed with water in a ratio of 1 part mill sludge to 10 parts water.

The mill sludge slurry may be accelerated through the jet pump. The jet pump may subject the mill sludge to a high intensity wash. The jet pump may vigorously mix the mill sludge slurry thereby causing attrition between the mill sludge particles. The jet pump may break up mill sludge clumps, lumps or particles into smaller particles.

Attrition of particles may occur when particles are forced against each other or forced against the surfaces of the mixing equipment. The collision of the particles may result in a scrubbing action which acts to remove contaminants from the surfaces of the particles. The attrition may promote separation between contaminants such as oils and the surface of the mill sludge particles.

Sufficient attrition of the contaminated mill sludge may remove some, if not substantially all, of the oil contained on the surface of the mill sludge particles. The oil so removed may be left emulsified, suspended, dissolved, or otherwise dispersed in the liquid.

There may be provided a receiving tank for receiving mill sludge slurry so output from the jet pump. The receiving tank may substantially slow and/or reduce the pressure of the mill sludge slurry output from the jet pump.

Reducing the flow rate of the mill sludge may prevent damage to, and improve the effectiveness of, downstream treatment units. The receiving tank may comprise a mixing element for mixing the material which may prevent the mill sludge particles from settling in the tank.

The mill sludge slurry may be subjected to electro- coagulation. The slurry may be subject to electro-coagulation in an electro-coagulation unit comprising one or more pairs of electrodes. For example, the electro-coagulation unit may comprise four pairs of electrodes, each pair comprising an anode and a cathode. Consecutive pairs of electrodes may be arranged substantially in-line. The electro-coagulation may cause contaminants, such as heavy metals, emulsified oils and suspended solids to coagulate, precipitate and/or form floc.

An advantage of using electro-coagulation to treat the mill sludge is that it is able to separate emulsified oil and grease and other small contaminants from the mill sludge without the need for chemicals, which can be expensive and raise environmental concerns, or mechanical filters, which need to be replaced and can be damaged by emulsified oil and grease. The rate of electro-coagulation may easily be controlled by altering the current supplied to the electrodes. For example, an increased rate of electro-coagulation can be achieved by increasing the potential difference between the electrodes.

Following and/or during electro-coagulation, contaminants such as oils, carbon and other smaller particles ("lights") may float to the top of the mill sludge slurry, forming floating floc. Bubbles created at an electrode may also increase the flocculation of such contaminants. The floating floc may be skimmed from the top of the mill sludge slurry.

Following and/or during electro-coagulation, heavy metals and larger particles ("heavies") may sink to the bottom of the mill sludge slurry, forming metallic sludge.

Metallic sludge, containing iron and iron oxide coagulates, may be removed from the mill sludge slurry by draining or pumping the sludge from the bottom of the mill sludge slurry.

The heavies and lights may be separated, as above, in the electro-coagulation unit itself. Preferably the heavies and lights may be separated in a separate unit. For example the heavies and lights may be separated in a flotation tank, which may comprise vertical baffles past which the mill sludge slurry passes. Alternatively, and preferably additionally, the heavies and lights may be separated in a lamella clarifier, which may comprise a set of inclined plates between which the mill sludge slurry flows. Heavies and lights may also be separated/removed using a filter and/or other means.

Typically the lights are disposed of as waste or sent for further processing (separately to the mill sludge) . The remaining liguid may be disposed of, sent for further processing and/or reused in the mill sludge treatment process. The heavies, i.e. the metallic sludge, so removed from the mill sludge slurry may substantially comprise iron and/or iron oxide particles and may therefore constitute treated mill sludge.

Ballast material may be added to the mill sludge.

Ballast material may be added to the mill sludge if oil levels exceed a maximum threshold level. Ballast material may comprise a material having 50 micron to 700 micron sized particles. Ballast material may comprise fine limestone sand. Ballast material may abrade the surface of mill sludge particles thereby increasing separation of oils from the mill sludge. Ballast material may also adsorb oils in the mill sludge slurry, thereby allowing them to be removed with the larger, more easily removed, ballast material particles.

Ballast material may be added to the mill sludge during screening of the mill sludge. For example, the ballast material may be added into the trornmel.

A chemical agent may be added to the mill sludge to increase the removal of contaminants, for example a surfactant may be added to increase the emulsification of oils in the mill sludge.

The treated mill sludge may be dried. The mill sludge may be dried by passing it through a dc-watering unit such as a heater, and/or be left to drain in drying beds.

Dried mill sludge may be beneficiated to remove ballast material from the mill sludge. The step of removing ballast material from the mill sludge increases the proportion of ferrous metals in the mill sludge. The mill sludge may be magnetically beneficiated, for example in a magnetic beneficiation unit comprising a rotating magnetised drum.

The magnetic particles may pass the rotating magnetised drum, sufficiently magnetic particles may be attracted and held to the sides of the rotating drum until they are carried out of the magnetic field and transferred to a collector, the nonmagnetic or less magnetic particles may be collected as waste.

The operation of some or all of the treatment units (i.e. the screening units, mixing units, attrition units and the like) may be controlled by a treatment control centre.

