GB2521021A

GB2521021A - Method of treating hot mill iron-containing sludge from iron and steel making processes

Info

Publication number: GB2521021A
Application number: GB1412862.3A
Authority: GB
Inventors: Gary Hunt
Original assignee: Cenin Ltd
Current assignee: Cenin Ltd
Priority date: 2013-07-21
Filing date: 2014-07-21
Publication date: 2015-06-10
Also published as: GB201412862D0; GB201313016D0

Abstract

A method of treating oil contaminated hot mill sludge by combining the sludge with a fibrous oil absorbent material which contains lime and/or calcium chloride to form a mixture and adding a cementitious binder containing calcium aluminate and sodium silicate to the mixture and then adding a liquid. The hydrated cementitious binder mixture can be processed to form spherical agglomerates or briquettes. The fibrous material can be paper ash or a incineration plant ash.

Description

METHOD OF TREATING HOT MILL IRON-CONTAINfNG SLUDGE FROM

IRON AND STEEL MAKING PROCESSES

Background to the invention

The present invention relates to a method of stabilising iron-containing sludge including oil-contaminated hot mill sludge waste. The invention further relates to the environmental recovery of the iron in the oil-contaminated hot mill sludge wastes.

The disposal of sludge waste from hot mill iron and steel making processes is a serious environmental issue. One method of disposal of hot mill sludge is to filter press the sludge to remove as much water as possible before transporting the waste to land-fill sites. However, this method of disposal is no longer environmentally acceptable as the oil contaminants within the pressed sludge remain unstable and subsequently leach out into the surrounding soil.

An alternative method of disposal of hot mill sludge waste is to transform the waste into aggregate by treating at high temperatures to remove all the moisture from the hot mill sludge. The dehydrated sludge dust is then graded before being subsequently mixed with Portland cement and made into pellets by means of a disc-type pelletiser. This is a lengthy and expensive process involving elevated temperatures with consequent high energy requirements. Although pellets made in this way cannot be disposed of in landfill due to the hot mill sludge oil-based contaminants remaining unstable, they can, if the aggregate strength and dimensions allow, be used in a rotary hearth furnace, However, this is a very expensive process and, given the extremely large volumes of contaminated hot mill sludge waste that requires disposal, is not a cost-efficient method that can be used effectively on a large scale sufficient to cope with demand.

There is therefore a need for an alternative method of treatment of oil-contaminated hot mill sludge waste.

Summary of the Invention

The present invention seeks to address the pmblems of the prior art, A first aspect of the present invention provides a method of treatment of oil contaminated hot mill sludge, the method comprising the steps of: a. mixing oil-contaminated hot mill sludge with a fibrous oil-absorbent material containing lime and/or calcium chloride to produce a mixture.

The addition of the oil-absorbent material serves to stabilise the oil-contaminated hot mill sludge.

The method may optionally include the step of drying the mixture. For example, the mixture may be air-dried in the absence of applied heat. Therefore, by comparison with prior art treatment methods, the method of the present invention is more cost effective and has a lower carbon footprint due to the reduced energy requirements. The use of an absorbent material containing lime and/or calcium chloride results in drying of the mixture by means of an exothermic reaction between the oil-contaminated hot mill sludge and the absorbent material, This results in the self-drying of the mixture.

Further, during the exothermic reaction a by-product, hydrocalumite, is formed.

Hydrocalumite is a known additive to cement and has chloride capturing properties that will prevent the leachate of chloride from concrete or aggregate produced from the stabilised material, In addition, hydrocalumite is known to minimise the risk of alkali silica reaction within the concrete, Hydrocalumite also reduces the risk of corrosion to steel reinforcing as the hydrocalumite absorbs the chloride and other metal ions surrounding the reinforcement Both of these effects of hydrocalumite benefit the concrete by improving the durability of the concrete.

The dried, stabilised mixture can then be further processed to enhance the iron content for re-use in the iron and steel making process and/or as an additive to the cementitous binder.

