WO2006040135A1

WO2006040135A1 - Treatment of sludge

Info

Publication number: WO2006040135A1
Application number: PCT/EP2005/010950
Authority: WO
Inventors: Peter Farkas Binderup Hansen
Original assignee: Rockwool International A/S
Priority date: 2004-10-13
Filing date: 2005-10-12
Publication date: 2006-04-20
Also published as: EP1799620A1

Abstract

A method of mineralisation of a sludge comprising providing a sludge and allowing mineralisation to occur, characterised in that the process comprises mixing man-made vitreous fibre material with the sludge before or during the mineralisation process.

Description

ROCKWOOL INTERNATIONAL A/S

TREATMENT OF SLUDGE

The invention relates to methods of treating sludge materials, by mineralisation which involves aeration of the sludge and decomposition of the organic components, and in particular to methods of improving the mineralisation process .

It is known commercially to mineralise sewage sludge material for a variety of purposes including provision of a raw material for production of a granulated slag. Such a method is described in WOOl/42154. This describes a process of treating municipal sewage sludge by mineralisation, which involves aeration of the sludge and decomposition of the organic components. The mineralised material is pressed into briquettes and these briquettes are combusted in a furnace such as a blast furnace. Melting of these briquettes results in formation of a glass, which is quenched to a slag which is at least partially self-granulating. The granulated slag can be used for various purposes including, after crushing and dividing, for sandblasting; or for production of slag wool.

It is suggested that, during the mineralisation process, mineral-containing components other than the sewage sludge-based material can be added, including "mineral-containing waste products" and natural rocks.

Examples given are car shreds, hammer scales, molding sand, garnet, aluminium silicate, corundum (these last three all being used sandblasting sand) , fireproof MgO bricks, chamotte bricks, ash from PVC and paper waste. In practice the examples use a combination of these exemplified materials with sludge ash and/or mineralised sludge in admixture to form the molten glass and granulated slag. In two examples the mixture is molten without briquetting. In one industrial scale example mineralised sludge is mixed with a number of other mineral-containing materials and the mixture briquetted. In another industrial scale example mineralised sludge is mixed with foundry sand, olivine sand, wood, treated grain remainings, used garnet and limestone and this mixture mineralised for 40 days. The mineralised mixture is then briquetted.

In commercial practice methods of this type involve mineralisation for up to one year before briquetting occurs and woodchips are commonly included in admixture with the sludge material to provide structure to the stack of sludge material as it is mineralised.

In practice it would be desirable to improve the mineralisation process. In addition, it would be desirable to improve the properties of the briquettes which are provided to the melting furnace. A separate and unrelated problem is in the field of mineral wool use. It is known to make a variety of products from man-made vitreous fibres. One particular man-made vitreous fibre (MMVF) product is growth substrate, for instance as used in the horticultural industry. A block or slab of growth substrate is used for one to two years and cannot then be further used. This presents a problem of disposal of the used growth substrate. Partly as a result of its characteristics as initially produced and partly as a result of changes during its use, disposal of used growth substrate is particularly difficult. It tends to have high content of organic materials including binder, wetting agent, remnants of plant materials such as roots, and fertilisers. Simple disposal of the used growth substrate to landfill is possible but undesirable, especially as a long-term solution. However, other disposal possibilities involving recycling of the growth substrate material are hampered by its content. In theory it could be recycled to the production of further growth substrate by melting with the other raw materials. However, it would normally have to be incorporated in the form of briquettes and the briquetting process is hampered by the presence of organic materials. It is presently used as reinforcement in bricks, but the volume which can be disposed of in this manner is limited.

The invention brings together the two fields mentioned above and finds solutions to the problems discussed above in combination.

According to the invention we provide a method of making a mineral product, the method comprising providing a sludge and allowing mineralisation to occur to form mineralised sludge, characterised in that the process comprises mixing man-made vitreous fibre material with the sludge before or during the mineralisation process.

In a preferred embodiment the invention also provides a method of producing a briquette from the mineralised sludge and a mineral melt from the briquette. Hence, the mineral product of the invention can simply be the mineralised sludge, or can be a material formed from it such as a briquette or mineral melt .

Mineralisation of sludge involves aeration of the sludge and decomposition of the organic components. We find that the inclusion of man-made vitreous fibre material with the sludge has a number of advantages . When added to the sludge before or during mineralisation, it provides additional structure and porosity to the stack of sludge, thus improving the potential for aeration and improving the mineralisation process. When the sludge is, after mineralisation, formed into briquettes, the inclusion of man-made vitreous fibre material may increase the strength of the briquettes. A particular advantage of the inclusion of used growth substrate material in processes of the type used commercially which ultimately result in a granulated slag material is that the used growth substrate has composition very similar to that of the granulated slag material and thus can easily be incorporated into the process. Furthermore, in a preferred embodiment where the man- made vitreous fibre material is waste growth substrate, the processes above have the advantage that the used growth substrate material can be included in the mixture with the sludge with the minimum of pretreatment, in contrast with previous recycling suggestions.