The flow of material through the treatment units may be automatically controlled. The level of fluid in the treatment units may be automatically controlled. The treatment units may comprise fluid level sensors, for example ultrasonic level sensors. The level of fluid in the units may determine the operation of fluid input and output pumps which pump fluid to and from the units. For example: if the fluid level is higher than a certain threshold level then the output pump alone may operate to remove water from the unit, if the fluid level is lower than a threshold level the input pump alone may operate to add fluid to the tank, and/or if the fluid is within a specific range both pumps may operate to ensure a continuous flow of fluid through the unit. The treatment control centre may automatically operate valves and/or pumps to control the flow of surfactants.

The treatment control centre may control the speed of the motors, mixer, and/or rotational shafts. The treatment control centre may control the voltage across the electrodes of the electro-coagulation unit. The operation of valves for the removal of sludge, for example from the bottom of a flotation unit, may be controlled by the treatment control centre and/or by a timer.

The turbidity of the treated fluid may be measured. The turbidity of the slurry may be measured. The turbidity of the liguid (process water) output from the lamella clarifier may be measured. The turbidity of the liquid (process water) output from the lamella clarifier may be used to control the operation of one or more treatment units. For example, a feedback loop may exist wherein the turbidity measurement influences the voltage between the electrodes of the electro-coagulation unit, for example if the level of turbidity exceeded a threshold level the voltage may be increased.

The treatment control centre may be configured so that it can be remotely accessed and/or controlled, for example -10 -via a web portal. The treatment control centre may alert one or more designated operators in the event of a fault or if certain predefined criteria are met. The treatment control centre may alert one or more designated operators via an automatic SMS and/or email alert.

Contaminants separated from the mill sludge may be sent to an emulsion splitter. More particularly the light particles and any oily water collected may be sent to an emulsion splitter for separation of the oil from the process liquid. The oil may be further processed and/or used as a fuel. l\xiy water used in the process may be sent to a water treatment plant. The water treatment plant may treat the water and return it to the process for reuse in one or more stages of the treatment process.

The present invention further provides an apparatus for carrying out a method as defined above.

The apparatus may comprise a jet pump, an electro-coagulation unit, a flotation tank, a lamella clarifier, a particle size separator, an attrition unit,a magnetic beneficiation unit, an air separator, a flotation unit, an eddy current separator and/or a treatment control centre.

It will of course be appreciated that features described in relation to the method of the present invention may be incorporated into the apparatus of the present invention.

Description of the Drawings

Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: -11 -Figure 1 shows a schematio diagram showing the steps taken / flow of material in relation to a method embodying the invention

Detailed Description

A method of treating contaminated mill sludge will now be described, with reference to Figure 1.

Unprocessed contaminated mill sludge 402 is provided; the mill sludge having been provided from a steel rolling mill. By way of example, the contaminated mill sludge comprises: approximately 751 iron oxide particles and flakes having an average particle size of less than 1 millimetre, and approximately 61 oils.

The contaminated mill sludge 402 is loaded directly to the feed hopper of a first screening unit 404. The first screening unit 404 comprises an inclined vibrating mesh screen having 80mm openings. The mesh screen removes large particles of metal, debris and tramp material from the contaminated mill sludge 402 which may otherwise damage the machinery.

The contaminated mill sludge 402 which passes through the mesh screen of the first screening unit 404 drops into an auger screw 406. The auger screw 406 transports the mill sludge 402 to a second screening unit 408 comprising a rotating trommel. The rotating trommel comprises a cylindrical 3mm mesh screen. Mill sludge 402 is fed into the trommel at one side and oversized particles which cannot pass through the screen exit at the other side.

Ballast material 412 comprising fine limestone sand (50 to 700 micron sized particles) is also fed into the trommel.

-12 -Ballast material is added in the ratio of 1 part ballast material to 10 parts mill sludge (by mass) . The ballast material 412 is thereby mixed with the mill sludge. Water 410 is sprayed into the trommel to help the mill sludge particles 402 and ballast material particles 412 pass through the screen.

The mill sludge 402 and ballast material 412 which passes through the tronimel of the second screening unit 408 is fed into a jet pump 4114. The jet pump 414 is configured to receive mill sludge 402 and ballast material 412 and water 410 in the ratio 1 part mill sludge 402 and ballast material 412 to 10 parts water 410. The mill sludge 402, ballast material 412 and water 410 mixture forms mill sludge slurry.

The action of the jet pump 414 vigorously mixes the mill sludge slurry. The jet pump 414 causes attrition between the mill sludge particles, between the mill sludge particles and the jet pump 414, and between the mill sludge particles and the other surfaces of the jet pump 414. The collisions and attrition of the mill sludge particles break up clumps of mill sludge and separates oils from the surface of the mill sludge particles. The ballast material 412 abrades the surface of the mill sludge particles increasing separation of oils from the surface of the particles. The ballast material 412 also adsorbs oils in the mill sludge slurry.

The mill sludge slurry exits the jet pump 414 having a pressure of approximately lObar. The mill sludge slurry is then passed into a receiving tank 416. The receiving tank 416 reduces the speed and pressure of the mill sludge slurry output from the jet pump 414. The receiving tank comprises -13 -mixing equipment to prevent the mill sludge 402 and ballast material 412 from settling in the receiving tank 416.

