A PROCESS FOR RECOVERING INORGANIC MATERIAL FROM DEINKING SLUDGE

Abstract: A process for recovering inorganic material from a deinking sludge obtained as reject material in teh recovery of recycled fiber from waste paper. In accordance with the invention the process comrises the following steps: (a) the proportion of organic material in the form of fiber material in teh deinking sludge is 15-40 % (dry solids content); (b) the water content of the sludge is adjusted so that the dry solids content becomes 10-35 %, step (a) and step (b) being so adapted to each other that the sludge is pumpable; (c) the pressure of the sludge is increased to at least 22 MPa; (d) the temperature is increased to supercritical or near supercritical temperature; (e) a medium containing oxygen is added to the sludge to oxidize allorganic material at supercritical temperature and pressure in a reactor; (f) energy released at the oxidation of the organic material is recovered; and (g) the flow of material from which energy has been recovered and which comprises said inorganic material, gases and water, is subjected to material separation for recovery of the inorganic material.

Description: A process for recovering inorganic material from deinking sludge The present invention relates to a process for continuously recovering inorganic material from a deinking sludge obtained as reject material in the recovery of recycled fiber from waste paper. Such deinking sludge has a dry solids content that is normally less than 8% prior to dewatering and contains organic material in an amount normally in excess of 35%, the remainder being inorganic material, calculated on the dry solids content. The term waste paper refers to paper or cardboard which, after use, is recovered in the form of recycled fiber and then, after slashing, is re-used to produce paper or cardboard or for some other industrial purpose. Recycled fiber thus refers to fiber material that has previously been used in a paper or cardboard product. Paper used for manufacturing various types of printed matter, such as newspaper, magazine, L C and fine paper, contains a plurality of additives. The finished printed paper products also contain printing ink. The additives comprise primarily fillers, pigments and colorants, but other additives may also be included. The quantities and the relative proportions of the various additives vary depending on the type of printing paper. The main purpose of the fillers is to increase the opacity of the printing paper. Clay, calcium carbonate, titanium dioxide and/or talcum are generally used as fillers, but other materials may be used. Pigment is included in the coating layer of coated paper or cardboard. Clay and calcium carbonate are mainly used as pigments, either separately or in combination, however, titanium dioxide and talcum may also be used. When recovering recycled fiber from waste paper, the waste paper is subjected to a cleaning process in order to decolour the fibers and remove impurities. The various rejects thus obtained are then combined to produce a deinking sludge with relatively high water content . No industrially practicable method for recovering valuable material from this deinking sludge has so far been used or even proposed and, instead, the deinking sludge represents a residual product which, after dewatering, can be used in the cement and brick-making industry or be incinerated using conventional technology. The ash obtained from incineration is deposited in suitable locations or used as land fill. Some factories and countries even deposit deinking sludge that has not been incinerated, which is a load from the environmental point of view. In the article entitled "Supercritical water oxidation of pulp mill sludges" by M. Modell et al, 1991 Engineering Conference, pages 393-403, an alternative method to conventional incineration is described for treating sludge from pulp mills. The method, generally termed the SCWO process (Super-Critical Water Oxidation) , is based on carbonization-free oxidation of the organic part of the sludge in the presence of water in a supercritical state. This means that the oxidation of organic material takes place at a temperature and at a pressure above the supercritical point