GB2395162A

GB2395162A - A method of producing moulded products from waste paper`

Info

Publication number: GB2395162A
Application number: GB0320694A
Authority: GB
Inventors: David Malcolm Owen; Peter Christmas
Original assignee: SALVTECH Ltd
Current assignee: SALVTECH Ltd
Priority date: 2002-09-05
Filing date: 2003-09-04
Publication date: 2004-05-19
Anticipated expiration: 2023-09-04
Also published as: GB0220602D0; GB2395162B; GB0320694D0

Abstract

A method of processing waste paper sludge to produce moulded products. The method involves the following steps: (a) adjusting the consistency of the sludge to a suitable processing consistency, (b) adding a suitable polymeric coagulant or flocculant to the mixture, whilst subjecting the mixture to a non-shear or low shear mixing, (c) adding a suitable quantity of starch or like material to the sludge and blending the mixture and (d) applying pressure to the mixture to produce the moulded products, i.e. boards. The method also includes the step of heating the sludge mixture, either before it is pressed into a moulded product or after, to a temperature such that the starch cooks out. The boards produced by the present process are suitable for use in construction work. The fire retardant nature of the boards can be improved by the addition of proprietary fire retardants. Plastics may be added in ground or particulate forms.

Description

-1- 2395162

A METHOD OF PRODUCING MOULDED PRODUCTS FROM WASTE PAPER

Field of the Invention

The invention relates to the field of waste paper recycling and more

specifically to the production of moulded products, of improved strength, from the residues derived from waste paper recycling and de-inking processes.

Background of the Invention

The use of paper is widespread. The resultant build-up of waste paper, paired with environmental concerns, has given rise to a huge industry devoted to the recycling of waste paper.

Using modern Recycling and De-inking processes, a high percentage of waste paper can be reclaimed in the form of reusable fibre for papermaking. However, a not inconsiderable amount of material (typically between 25% and 40%) is rejected as waste material from these processes. This de-inking residue (DIR), which comprises ink, clay, chalk and many short, low quality fibres that are unsuitable for papermaking, represents a major disposal problem for the industry.

There is thus a desire for a process that can turn this residue into a useful product. In European Patent No 0686218, a process is disclosed in which the residue may be pressed into a board, or other such objects, using a two-stage pressing process with sequential drying. However, it is understood that the process of pressing the residue sludge according to the above patent is very slow due to the poor drainage of water from the material during forming. Also, the pressing of the residue in the initial wet (sludge) stage is subject to many problems associated with containment in the forming area due to hydraulic shear causing weak spots in the final board. Furthermore, it is a requirement of the process to undertake four basic steps to achieve the desired result, namely: pressing the sludge in a press mould with double drainage wires to allow release of water, through the wires, from the matrix, to produce a 'board- like' or 'moulded' product that is approximately 30% wet; drying the board or moulded product in air at ambient temperature or in an oven, such that the remaining moisture content is only that water that is associated with the cell wall of the cellulose contained therein (this is usually 20%); further press the

-2 board to achieve dimensional stability; further dry/cure the board or panel at elevated temperature to achieve a final moisture content of <5%.

The moulded product formed by the above patented process has a typical bending strength modulus of between 4.5MPa and 7MPa and can be used in the construction industry.

Reference to the a similar method of processing waste paper is made in UK Patent No 1275042 and UK Patent No 1405587, both of which were owned by the same Applicant. These patents teach that residue sludge may be taken and extra fibre added followed by a water removal step and heating to produce moulded products. The patents identify the need to use mesh type wires to allow release of water from the press so that steam and hydraulic explosion does not occur during pressing.

However, attempts to utilise these inventions (the patents having now expired) have proved to be commercially impractical due to the cost implications of the additives and also the fact that products produced by such methods lack some of the basic strength characteristics required for the construction industry.

Similarly, it has been discovered that in the case of moulded products made essentially from paper mill sludge with no fibrous material, the time taken to drain the moulded article from the beginning of the press cycle to completion of pressing is excessive and makes any commercial process extremely marginal.

There is a need for a method of processing waste paper residue in such a way that the pressing time is reduced and product strength is improved.

Summary of the Invention

The invention provides a method of processing waste paper sludge to produce moulded products, comprising the steps: a) adjusting the consistency of the waste paper sludge to a suitable processing consistency; b) adding a polymeric coagulant or flocculent to the sludge, whilst subjecting the sludge to a non-shear or low shear mixing, to form a sludge mixture; c) adding starch, or like material, to the sludge mixture; d) pressing the sludge mixture to produce the moulded product; and e) heating the sludge mixture or moulded product to a temperature such that the starch or like material is cooked out.

-3 It has been found that the above process results in a rapid removal of water from the mixture. As a result, the pressing times can be reduced by as much as 75%.