The material can be classified into particle size fractions and or magnetically separated. The advantage of this is that at certain size fractions and by magnetic separation you can obtain higher concentrations of iron in the form of hematite, magnetite, wustite and other iron compounds.

The composition of the hot mill sludge is variable due to the nature of its source and processing, the total iron content typically varies from less than 30% to more than 60%. As previously mentioned, iron content can be influenced by particle size fraction selection, i.e. the greater the particle fractions in the sludge the greater the total iron content In experiments it was found that the 75jtm fraction contained between 35-40% total iron, the 100pm fraction contained between 40-45% total iron and the! SOpm fraction contained between 55-60% total iron.

When the hot mill sludge is stabilised then the total iron content is reduced by between 5 and 10% due to the dilution effect of the stabiliser. Again the larger the particle size the greater the iron content The dried stabilised sludge can now be mechanically separated by classification into one or more size fractions with the larger size fraction containing the highest amount of total iron content The mechanically separated fractions can then be further treated by magnetic separation, the higher the fraction the greater the total iron content, The magnetic fraction will contain >30% total iron and the non magnetic fraction will contain < 30% total iron.

In experiments it was found that the 5üm fraction contained between 30 -35% total iron, the 75p,m fraction contained between 45 -50% total iron, the lOOp,m fraction contained between 50 -55% total iron and the 1 5Otm fraction contained between 60 -65% total iron.

it can be noted that the total iron content concentration has now increased compared to the concentration that was originally in the hot mill sludge. The magnetic fraction is now suitable for recycling into the iron and steel manufacturing processes via the methods described in this patent.

The hydrocalumite contained in the magnetic fraction may help to absorb the soluble chloride and undesirable metal ions.

The non magnetic fraction can be used as a fine aggregate in concrete to improve the durability of the concrete as described earlier in this patent or as an additive to cement for the same reasons mentioned above. A preferred partide size fraction for a cement additive would be <75 m. The non magnetic fraction would also be suited for use in the adhesive and coating industries, The dried, stabilised mixture may be successfully mixed with cement, if desired. it is well known that under normal circumstances, where the oil-contaminated hot mill sludge has not been stabilised, cement and oil do not mix. Therefore oil-contaminated hot mill sludge is not typically used with cement processes.

However, with the present invention, due to the stabilisation of the oil contaminated hot mill sludge, the dried mixture can be mixed with cement without compromising the strength of the resultant material. Alternatively, as the oil contaminated hot mill sludge has been stabilised, the hot mill sludge could now be safely disposed in landfill without fear of oil contamination leaching out into the surrounding soil.

The oil absorbent material preferahly comes from a recycled source such as, but not restricted to, industrial and/or municipal wastes from high heat processing, as such processes result in a lime based waste material. For example, in one embodiment, the oil absorbent material may be a lime based waste material, such as paper ash, However, it is to be appreciated that other lime based waste materials may be used for example from modern waste incineration plants that produce ash that is high in lime and calcium chloride concentrations.

Preferably, the oil absorbent material does not contain zinc or preferably contains less than or equal to 0.0 1% of zinc.

In one embodiment, the method further comprises the step of adding a cementitous binder containing, but not restricted to, one or more of cakium, silica, alumina, iron and magnesium oxides to the mixture. The addition of a binder solidifies the sludge into a hardened state for re-use within the iron and steel making process.

Preferably, the cementitious binder contains orthorhombic calcium aluminate or a mixture of orthorhombic and cubic calcium aluminate. The orthorhombic calcium alumina controls the setting times and workability of the mix, whereas the cubic calcium alumina benefits strength development during the hydration process, the addition of calcium sulphate can also be added to assist in the controlling of the setting and hardening of the mixture which is particularly useful as if the percentage of orthorhombic calcium alumina is not in balance this will cause false or flash setting when mixing.