In the invention it is essential that a man-made vitreous fibre material is mixed with the sludge. Man-made vitreous fibre materials are well known. They are produced by providing mineral materials, melting the mineral materials to form a mineral melt, and forming fibres from the mineral melt . Generally the man-made vitreous fibre material to be used is a waste material, that is a material which would otherwise have to be disposed of. Often it has previously been used for another purpose. It can have been used for any of the purposes for which man-made vitreous fibre materials are- known, including thermal insulation, sound insulation, fire protection, and as an artificial growth substrate.

One source of man-made vitreous fibre material is demolition waste and this can be used in the invention. If this is done, it is usual to pretreat the waste so as to shred, crush and/or grind the man-made vitreous fibre material. If required, other building materials such as iron can be removed. Addition of lime can be desirable to assist in processing of concrete which might be present in the demolition waste.

Preferably the waste man-made vitreous fibre material is waste growth substrate, usually growth substrate which has already been used for horticultural purposes. This may contain, within the matrix of man-made vitreous fibres: organic and/or inorganic binder; organic wetting agent; clay; fertiliser and nutrient materials; and/or plant materials such as roots and stems.

Used growth substrate can contain from 5 to 50 wt.%, preferably 10 to 40 wt.% and usually 15 to 30 wt.% organic material (dry basis) . Moisture content of used growth substrate is often in the range 10 to 70 wt.%, more usually 20 to 60 wt.%, most commonly 30 to 50 wt.%. The man-made vitreous fibre material may have any known composition but preferred upper and lower limits for components are set out below, with each content being expressed by weight of oxide. SiO₂ at least 30, 32, 35 or 37; not more than 51, 48, 45 or 43

Al₂O₃ at least 14, 15, 16 or 18; not more than 35, 30, 26 or 23 CaO at least 8 or 10; not more than 30, 25 or 20 MgO at least 2 or 5; not more than 25, 20 or 15

FeO (including Fe₂O₃) at least 2 or 5; not more than 15, 12 or 10

FeO + MgO at least 10, 12, 15; not more than 30, 25, 20 Na₂O + K₂O zero or at least 1; not more than 10 CaO + Na₂O + K₂O at least 10, 15; not more than 30, 25 TiO₂ zero or at least 1; not more than 6, 4, 2 TiO₂ + FeO at least 4, 6; not more than 18, 12 B₂O₃ zero or at least 1; not more than 5, 3 P₂O₅ zero or at least 1; not more than 8, 5 Others zero or at least 1; not more than 8, 5

Fibres can be used which have low content of aluminium, for instance below 5% or 3%, but in practice it is preferred to use fibres having the levels of aluminium mentioned above, especially when the amount of man-made vitreous fibre added to the sludge is relatively large, for instance at least 5% or at least 10%, based on the total mixture.

Prior to use in the invention, plastic coverings can be removed from the growth substrate. Moisture content can be reduced, for instance by a press screw.

Growth substrate can be supplied in the form of products of a variety of different dimensions, including products described as blocks and slabs. Standard products have dimensions about 10 cm x 10 cm x 7.5 cm (blocks) or about 90 cm x 15 cm x 7.5 cm (slabs) . An advantage of using blocks in the invention is that no size reduction pretreatment is required before mixing the growth substrate with the sludge. However, pretreatments can if desired include size reduction, for instance shredding, grinding and/or crushing. These treatments can be particularly valuable with the growth substrate is in the form of products of maximum dimension greater than 20 or 50 cm.

It is possible to carry out treatments such as heating to remove organic materials but preferably no treatments are carried out to remove organic materials from the used growth substrate before it is added to the sludge in the invention.

The sludge will generally contain organic material, inorganic material and water.

For instance it can be sewage sludge from municipal purification plants. For instance it can be sludge generated from chemical treatment of sewage water, followed by dewatering. Dewatered sludge typically consists of 70 to 80% water, 10 to 15% organic material and 10 to 15% mineral components .

In the first aspect of the invention the sludge is allowed to undergo mineralisation. During the process the sludge is contacted with oxygen and often the mixture will be self-ignite. During the process the organic components are generally decomposed, usually to form materials such as methane which are emitted from the sludge. The process includes decomposition of fat, protein and soluble carbohydrate. By the end of the mineralisation process the mineralised sludge consists primarily of inorganic material .

The sludge is generally provided in the form of one or more stacks. The stack can be of height up to 19 metres.

In practice the sludge is allowed to mineralise for at least 10, preferably at least 20, more preferably at least 30 days, in particular at least 60 days, and often more than six months or nine months and can be up to one year. Preferably the man-made vitreous fibre material is mixed with the sludge before any substantial mineralisation has occurred. Preferably mineralisation occurs for at least 40 or 60 days, preferably at least 3 months or 6 months, after the man-made vireous fibre material has been added.

In a common commercial process of the type described in WOOl/42154, the sludge is provided in a stack which is periodically mixed and, as a result of the mixing, moved to a new position.