The resultant moulded product has been found to have significantly improved strength over the products of the earlier referenced processes.

Preferably, steps d) and e) of the above method may be carried out using a heated press with drainage wires to permit the release of water and steam.

Alternatively, step e) of the above method may be carried out using an oven.

Preferably, a further flocculation promoter may be added to the sludge mixture at step b) of the above method. However, the flocculation promoter may also be added at a different stage of the method if required. Such further flocculation promoter may be a clay material or suitable alternative thereof.

It will be understood that the addition of the clay material promotes super flocculation of the sludge mixture. Preferably, the clay material or a suitable alternative should be bentonite or an equivalent material.

Preferably, the above method may include the further step of admixing a suitable proprietary fire retardant material, such as aluminium trihydrate or like material, into the sludge mixture. Further preferably, the proprietary fire retardant material may comprise a combination of fly ash and alum.

It will be understood that the order in which the steps of the above process occur can be varied as required.

Preferably, the starch used should be raw. However, cationic starch may also be used.

A further process step of shearing the mixture may be used to reduce lumps and to homogenise the mixture to help eliminate 'weak spots' in the final product. It should be noted that this process is similar to, but not as energetic as, refining in the paper industry.

In the initial stages of the process, the consistency of the sludge and additives mixture may preferably be adjusted to be between 30% and 40%, by water addition or removal as appropriate.

Preferably the amount of added fire retardant material would have an addition level of no more than 30% by weight relative to the total solids of the final sludge mixture. More preferably, the amount of fire retardant material added is limited to an addition level of about 10 and 15%.

-4 Preferably, the amount of coagulant or flocculant added to the mixture is between 0.25 and 10kg per dry tonne of treated solids.

Preferably, the mixture may be pre-heated in the mixing equipment, heated while in the press, and may also be heated subsequently in an oven. In all cases the mixture may preferably be heated to a temperature of >50 C. However, higher temperatures may be used to more rapidly achieve the desired result and to promote curing. Also, it may be preferable to press the product of the process a second time to establish dimensional stability.

Preferably, the process may include the step of adding plastics materials, in ground or particulate form, to the sludge before starch is added. Plastics materials that are considered suitable include PVC, polyethylene, polyvinyl acetate, and urea formaldehyde resins.

Additionally, it may be necessary to oven heat the board after pressing to a temperature of >1 00 C, followed by a second pressing at, or close to, that temperature. By doing this, the distribution of the melted plastic into all porous areas is facilitated, thus producing a product that is both weather and water resistant.

During the development of the present invention a number of experiments were carried out. The experimental details and accompanying results are presented below. De-watered sludge was taken from the outlet press at a UK paper mill. This material was a grey solid material containing approx 60% solids. This was the master batch (or blank) of the raw material.

Experiment 1 A slurry was made to approximately 30% solids by mixing with fresh water.

The resultant slurry was then fed through a shearing mixer to de-lump and smooth the formed paste.

The paste was then admitted to a press with two wire elements mounted top and bottom as described in prior patent.

A board was pressed of typical 14mm thickness according to the method of

Prof. Skjelmose. The time for the initial pressing at 102 bar was typically of the order of 6 minutes. This produced a board consistent with the product made in the primary process of Skjelmose, at approx. 70% solids. (30% wet).

Experiment 2 A slurry was made, as per experiment 1, except that some of the added water was retained to mix into the additives.

Using the retained water volume dilute solutions of: 1. Cationic polyamine 576C manufactured by 3f Chimica, Sandrigo, Venezia, Italy, and 2. Locally available Bentonite were made.

The mixing of the sludge additions to 1 and 2 above were made sequentially, allowing a suitable time for each flocculation to take place.

Part of the sample was sheared as per experiment 1.

Pressing was undertaken of both the sheared and un-sheared samples It was noted that the pressing time of a sheared sample was identical to that of experiment 1. However, the un-sheared sample, at an appropriate dose rate of each of the additives, showed a significant improvement in drainage time.

Experiment 3 A slurry was made as per experiment 2 except that the addition was of a single cationic polymer.

Several different types were used, but all types gave the same order of drainage time. For a particular example a cationic polyamine 576C manufactured by 3F Chimica, Sandrigo, Italy was used at various addition rate levels from 1-10kg/tonne dry weight of sludge solids.

-6 There was an improved drainage time in the press, but it was significantly slower than 2 above.

The results are detailed below as typical.

Chemical dose Kg/dry tonne solids Typical press time to gauge (drainage) Blank (sheared, no additives) 6 min 2sec Polymer only (No shear) Polyamine 576C 1 kg/tonne 4 min 17sec Polyamine 576C 3kg/tonne 3 min 48sec Polyamine 576C 5kg/tonne 3 min 39sec Polymer and Bentonite (no shear) Polyamine 576C 1 kg/tonne 2 min 27 sec Polyamine 576C 3kg/tonne 1 min 15 sec Polyamine 576C 5kg/tonne 1 min 31sec Sheared samples always same as blank It can be seen that de-watering is enhanced only by appropriate mixing (low shear) and by minimal shear after achieving correct flocculation.