In a further embodiment, the cementitious binder has hydraulic and/or pozzolanic properties. These properties of the binder help solidify and harden the mixture, thereby minimising any environmental impact that may have resulted from leaching of the oil contamination into the environment from the treated hot mill sludge.

Due to the environmental impact of Portland cement, it is preferable that the cement binder used contains less than 30% Portland cement in order to minimise environmental impact. More preferably the cement binder used contains less than 20% Portland cement, and the cement binder may contain as little as 1 0% Portland cement. However it is most preferable of all if the cement binder used is substantially free of Portland cement. This is only possible due to the stabilisation of the oil contaminated hot mill sludge by the addition of a lime-containing oil absorbent material.

It is to be appreciated that other wastes from the iron and steel manufacturing process and alternative industries can be added to the oil contaminated hot mill sludge in the mixture to strengthen the hardened sludge and improve the chemistry of the end recycled material.

The end recycled material may then be re-used in the iron and steel manufacturing industry by feeding it into the furnaces, The final material must have sufficient strength to hold together when the material is transported by conveyor or other transportation means into the furnaces. The material must also be able to bind together until it reached the appropriate point within the furnace where it is required to gradually break down and melt, thereby releasing the oil which is used as an energy source within the furnace in the usual way.

For example, the oil contaminated hot mill sludge aM/or oil absorbent material mixture may be mixed with the cementitious binder and then poured into large slabs and left to harden until sufficient strength is obtained to be transported into the furnace. The hardened sludge can then be broken up into fragmented pieces of aggregate type material before being crushed and sized to the required grading for the furnace type in which it is to be used.

In a further embodiment, the method further comprises the step of adding liquid to the cementitious binder mixture. The liquid may include, but is not restricted to, super plasticiser admixtures to improve workability and strength enhancement and/or a calcium chloride based adniixture to enhance setting times and early strength development, particularly in cold weather.

The method may further comprise the step of processing the hydrated cementitious binder mixture to produce substantially spherical aggregate agglomerates. It is preferable to produce spherical shaped aggregate type material that can be graded in accordance with the requirements of the iron and steel making process and furnace type within which it is to be used. This is particularly important as the spherical aggregate has to remain intact up to temperatures of 1100°C whilst travelling through the upper hyers of the furnace before breaking down, One benefit of using spherical shaped aggregates within a furnace is that the aggregate material will be more robust and therefore easier to convey into the furnace.

Further, once in the furnace, spherical aggregate will roll down slowly through the layers within the furnace and avoid blockages within the furnace that could occur in the event that irregular shaped aggregates were used.

A further aspect of the present invention provides an aggregate comprising a hardened mixture of stabilised oil contaminated hot mill sludge waste, fibrous lime-containing oil absorbent material, and calcium aluminate containing cementitious binder, A further aspect of the present invention comprises producing one or more briquettes from the recovered iron rich mixture for reuse in for example the furnaces. The briquettes are preferably formed by binding the recovered iron rich mixture with at least one cementitious binder. The cementitious binder may comprise Portland cement, or a combination of Portland cement and lime. The binding of the recovered iron rich mixture with at least one cementitious binder is is a costly operation and has a high associated carbon footprint.

The at least one cementitious binder used to prepare the briquettes may comprise one or more low carbon binders such as for example a calcareous fly ash, for example Cenpave produced by Cenin. The one or more low carbon binders may partly or wholly replace the Portland cement within the cementitious binder, The briquette mixture may comprise Portland cement as a minor constituent so as to help harden the briquette for early handling on conveyor or other transportation methods.

The cementitious binder mixture may further comprise sodium silicate to improve the durability of the aggregate, for example spherical aggregate agglomerates or briquettes. The sodium silicate may be converted to calcium silicate from the lime in the calcareous fly ash resulting in improved hardness and durability of the briquettes. Briquettes or agglomerates produced with this method advantageously do not break down when wet and can be stored outside in bays until required.

An embodiment of the invention will now be described, by way of example only.