in the phase diagram of the water, i.e. 374°C/22 MPa. Oxidation of organic material at the relatively low temperatures is possible due to the high pressure and the presence of water as reaction medium. At its supercritical point 374°C/22 MPa and in its supercritical area above this point, the water behaves as a supercritical fluid and in this state the supercritical water acts as an efficient solvent for organic material and gases. The article only discusses sludges obtained in the conventional manufacture of pulp and thus mentions nothing about the specific deinking sludge obtained from the recovery of recycled fiber from waste paper, nor does it mention anything about the use of the method for recovering inorganic material from deinking sludge. An article entitled "Nassoxidation von Deinkingschlammen" by U. Hamm and L. Gδttsching, Wochenblatt fύr Papierfabrikation 1, 1998, pages 15-23 describes results from experiments in deoxidizing deinking sludge using the SCWO process mentioned above. It is clear from the article that, even at extremely high oxidation of the organic material in the deinking sludge, an inorganic material is obtained with such low brightness, less than 60% ISO, that it cannot be used as filler. The state of the art discussed does not therefore show or intimate the recovery of filler from deinking sludge for re-use in a paper-manufacturing process making use of oxidation in accordance with said SCWO process . The object of the present invention is to provide a process for recovering inorganic material from deinking sludge, said material being of such quality as to be suitable for re-use in the paper-manufacturing process. The process in accordance with the invention is characterized in that it comprises the following steps: (a) controlling and/or adjusting the proportion of organic material in the form of fiber material in the deinking sludge so that it is or becomes 15-40%, calculated on the dry solids content, (b) adjusting the water content of the deinking sludge so that the dry solids content becomes 10-35%, step (a) and step (b) being so adjusted to each other that the deinking sludge is pumpable, (c) increasing the pressure of the pumpable deinking sludge to at least 22 MPa, (d) increasing the temperature of the pumpable deinking sludge to supercritical or almost supercritical temperature , (e) adding a medium containing oxygen to the deinking sludge in a sufficient quantity to oxidize all organic material, which takes place at supercritical temperature and pressure in a reactor, (f) recovering energy released at the oxidation of the organic material in a suitable form, and (g) subjecting the flow of material from which energy has been recovered and which comprises said inorganic material, gases and water, to material separation comprising recovery of the valuable inorganic material. The invention is described further in the following with reference to the drawings . Figure 1 shows a flowchart for a recovery plant for recovering inorganic material from deinking sludge in accordance with the invention. Deinking sludge from a cleaning process for waste paper is charged into a storage tank 1 and stirred constantly to prevent sedimentation. If the deinking sludge has not been treated in special dewatering equipment, it normally has a dry solids content below 8%, usually about 5% or lower, the remainder being water. The deinking sludge normally has an organic material content over 35%, calculated on the dry solids content, usually within the interval 45-50%, the content of inorganic material thus being below 65% and 55-50%, respectively. The organic material comprises fiber material, printing ink, other colorants, various types of organic additives and binders, the fiber material thus constituting the largest proportion. The fiber material consists of fibers and fiber parts, so-called "fines". The inorganic material comprises primarily filler, but even other inert materials may be included, such as pigment, depending on the type of paper or cardboard product constituting the waste paper. The deinking sludge is conveyed continuously to a pretreating plant 2 for suitable pretreatment so that the proportion of fiber material of the dry solids content is reduced to a