In a preferred aspect of the invention the sludge, after mineralisation has been carried out to the desired extent, is formed into briquettes. The man-made vitreous fibre material can be added at any stage before or during mineralisation, and in particular before formation of briquettes.

Any method of mixing the man-made vitreous fibre material with the sludge can be used, provided that it allows the desired degree of distribution of the man-made vitreous fibre material throughout the organic material . In processes which already involve mixing of the sludge, the man-made vitreous fibre material can be added in a manner which allows this mixing to effect distribution of the man- made vitreous fibre material through the sludge.

The amount of man-made vitreous fibre material can be chosen within wide ranges, for instance 0.1 to 99 wt.%, based on the total weight of the mixture, but is generally in the range 0.5 to 50 wt.%, more preferably 1 to 20 wt.%, especially 1 to 10 wt.%. These amounts are based on the total weight of the mixture of man-made vitreous fibre material and sludge at the time when the man-made vitreous fibre material is addded. If the man-made vitreous fibre material is added as a mixture with other waste materials, as when demolition waste is used, these percentages refer to the amount of man-made vitreous fibre material added.

The mineralisation process is generally carried out until the moisture content of the sludge is within a predetermined desired range, for instance from 20 to 35 wt.%, in particular from 27 to 33 wt.%. Generally water content is monitored during the process so that the process may be stopped once the desired water content has been achieved.

After mineralisation is complete the mineralised sludge is formed into briquettes by pressing. At this stage other materials may be mixed with the mineralised in order to aid formation of briquettes, for instance as described in WO01/42154. The briquettes generally have a minimum dimension of at least 50 mm, often at least 60 mm.

The man-made vitreous fibre material can be added at any point in the process up to formation of the briquettes.

Preferably the briquettes are allowed to harden, as described in WO01/42154. For instance they can be hardened at a temperature of from 75 to 110⁰C. Hardening time can for instance be from 2 to 10 hours, preferably 3 to 6 hours . Hardening takes place until the water content of the briquettes is preferably between 15 and 20 wt. %.

The briquettes are added to a furnace and melted, under conditions whereby the mineral content of the melt is in oxide form. Suitable furnaces are described in WO01/42154. Generally a blast furnace is used but other types of furnace can also be used such as other shaft furnaces including cupola furnaces .

Once the briquette material is molten, at a temperature of at least 1,400⁰C, a material which can be described as a glass is formed. This glass melt is then quenched to form a slag. The slag can be at least partially self-granulating but can also be crushed and divided according to the intended final use.

The composition of the granulated slag can be as follows; as described in WOOl/42154.

SiO₂ 35 to 50 wt.%

Al₂O₃ 15 to 25 wt.%

Fe₂O₃ 5 to 15 wt . %

CaO 5 to 20 wt. % MgO 1 to 10 wt . %

MnO₂ less than 1 wt .%

TiO₂ less than 3 wt . % P₂O₅ 1 to 10 wt. % K₂O less than 2 wt . % Na₂O less than 2 wt . % other elements less than 5 wt . %. All elements are expressed by weight of oxides. Some microelements may be present, for instance: Sb less than 0.007 wt.% Pb less than 0.020 wt . % Cd less than 0.009 wt. % Sn less than 0.043 wt . %.

Levels of carcinogenic microelements are very low: As less than 0.009 wt. % Be less than 0.007 wt . % Cr less than 0.001 wt. % Co less than 0.007 wt . % Ni less than 0.22 wt. %.

There are a variety of uses for the granulated sludge slag. For instance it can be used as a blowing agent in sandblasting. This material, once used for sandblasting, can be used as a raw material in the production of man-made vitreous fibre products, including growth substrates. Alternatively, it can be used as a raw material for man- made vitreous fibre products even without prior use as sand blasting blowing agent.

Claims

1. A method of making a mineral product, the method comprising providing a sludge and allowing mineralisation to occur to form mineralised sludge, characterised in that the process comprises mixing man-made vitreous fibre material with the sludge before or during the mineralisation process.

2. A method according to claim 1 in which the sludge is sewage sludge.

3. A method according to claim 1 or claim 2 in which mineralisation occurs for at least 40 days, preferably at least 60 days, after the addition of the man-made vitreous fibre material.

4. A method according to any preceding claim in which substantially no mineralisation takes place before addition of man-made vitreous fibre material .

5. A method according to any preceding claim in which the man-made vitreous fibre material is waste growth substrate.

6. A method according to any preceding claim in which the amount of man-made vitreous fibre material is from 1 to 20%, preferably 1 to 10%, based on the total mass of the sludge and man-made vitreous fibre material .

7. A method according to any preceding claim in which the man-made vitreous fibre material is not pretreated prior to addition to the sludge material by removal of organic components .

8. A method according to any preceding claim, the method additionally comprising the step of forming briquettes from the mineralised sludge.

9. A method according to claim 8, the method additionally comprising the step of melting the briquettes in a furnace to form a mineral melt .