Detailed Description of the invention

Stabilisation of oil contaminated hot mill sludge Oil contaminated hot mill sludge was mixed with paper ash and or other ashes in proportions of 75% hot mill sludge and 25% paper ash or combination of ashes, to produce a slurry. The temperature of the mixture increases rapidly to >70°C, and the mixture is then mixed for several minutes or until the materials are adequately mixed. The stabilised sludge can then be stored until it is required for recycling or remain in a slurry form, In this case, the paper ash is obtained from a recycled source i.e. it is an industrial waste produce from high heat processing. However, it is to be appreciated that the lime-based oil absorbent material need not be obtained from a recycled source, The resulting thick slurry was then mixed with other construction materials and/or a cementitous binder to produce a concrete mix. The amounts of construction materials and binder will be dependent on the application the final aggregate it is intended for. When the hot mill sludge has been stabilised, a number of standard binders can be used including, but not limited to, Portland cement, Portland composite cements with limestone (PLC), pulverized fuel ash(PFA), ground granulated blast thrnace slag (GGBS) or any other suitable cementious binders such as those produced by Cenin Limited. After mixing, the slurry or concrete mix is transported to its place of use or poured into moulds where it is left to solidify into a hardened state.Once in a hardened state, the oil-based contaminants are stabilised and the hardened material can be re-used within the iron and steel making process, In addition to the lime-containing oil-absorbent material and the binder, it is possible to mix other waste products resulting from the iron and steel manufacturing process and/or allernative industries can be added to the oil contaminated hot mill sludge to strengthen the slurry when in its hardened state and improve the chemistry of the final recycled material, For example, by selecting the lime-containing oil-absorbent material specifically, it is possible to produce a slurry with the oil-contaminated hot mill sludge that, when subsequently hardened on additional by a binder, results in a final hardened product with low levels of zinc, in the production process any zinc in the raw materials can create a coating on the walls of the blast furnaces which will impact on the efficiency and life span of the furnaces. Therefore it is prefened to have the minimum possible level of zinc present in the recycled materials.

The production of the pellets occurs in a pellitiser, where the stabilised hot mill sludge (preferably magneticafly separated) can be mixed with other high iron content materials and a cementitous binder. A fine water spray is then added to the mixture for the hydration process, after which the material will then ball into spherical particles. The spherical particles are then stored and allowed to harden until they are required for use in the furnace. The curing time for the spherical particles is dependant on the strength of end product required. The strength is typically assessed by a tumble test.

Alternatively briquettes may be produced and added to the process.

The final hardened product can take the form of slabs, briquettes and/or pellets, including spherical aggregate. Stabs provide the advantage of easy stackable storage and/or transportation, Spherica' aggregate offers the advantage that the final hardened product can be re-used in the iron and steel making industries by feeding into the furnaces, where the oil can be rekased at the appropriate time within the furnace process and used as recycled fuel.

Preparation of slabs When in a dry state the sludge can be blended with dry fume dust and a cementitious binder and mixed thoroughly, at this stage dry sinter aggregate and dust(s) can also be added to the blend as part of the recycling process. The concrete slabs are left to harden which normally takes 28 days, although this time can be reduced by increasing the specification of the concrete strength. The slabs can be prepared by pouring in situ and used in hard standing for storage areas or for road sub-bases.

Alternatively pre-cast walling such as lego blocks can be produced.

Preparation of spherical aggregate Alternatively, once the sludge has been mixed with the lime-containing oil-absorbent material and binder, the blended material can then be transferred to a balling or pelletizing apparatus for balling/pelletizing. The size of the balled/pelletized material can be influenced (and therefore selected) by the addition of a desired amount of water during production of the recycled spherical aggregate.

Typically, the addition of more water leads to larger pellets. However, the pellet size is also influenced by the length of the mixing time, i.e. the longer the mixing time the greater the particle size. It is important that, in addition, the moisture content is correct in order to prevent the pellets from sticking together.