predetermined level. Several different methods are available as suitable pretreatment to achieve such a reduction in the fiber material content, such as flotation, sedimentation, vortex cleaning, screening, centrifuging, biological decomposition (aerobic or anaerobic) of a part of the fiber material, converting a part of the fiber material to ethanol through enzymatic or some other type of hydrolysis followed by fermentation, or a combination of two or more of these technologies, which may be performed in one or more steps. The pretreatment of the deinking sludge in order to reduce the proportion of fiber material to a predetermined level may also be integrated in the deinking process. In this case no separate pretreatment plant is required. Reducing the content of fiber material so that the amount of organic material remaining is between 15 and 40% is important if a process is to be obtained that has acceptable operating economy. A reduction in the content of organic material increases the capacity of a SCWO process plant since deinking sludge with a higher dry solids content can be used. This is in turn a result of the COD content being lower, and of better pumpability of the deinking sludge. Furthermore, the cost of oxygen gas is lower if the ratio between organic and inorganic material decreases . The deinking sludge treated in this manner is then conveyed to a plant 3 to increase the concentration of dry solids to a value within the range 15-35%. This increase in the concentration may be achieved by performing a suitable treatment that either directly produces this concentration interval or first a considerably higher concentration after which the deinking sludge is diluted so that a concentration within the interval 15-35% is obtained. Several methods are available as suitable treatment to achieve such an increase of the dry solids content, such as sedimentation, vortex cleaning, centrifuging, pressing in a suitable press, or a combination of two or more of these technologies. Thickening of the deinking sludge is necessary in order to reduce the amount of water present during the oxidation performed down-stream. When the dry solids content is increased, or immediately thereafter, the deinking sludge may be subjected to washing, particularly if the salt content in the sludge is too high. Metal ions are removed during this wash and may form metal oxides in the subsequent oxidation which may discolour the inorganic material recovered, thereby reducing its brightness. In order to avoid salt deposits, so-called encrustations, in the plant the salt content should be kept as low as possible, preferably below 1000 ppm. The deinking sludge must still be pumpable after the increase in dry solids content in order to enable continued processing. The two main initial steps must therefore be adjusted to each other so that the deinking sludge leaving the plant 3 is in pumpable state and also has a COD content of 40-200 g/1, preferably 60-150 g/1. Dispersion methods may be used to increase the pumpability of the deinking sludge. 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The outgoing flow of material then passes a steam generator 10 (or some other system for recovering energy) , where the remaining energy can be recovered, a final cooler and then a pressure-reducing device. The material flow cooled and pressure-relieved in this way is finally conveyed to a separation plant comprising a separator 12 for separating the gas from the aqueous inorganic material. The gas phase is carried through a pipe 13 out into the atmosphere or is collected in suitable manner to make use of its constituents, e.g. carbon dioxide, while the aqueous inorganic material in the form of a suspension is withdrawn through a pipe 14. If necessary, the suspension may be thickened in suitable manner, e.g. in a sedimentation tank or screening device (not shown) . Washing may also be performed either before or after the thickening in order to remove any residual soluble compounds. The separated water is conveyed to recipients or is returned to one of the process steps in the mill . 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Pi 3 Φ ^ Ω o H{ Ω rf SD μ- cf O Ό cf 3 φ < CQ cf Φ Φ LQ P. tr ϋ ? TJ P' TJ Ω 0 cr P f SD SD φ μ- cf SD Hi φ CQ μ- μ- • 3 Hi cf 0 Φ μ- Hti SD CQ rf 0 3 - 0 μ- μ- O Hi 0 3 Hi • o P CQ φ T μ- 0 SD <! 