However, whilst speed of drainage is important to production efficiency, it was necessary to address the issue of board/panel strength. In addition, the fire retardant qualities of the board were considered.

The fire redundancy properties of the board are easily adjusted by the addition of a suitable fire retardant such as aluminium trihydrate. Addition levels can be up to 30% depending on the fire rating required in the final board.

Various low cost additives were considered in order to strengthen the board. A suitable candidate was starch as used in the paper industry. Raw starch is normally

-7 cooked to produce strength. However attempts to dewater the sludge after additions of cooked starch solutions significantly increased dewatering times.

It was found that if raw starch of any origin, i.e. wheat, potato, corn etc was added to the slurry by way of the water addition or by dry admixture followed by mixing, that there was no significant effect on the drainage rate that would have been achieved, with or without addition of chemicals.

Additionally if the board produced by cold or hot pressing was further heated, a cooking process took place inside the board that gelatinised the starch to produce strength in the board. It is not immediately obvious that this reaction will take place as it is well known in the paper industry that the addition of raw starch to paper making stock does not necessarily produce dry strength in the finished paper. This non- effect is due to a lack of retention and curing on the paper machine during the drying operation. It was uncertain that the low water content of the pressed material would be adequate to produce the final strength.

Therefore it is not obvious to use raw starch in the board manufacturing as dry strength is produced at the first pressing stage if that pressing is conducted with heating. For the purposes of manufacture it is preferable, but not absolutely vital, to heat the press, as subsequent heating may be employed to cure the starch into the board.

Board manufactured by this process had an intrinsically higher strength than previously reported. Typically the bending strength modulus was 15MPa compared with 4.5MPa to 7MPa reported previously.

The basic properties are incorporated into the board at this stage and further pressing is only necessary to achieve dimensional stability and bulk density.

In a further inventive step, the first starch gelatinization may be conducted in either the press or in an oven at up to 1 00 C or thereabouts, such that the board dries to an appropriate degree so that added plastic material is not melted, but after drying out the water, the temperature may be raised in the board such that the dispersed plastic material reaches its thermoplastic/thermo-cure temperature.

-8 The board would then require a further consolidation press to squeeze the melted plastic into the interstitial spaces and consolidate on cooling to a water/weather resistant board or moulding.

Detailed Description of the Preferred Embodiments

The present process has a number of potential variations based on the fundamental steps of the invention. Listed below are more detailed descriptions of some of these

variants. Variation 1.

A method for the manufacture of a moulded material from waste sludges especially those derived from the de-inking process in the paper industry. Adding to the sludge, at an appropriate consistency, a fire retardant such as aluminium trihydrate or similar aluminium salt derivative, and a strengthening aid such as raw starch or starch derivatives, or other recognised poly hydroxy organic molecules, usually between 0.1 and 50 kg/dry tonne solids, in admixture with a sequential addition of a cationic polymer natural or synthetic usually between 0.25 and 40 kg/dry tonne solids, capable of producing a drainage reaction and flocculation, followed by admixture of a bentonite or similar microparticle effect material, natural or synthetic, usually between 0.5 and 50 kg/dry tonne solids, under conditions to produce mixing but low shear. Admitting the treated slurry to press containing filter belts or moulds and removing the water therein by pressing to no free water removal with or without the addition of heat.

The order of addition may be varied insomuch as the starch may be added at any appropriate stage so long as the incorporation into the sludge is evenly distributed.

The board may be cured in the press or subsequently in an oven at temperatures, normally greater than 50 C to produce a dry board. The board may be subsequently pressed, with or without the application of heat to consolidate the board properties.

Variation 2.

A method for the manufacture of a moulded material from waste sludge, especially those derived from the de-inking process in the paper industry, by adding

-9- to the sludge, at an appropriate consistency, a fire retardant such as aluminium trihydrate, or similar aluminium salt derivative, and a strengthening aid such as raw starch or starch derivatives or other poly hydroxy organic molecules, usually between 0.1 and 50 kg/dry tonne solids, mixing the said slurry until an even consistency is achieved. Admitting the treated slurry to a press containing filter belts or moulds and removing the water therein by pressing to no free water, with or without the addition of heat.

Variation 3.

A further modification to the method whereby admixture of plastic agents of virgin or recycled origin may be made at the original mixing stage. Incorporation of the said material into the matrix as a thermoplastic or thermosetting effect is achieved by drying to less than 10% moisture and heating to the critical temperature of the specific material followed by a second stage pressing to achieve consolidation and pore filling. Final properties will be determined by the percentage of added material and properties associated therewith.