Preparation of briciuettes Once the sludge has been mixed with the oil absorbent material, such as for example a lime-containing oil-absorbent material, and binder (such as for example a cementitious binder), the blended material can then be transferred to a briquetting machine, The briquetting machine is arranged to press the mixture under high pressure into the desired shape for the furnace. The high pressure conditions also help to harden of the mixture. It is advantageous for the blended material to have a low moisture content. Preferably, the blended material comprises between 5% and 20% moisture dependent on the blended material, Sodium silicate may also be added to the mixing water to provide the blended material. The addition of sodium silicate has the additional benefit of providing improved adhesion and chemical reaction properties. Typically the sodium silicate reacts with the lime in the binder to form calcium silicate.

At this stage (i.e. the hardening of the blended mixture), there is a fine balance between flash and false setting that is required to achieve the desired hardening for the recycled spherical aggregate or briquettes. This is due to the blended material hardening by the chemical reactions of the cementitious material that contains calcium aluminates alone in combination with calcium aluminate (Cubic), rather than calcium aluminate (Cubic) alone. Calcium aluminate (Orthorhombic) will create flash or false setting in the mixer, Therefore the balance of Cubic and Orthorhombic) is required at this stage. As the liquid is added to the blended material, the blended material will produce spherical aggregate agglomerates or briquettes that will rapidly harden. This process can be speeded up by passing the agglomerates or briquettes through a drying system.

One or more cementitious binders containing hydraulic and/or pozzolanic properties is added to the slurty at pre-determined proportions dependant on the setting times required for the hardening of the slurry. This is determined by the process that is to be used for the formation of the recycled aggregate type material.

The preferred method is to produce spherical aggregate type materials or briquettes that are a preferred shape and/or dimensions for the handling and dispersion in the furnace.

For the mixing of the materials a typical concrete mixer can be used to produce a consistent sludge to feed into the process.

Another method is to use a rotary revolving drum or kiln where the sludge can be poured into the rotating drum and will then ball into spherical shaped aggregates as the sludge is transferred from one end of the drum to the other, Alternative methods There are two further methods that can be used at this stage which shall be referred to as the wet method and the dry method: 1. Wet Method (a) The wet method is to stabilise the oil contaminated hot miH sludge with oil absorbent material, as previously described. Other waste materials can be added according to the desired properties of the end product. Cementitious material is then added to harden the sludge. This can be done in a specialised mixer that will form the spherical shaped aggregate as required. This method will require a high percentage of cementitious material to harden the sludge.

2. Wet Method (b) An alternative wet method is to use a two stage process where the oil contaminated hot mill sludge is stabilised and other wastes added in a standard concrete mixer for example, then a lower percentage of cementitious material added. The sludge is then transferred to the specialised mixer to form the spherical aggregate type material and a second cementitious material added that contains calcium aluminates, preferably calcium aluminate (Orthorhombic) alone or in combination with calcium aluminate (Cubic).

As previously discussed, calcium aluminate (Orthorhombic) will create flash or false setting in the mixer, It is the fine balance between flash and false setting that is important at this stage to determine the hardened properties of the final recycled material. As the spherical aggregate material or briquette leaves the mixer it will rapidly harden.

A variation on this method is to transfer the sludge containing the second cementitious material added into a rotary drum or kiln. The spherical aggregate material will rapidly harden as it leaves the drum or kiln.

The advantage of the two stage process is that it will reduce the percentage of cementitious material required and therefore reduce the overall cost and associated carbon footprint of the process.

3. Dry Method (a) The dry method is to stabilise the oil contaminated hot mill sludge with lime-containing oil absorbent material. Other waste materials can be added according to the desired properties of the final waste product. Cernentitious nrnterial is then added to harden the sludge. This can be done in a specialised mixer that will form the spherical shaped aggregate or briquette as required. Alternatively the slurry and cementitious material can be mixed in a conventional mixer before transferring the sludge to a rotary drum or kiln. The sludge is preferably moister than the wet method when leaving the mixer.