3 TJ 0 SD μ- 3 Φ SD Hh 0 3 Φ 3 0 P. LQ φ 3 0 φ 3 SD H Hi μ- LQ o φ 3 TJ SD rt 3 μ- Hj 3 Hi μ- i HI Hi Ω cf Hi CQ i i Ω rf Hi μ- Hh μ- tr Hi Φ cr 3 0 μ- Ό < Φ μ- LQ 0 cf cf μ- Φ CQ SD Pi μ- ISl Φ 0 ϋ 3 cr fi 3 Φ Hi Φ μ- CQ CQ SD 3 <! μ- • H 1 rt Hi CQ φ rf c o Φ fi cf ϋ SD X μ- CQ 3 cf 3 P. HI 3" O μ- cf P " μ- SD ϋ Φ rt rt 0 Φ p- Φ CQ Ό 3 CQ Hi μ- tr SD SD P- Pi 0 Ω Ω φ SD 3 P. SD Φ Hi $ Φ Cf CQ SD φ Ω Φ <! Hi Φ 3 LQ 3 rt o H Hi ^ LΠ cr Φ cr Cf μ- Hi μ- SD Ω tr 1 SD μ- ft SD φ 0 3 LΠ μ- r μ- 3 Pi rf Hi cf μ- 3 Cf Ω SD £ 3 Hj Hi 3 0 ^ rf Hi rt Ω 0 0 o\° 0 3- 3 Φ Φ 0 Φ 3^ 3 LQ μ- o < CQ SD cf μ- LQ 3 ^ H{ £ Hj ^ Φ 0 rf TJ 3 Φ ?r 3 μ- SD Φ CQ Hj Φ SD cf Hj SD tr SD Φ 3 3 SD rt 0 μ- cf 3 Hj Cf SD 3 Ό Hi Si tr Φ Ό CQ CQ <! Hi φ CQ SD μ- rt rf rr μ- Hi rf ϋ 3 .Y Φ Φ f Hi SD SD SD Hi μ- Cf 3 μ- Hj rf 3 • Φ μ- 3 μ- Hi Hi LQ CQ μ- P. ϋ Ω CQ 0 Ω 3 h- 1 Φ μ- LQ SD 3 SD TJ μ- 3 3 0 μ- LQ φ ^ 0 3 P 0 O Cf Ω CQ SD 3 O I- 1 LQ Ω Hi 3 Φ Φ HI rt 3 N μ- SD CQ 3 LQ Ω 0 3 Ό Φ 0 φ 0 SD H-" φ SD Cf 0 Ω 3 X tr Φ 3 cr CQ Φ Φ 3 < Φ Hi Hi CQ 3 Hj Φ μ- Hi μ. rf TJ μ- 0 TJ P. SD 3 0 c CQ Φ fi Cf ■ ; 3 LQ Hj cf Cf Hi SD 0 μ- φ Φ 0- Hi μ. Φ P. 0 cf SD 3 Hi μ- μ- tr Hi Φ fi 3 rt SD rt Hi o P. 1— ' 3 μ- LQ Hi P tr rt 0 Φ HJ rt 0 μ- Ω $, rt T) LQ SD CQ tr μ- <! SD Ω P * cr P. Φ P. Φ • μ- H CQ Φ 3 ^ ≤ 0 tr tr SD Φ cf tr Φ Φ 3 SD tr Φ Hj • CQ Cf LQ μ- 0 3 • CQ 3 <! μ- φ fi tr ϋ Φ \-~ 0 CQ μ- r tr Φ 3 φ P cf Φ φ Ω 3 rt Φ φ p. 3 C c CQ 3 HI 0 HI φ • 0 SD Si tr rf Hj Hi tr μ- •- rr Hj rf Φ rt LQ tr P' rt Cf Φ tr φ Ω O ^ Φ Φ μ- μ- Si rr 3 3 cf 3 φ 0 μ- 3 3 SD 3 O LO to to μ> μ> π o LΠ o LΠ o LΠ LO H cf μ- . tr Hj 3 LΠ μ- Φ o\° ra SD cf 1 Cf tr P " cf Φ Φ 3 Hi ϋ μ- φ Hi cf SD μ- c ω cf 3 H- 1 3 H-" H Φ SD 1 to 3 CQ O Cf 3 Ω SD SD LQ LQ H- 1 SD 3 Φ <! Φ Φ cf Φ μ- X μ- Ω TJ 3 φ Ω CQ Hi Hi Φ μ- φ TJ 3 SD SD φ CQ Hi 3 Φ SD cf ϋ Cf o μ- tr Hj 3 fi μ- o cf LQ Hi tr Φ cf tr 3 μ- TJ Φ LQ SD CQ tr TJ CQ 1 Φ LQ Hi 0 Hj Hi SD 3 o. SD μ- Ω TJ Φ " 3 μ- cf cf 3 0 Φ cf τ Φ Φ fi Φ Experiments were also performed with the above-mentioned filler, with dithionite bleaching. This gave increased brightness of 1-2%-units ISO.

Claims: C L A I M S 1. A process for continuously recovering inorganic material from a deinking sludge obtained as reject material in the recovery of recycled fiber from waste paper, characterized in that it comprises the following steps : (a) controlling and/or adjusting the proportion of organic material in the form of fiber material in the deinking sludge so that it is or becomes 15-40%, calculated on the dry solids content, (b) adjusting the water content of the deinking sludge so that the dry solids content becomes 10-35%, step (a) and step (b) being so adjusted to each other that the deinking sludge is pumpable, (c) increasing the pressure of the pumpable deinking sludge to at least 22 MPa, (d) increasing the temperature of the pumpable deinking sludge to supercritical or almost supercritical temperature, (e) adding a medium containing oxygen to the deinking sludge in a sufficient quantity to oxidize all organic material, which takes place at supercritical temperature and pressure in a reactor, (f) recovering energy released at the oxidation of the organic material in a suitable form, and (g) subjecting the flow of material from which energy has been recovered and which comprises said inorganic material, gases and water, to material separation comprising recovery of the valuable inorganic material . 2. A process as claimed in claim 1, characterized in that the quantity of organic material in said step (a) is 15-30%. 3. A process as claimed in claim 1 or claim 2, characterized in that said step (a) and step (b) are adapted to each other so that, after said step (b) the deinking sludge has a COD content within the interval 40-200 g/1, preferably 60-150 g/1. 4. A process as claimed in any one of claims 1-3, characterized in that in said step (d) the temperature is increased to at most 425°C. 5. A process as claimed in any one of claims 1-4, characterized in that the temperature in the reactor after said step (e) is at most 650°C. 6. A process as claimed in claim 5, characterized in that the temperature in the reactor is between 500 °C and 600°C. 7. A process as claimed in any one of claims 1-6, characterized in that gaseous reaction products are separated from the flow of material in said step (g) and are released into the atmosphere or collected. 