The mill sludge slurry is then fed into an electro-coagulation unit 418. The electro-coagulation unit 418 comprises four consecutive pairs of aluminium electrodes, past which the mill sludge slurry passes. Electro-coagulation causes the contaminants in the water, including metal ions, emulsified oils and suspended solids, to coagulate, precipitate and form floc.

Following electro-coagulation the mill sludge slurry is fed to a flotation tank 420. The flotation tank 420 comprises a series of vertical baffles under and over which the mill sludge slurry flows.

Lighter coagulated and precipitated material floats to the top of the flotation tank 420 forming floating floc.

Skimmers skim the floating material from the top of the floatation tank 420. The lighter material ("lights") 422 typically comprises oils, carbon and other small particles.

Heavier coagulated and precipitated material ("heavies") 424 skinks to the bottom of the flotation tank 420 forming sludge. The bottom of the tank is configured such that the sludge drains to toward an outlet valve. The outlet valve is controlled by a timer and periodically opens to release the sludge. The sludge typically comprises metal and metal oxide particles, principally iron and iron oxide, as well as ballast material 412 and other large and heavy particles.

The mill sludge slurry, which comprises the remaining lights 422 and heavies 424, is fed into a lamella clarifier 426. The lamella clarifier 426 comprises a set of inclined plates between which the mill sludge slurry flows. As in the -14 -floatation tank, heavies 424 sink to the bottom of the lamella clarifier 426 forming sludge which is periodically drained cut, and Lights 422 float to the top of the lamella clarifier 426 forming floating floc which is skimmed off.

The remaining water which passes through the lamella clarifier is sent for further processing and is subsequently reused in the treatment prccess, fcr example by being sprayed into the trommel or fed into the jet pump.

The sludge (treated mill sludge), comprising heavies 424 drained from the bottom of the floatation tank and lamella clarifier, is dried by passing it through a hot drier 428. The hot drier 428 comprises an auger screw which transports the treated mill sludge through a heated pipe.

The dried treated mill sludge is then beneficiated to remove the ballast material and increase the proportion of ferrous metals in the treated mill sludge. The mill sludge is beneficiated in a magnetic beneficiation unit 430 comprising a rotating magnetised drum. The mill sludge particles are fed onto the rotating drum, a stationary magnet inside a portion of the drum creates a magnetic field to attract magnetic particles 432, principally the iron/iron oxide particles, to the drum. The magnetic particles 432 are magnetically held to the sides of the rotating drum until they are substantially carried out of the magnetic field created by the stationary magnet. The magnetic particles 432 then fall from the drum and are collected. Non-magnetic particles, including the ballast material and particles containing only a small amount of magnetic material, are not attracted to the rotating drum as they pass it and are sent to a waste collector.

-15 -

Where in the foregoing description, integers or

elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such eguivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims (16)

1. -16 -Claims 1. A method of processing mill sludge, the method comprising steps of: providing contaminated mill sludge, the contaminated mill sludge comprising particles of ferrous material contaminated with one or more contaminants; diluting the contaminated mill sludge with process water to form mill sludge slurry; subjecting the contaminated mill sludge slurry to attrition using a jet pump; treating the mill sludge slurry by electro-coagulation; substantially separating the ferrous material from the contaminants, thereby producing treated mill sludge.
2. 2. A method according to claim 1, wherein the contaminants include one or more of: oil, grease, carbon, silica, and/or salts.
3. 3. A method according to any preceding claim, wherein the step of providing contaminated mill sludge comprises providing contaminated mill sludge comprising 1% to 25% oil.
4. 4. A method according to any preceding claim, wherein the contaminated mill sludge is diluted with process water in the jet pump.
5. 5. A method according to any preceding claim, wherein the ferrous material is substantially separated from the contaminants in a flotation tank and/or a lamella clarifier.

   -17 -
6. 6. A method according to any preceding claim, wherein the method includes a step of magnetically beneficlating the mill sludge.
7. 7. A method according to any preceding claim, wherein the method includes a step of adding a ballast material to the contaminated mill sludge.
8. 8. A method according to any preceding claim, wherein the method includes a step of adding a chemical agent to the contaminated mill sludge.
9. 9. A method according to claim 8, wherein the chemical agent is a surfactant.
10. 10. A method according to any preceding claim, wherein the method includes a step of removing debris and tramp material from the contaminated mill sludge.
11. 11. A method according to any preceding claim, wherein the method includes a step of drying the treated mill sludge.
12. 12. A method according to any preceding claim, wherein the method includes a step of sending contaminants, separated from the mill sludge, to an emulsion splitter.
13. 13. A method according to any preceding claim, wherein the method includes a step of removing ballast material from the mill sludge.
14. 14. A method according to any preceding claim, wherein the process water is cleaned and reused in one or more steps of the method.

    -18 -
15. 15. A method according to any preceding claim, wherein the method includes a step of providing a treatment control centre to control at least some of the treatment units.
16. 16. An apparatus for carrying out a method according to any preceding claim.