In the thy method the cementitious material that contains one or more calcium aluminates, preferably calcium aluminate (Orthorhombic) rather than calcium aluminate (Cubic) or a combination of both, is fed into the rotary drum or kiln separately to the slurry mixture, The blended slurry, binder mixture is then poured into the rotary drum or kiln and as it balls into the spherical shaped aggregate it will coat the aggregate with the cementitious powder and flash or false set, It is advantageous at this stage to add waste fume dust containing a high percentage of iron from the iron and steel process to the cementitious material in the rotary drum or kiln, Tron ore fines may also be added to the fume dust or as an alternative to fume dust at this stage. A fine spray should be applied to the coated spherical aggregate particles at the back end of the process to ensure all the fine powders are mist and can fully hydrate, 4. Dry Method (b) A further dry method is to use a two stage process where the oil contaminated hot mill sludge is stabilised as discussed above by the addition of a lime-containing oil absorbent material, and other waste materials (as desired) added (for example in a standard concrete mixer) then a lower percentage of cementitious material added.

The blended sludge is then transferred to a specialised mixer to form the spherical aggregate type material and a second cementitious material added that contains calcium aluminates, preferably calcium aluminate (Orthorhombic) alone or in combination with calci urn al urninate (Cubic).

Calcium aluminate (Orthorhombic) will create flash or false setting in the mixer, it is the fine balance between flash and false setting that is required at this stage. As the spherical aggregate material leaves the mixer it will rapidly harden. An alternative is to transfer the sludge with the second cementitious material added into a rotary drum or kiln having similar effect when the spherical aggregate material leaves the drum or kiln.

Heat can be used in this process where heat is put into the kiln or rotary drum to accelerate the curing time of the spherical shaped particles, It is preferable that the heat used is from a recycled source.

The advantage of this method is that the spherical shaped aggregate or briquettes can be transferred to a furnace for re-use in a shorter time frame, thereby reducing the quantity of material that would have to be stored between the waste material being produced and the recycled waste material being used.

Although aspects of the invention have been described with reference to the embodiment shown in the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiment shown and that various changes and modifications may be effected without further inventive skill and effort, for example, it is to be appreciated that the second cementitious material referred to in the methods above may be the same as or different to the first cementitious material.

Claims

CLAIMS1. A method of treatment of oil-contaminated hot mill sludge, the method comprising the steps of: a. mixing oil-contaminated hot mill sludge with a fibrous oil-absorbent material containing lime and/or calcium chloride to produce a mixture.
2. A method according to claim 1, further comprising the step of drying the mixture,
3. A method according to claim 1, wherein the oil absorbent material is a lime based waste material.
4. A method according to claim I or claim 2, wherein the oil absorbent material contains less than or equal to 0.01% of zinc.
5. A method according to any preceding claim, wherein the method further comprises the step of: b. adding a cementitious binder containing calcium auminate to the mixture.
6. A method according to claim 5, wherein the cementitious binder contains orthorhombic calcium aluminate or a mixture of orthorhombic and cubic calcium aluminate.
7. A method according to claim 5 or claim 6, wherein the cementitious binder has hydraulic and/or pozzolanic properties.
8. A method according to any one of claims 5 to 7, wherein the cementitious binder is a cement other than Portland cement.
9. A method according to any one of claims 5 toS, wherein the method further comprises the step of: c. adding liquid to the cementitious binder mixture.
10. A method as claimed in any one of claims 5 to 9, further comprising adding sodium silicate.11 A method according to any one of claims 5 to 10, the method further comprising the step of: d. processing the hydrated cementitious binder mixture to produce substantially spherical aggregate agglomerates and/or briquettes.12, Aggregate comprising a hardened mixture of stabilised oil contaminated hot mill sludge waste, fibrous lime-containing oil absorbent material, and calci uni alurninate containing cernentitious binder,