8. A process as claimed in claim 7, characterized in that carbon dioxide is recovered from the gases collected. 9. A process as claimed in any one of claims 1-8, characterized in that the deinking sludge is subjected to washing prior to said step (c) . 10. A process as claimed in claim 9, characterized in that the deinking sludge has a salt content of at most 1000 ppm after said washing. 11. A process as claimed in any one of claims 1-10, characterized in that the oxidation time in the reactor is about 10 minutes or less, preferably about 5 minutes or less and most preferably about 1 minute . 12. A process as claimed in any one of claims 1-11, characterized in that the fiber material is removed from the deinking sludge in order to achieve a content within the interval in step (a) by means of flotation, sedimentation, vortex cleaning, screening, centrifuging or a combination of two or more of these techniques. 13. A process as claimed in any one of claims 1-11, characterized in that the fiber material is removed from the deinking sludge in order to achieve a content within the interval in step (a) by means of flotation, sedimentation, vortex cleaning, screening, centrifuging, or a combination of two or more of these techniques being integrated into the deinking process . 14. A process as claimed in claim 12 or claim 13, characterized in that recycled fiber is recoverec].^j m ι the fiber material removed. 15. A process as claimed in any one of claims 1-11, characterized in that the fiber material is removed from the deinking sludge in order to achieve a content within the interval in step (a) by means of biological decomposition (aerobic or anaerobic) of a part of the fiber material, converting a part of the fiber material to ethanol through enzymatic or some other type of hydrolysis followed by fermentation, or a combination of two or more of these techniques . 16. A process as claimed in any one of claims 1-15, characterized in that the water content is adjusted in step (b) by means of sedimentation, vortex cleaning, centrifuging, pressing in a suitable press, or by a combination of two or more of these techniques. 17. A process as claimed in any one of claims 1-16, characterized in that the inorganic material obtained in step (g) has a brightness of at least 68% ISO. 18. A process as claimed in any one of claims 1-17, characterized in that the inorganic material obtained in step (g) is subjected to after-treatment in order to increase the brightness by 2 or more % ISO. 19. A process as claimed in claim 18, characterized in that the after-treatment comprises cleaning to remove brightness-reducing metal oxides and/or bleaching with a bleaching agent. 20. A process as claimed in any one of claims 1-19, characterized in that the valuable inorganic material recovered also contains pigment having a smaller particle size than the particle size of the filler, the inorganic material being divided into a filler fraction and a pigment fraction. 21. A process as claimed in any one of claims 1-20, characterized in that the energy in step (f) is recovered in the form of vapour and/or hot water. 22. A process as claimed in any one of claims 1-21, characterized in that the incoming deinking sludge has a dry solids content that is normally less than 8% prior to dewatering and contains organic material in a quantity that is normally over 35%, the remainder being inorganic material, calculated on the dry solids content.

Inventors: AELMQVIST ARNOLD, DAHLBLOM BORIS, DAHLIN JOERGEN, LARSSON MATS ERIK, MATTSSON ERIK, SUNDKVIST ARNE, AELMQVIST, ARNOLD, DAHLBLOM, BORIS, DAHLIN, JOERGEN, LARSSON, MATS, ERIK, MATTSSON, ERIK, SUNDKVIST, ARNE
Applicants: AELMQVIST ARNOLD, DAHLBLOM BORIS, DAHLIN JOERGEN, LARSSON MATS ERIK, MATTSSON ERIK, STORA KOPPARBERGS BERGSLAGS AB, SUNDKVIST ARNE, AELMQVIST, ARNOLD, DAHLBLOM, BORIS, DAHLIN, JOERGEN, LARSSON, MATS, ERIK, MATTSSON, ERIK, SUNDKVIST, ARNE
Application year: 2001
Main classification: D21C5
IPC codes: B01J3, C02F11, C12S3, D21C5, D21F1, D21H17