Variation 4.

A method for rapidly de-watering said slurry, whereby a cationic flocculating polymer, of natural or synthetic origin, usually between 0. 25 and 40 kg/dry tonne solids, is added to the slurry whilst mixing is taking place with low shear. Removal of the treated slurry from the mixer without shear and pressing as described above.

Variation 5.

A method for rapidly de-watering said slurry, whereby a cationic flocculating polymer, of natural or synthetic origin, usually between 0. 25 and 40kg/dry tonne solids, is added to the slurry whilst mixing is taking place with low shear. A further admixture, with low shear mixing, of Bentonite or some other similarly active micro particle material of synthetic or natural original at doses appropriate to achieve the

-10 effect usually between 0.25 and 50 kg/tonne dry solid. Removal of the treated slurry from the mixer without shear and pressing as described above.

Variation 6 An alternative fire retardant can be used in the form of 'neutralized fly-ash' from the Kemsley Combustor. Alum is added to the fly ash to neutralise the high causticity of the mainly calcium oxide (quick lime) ash. This solution can then be added to the slurry instead of a proprietary fire retardant and acts effectively in promoting fire retardancy in the final board. Salvtech work has demonstrated that this fly ash retardant' can be added up to levels of 30% by weight of the final mix without any adverse effect on the final board strength. This phenomenon is attributed to the cementations properties associated with the Quick Lime (Calcium oxide) in the fly ash being passed onto and incorporated into the board product.

Currently the fly ash is classified as a 'hazardous waste' and disposal is expensive. It can contain around 20% of calcium oxide. Salvtech study work has shown that a solution of Alum (as used in the papermaking process) can be added to a 30% solution of the fly ash in a propriety low shear mixer until the initial pH (around pH11.5) falls below pH 8. At this point the neutralized ash can be added to the DIR board mix as a substitute for the propriety fire retardant. Neutralisation of the fly ash with Alum also declassifies the waste to non-hazardous and facilitates cheaper alternative disposal routes for the combustion process.

Claims

-11 CLAIMS

1. A method of processing waste paper sludge to produce moulded products, comprising the steps: a) adjusting the consistency of the waste paper sludge to a suitable processing consistency; b) adding a polymeric coagulant or flocculent to the sludge, whilst subjecting the sludge to a non-shear or low shear mixing, to form a sludge mixture; d) adding starch, or like material, to the sludge mixture; d) pressing the sludge mixture to produce the moulded product; and e) heating the sludge mixture or moulded product to a temperature such that the starch or like material is cooked out.

2. The method of claim 1, wherein steps d) and e) are carried out using a heated press with drainage wires to permit the release of water and steam.

3. The method of claim 1, wherein step e) is carried out using an oven.

4. The method of claim 1, 2 or 3, wherein a flocculation promoter is added to the sludge mixture at step b).

5. The method of claim 4, wherein the flocculation promoter is a clay material or suitable alternative thereof.

6. The method of claims 4 or 5, wherein the clay material, or suitable alternative thereof, is bentonite or an equivalent material.

7. The method of any of claims 1-6, wherein the method includes a further step of admixing a fire retardant material into the sludge mixture.

8. The method of claim 7, wherein the fire retardant material is aluminium trihydrate or a similar material.

9. The method of claim 7, wherein the fire retardant material comprises a combination of fly ash and alum.

-12

10. The method of claims 7, 8 or 9, wherein the amount of fire retardant material added has an addition level of no more than 30% by weight relative to the total solids of the final sludge mixture.

11. The method of claim 10, wherein the amount of fire retardant material added is limited to an addition level of about 10 to 15%.

12. The method of any of the preceding claims, wherein the starch used is raw.

13. The method of any of claims 1 to 11, wherein the starch used is cationic starch.

14. The method of any of the preceding claims, wherein the method comprises the further step of shearing the sludge mixture to reduce lumps and to homogenise the sludge mixture.

15. The method of any of the preceding claims, wherein the consistency of the sludge is set at about 30% by the addition and removal of water as appropriate.

16. The method of any of the preceding claims, wherein the amount of coagulant or flocculent added to the sludge mixture is between 0.25 and 10kg per dry tonne of treated solids.

17. The method of any of the preceding claims, wherein the sludge mixture is heated to a temperature of above 50 C.

18. The method of any of the preceding claims, wherein the product of the process is subjected to a further pressing to establish dimensional stability.

19. The method of any of the preceding claims, wherein the method comprises a further step of adding plastics material, in ground or particulate form, to the sludge before the starch is added.

20. The method of claim 19, wherein the plastics material is selected from a group comprising, PVC, polyethylene, polyvinyl acetate, and urea formaldehyde resins.

21. The method of processing waste paper sludge to produce moulded products,!-

substantially as described herein with reference to the examples. I O 1w