SE528693C2 - Process and system for producing precipitated calcium carbonate - Google Patents

Abstract

A method for manufacturing precipitated calcium carbonate (PCC) comprises the steps of increasing the pressure and temperature of a stream containing oxidizable material and water; causing the stream to flow in a first reaction chamber system; adding oxidant to the first reaction chamber system, oxidizing the oxidizable material of the stream through supercritical water oxidation (SCWO) to form carbon dioxide and water; causing the stream to leave the first reaction chamber system; causing the stream to flow in a second reaction chamber system; adding lime to the second reaction chamber system so that the carbon dioxide of the stream and the lime react to form PCC; and thereafter reducing the pressure of the stream. Preferably, the stream is a deinking sludge, wherein the method improves the brightness of paper filler recovered from the deinking sludge by means of precipitating PCC onto the paper filler.

Description

528 693 ash particles. This results in a "recycled" PCC pigment containing a "core" of ash. The properties of the recycled PCC are similar to unused PCC, ie. it is non-abrasive and light in color.

Various common methods for manufacturing PCC are described in e.g. Manufacture of_precipitated calcium carbonate, J. Laine, Paperi ja Puu - Paper and wood, Nr. 11, p. 725, 1980, and in references therein.

SUMMARY OF THE INVENTION A limitation of the described methods of making PCCs is that they are all time consuming due to the slowness of the chemical reactions involved.

A disadvantage of incinerating waste paper sludge is that the above-identified, patented method, for enclosing the incinerator ash in PCC, is required to be able to reuse the ash at all in the papermaking process.

Furthermore, large amounts of PCC must be created to improve the brightness of the combustion ash. U.S. pat. no. 5,759,258 mentions that the proportion of incineration ash can be up to about 50 percent of the weight of the product. In a typical product, the proportion of ash will be between 5 and 30 percent, and preferably between 10 and 25 percent. Thus, large amounts of PCC must be created to reuse the ash that remains after incineration of waste paper sludge.

In addition, the incinerator ash may not be suitable for reuse a large number of times as it is abrasive and gray. The PCC that precipitates on the ash can be worn off over time.

According to a first aspect of the present invention, precipitated calcium carbonate is produced in a high pressure section of a supercritical water oxidation system (528 693 SCWO). The pressure and temperature of a stream containing oxidizable material and water are increased to reach a supercritical state of water (ie a pressure of [Yes at least 22 Mæa and a temperature of at least 374 ° C) or the vicinity thereof. The stream is allowed to flow in a first reaction chamber system in the SCWO system, whereby an oxidant, preferably oxygen, is added to the first reaction chamber system so that the oxidizable material in the stream is oxidized by SCWO to form carbon dioxide and water. The oxidized stream is then made to leave the first reaction chamber system.

Then, the oxidized stream is caused to flow into a second reaction chamber system, with lime being added to the second reaction chamber system so that the carbon dioxide in the oxidized stream and the lime react to form precipitated calcium carbonate. The pressure inside the second reaction chamber system is maintained above atmospheric pressure, e.g. maintained above the pressure required to obtain supercritical water, while the temperature may be lower. Finally, the pressure in the stream can be reduced and the various components of the stream separated.

SCWO is an efficient method for the destruction of organic matter into carbon dioxide and water. The process can be used for the treatment of various wastewater and sludge. The products, carbon dioxide and water, can both be used in the subsequent carbonation process.

The lime is preferably added as a slurry of calcium hydroxide, also known as lime milk or slaked lime.

The slurry of calcium hydroxide reacts with the carbon dioxide in the stream to form PCC. 528 693 Optionally, if calcium hydroxide is added in stoichiometric excess, additional carbon dioxide may be added to the calcium hydroxide stream to create larger quantities of PCC.

Alternatively, lime can be added as dry, quicklime, CaO, which reacts with the water to form a slurry of calcium hydroxide, and the carbonation reaction is then carried out as described above.

The high pressure has proven to be very advantageous in terms of reaction rates. Full-scale experiments have shown that injected calcium hydroxide can be converted to PCC quickly and efficiently.

The presence of water and carbon dioxide at high pressure provides excellent opportunities to incorporate a PCC manufacturing system into the SCWO system relatively easily. In principle, only a lime supply source is needed and a reaction chamber system is added to the SCWO system to provide a combined system for SCWO and PCC manufacturing. The high pressure in the reaction chamber system significantly shortens the reaction time for the carbonation, compared to conventional low pressure processes.

According to a second aspect of the present invention, the process for producing precipitated calcium carbonate, as above, is applied to a sludge stream containing paper fillers. The organic part of the sludge is oxidized by SCWO, while the paper filler transported in the stream through the first reaction chamber system remains essentially unconverted in chemical composition. Then, the paper filler is transported in the stream through the second reaction chamber system while the precipitated calcium carbonate formed is at least partially precipitated on the substantially unconverted paper filler. Finally, the paper filler is recycled. 528 695 When sludge, such as e.g. Recycled paper sludge treated with SCWO can be recycled very light filler, in some cases almost as light as new filler. However, the brightness of the recycled filler is more or less determined by the raw material, i.e. the quality of the filler in the recycled paper and the amount of inorganic impurities in the raw material or obtained in the decolorization process.

The process as above increases the brightness of fillers recovered from sludge by SCWO. The brightness is improved by manufacturing PCC in the SCWO process, after the SCWO reaction, but before the pressure is reduced to ambient pressure. The process of the present invention uses the high carbon dioxide content and the high pressure in the wastewater from the SCWO process, before the pressure is lowered, to quickly and efficiently convert calcium hydroxide to PCC.

If a high brightness calcium hydroxide is used, the PCC formed is lighter than the recovered filler. This means that the PCC will increase the overall brightness of the mixture of the formed PCC and the recovered filler compared to the brightness of the recovered filler alone. The increased brightness is believed to be a result of a combination of mixing effect and precipitation of the manufactured calcium carbonate on the less bright, recycled filler.

According to further aspects of the present invention, there is provided systems for manufacturing PCC according to the methods identified above.

These and other objects are achieved by methods and systems according to the appended claims.

Further features and advantages of the invention will become apparent from the detailed description taken in conjunction with the appended claims and the appended figure. 528 693 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of a system for making precipitated calcium carbonate according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Fig. 1, a system for producing precipitated calcium carbonate in a SCWO system will be described.

A stream of wastewater containing oxidizable material is collected in a feed tank 11 equipped with a mixer.

The feed tank 11 is connected to an auxiliary pump 12 and a high pressure pump 13 for feeding. The auxiliary pump 13 supplies the high-pressure pump 13 for supply with a constant intake pressure. The high pressure pump 13 for supply raises the supply pressure to a pressure required for water to be supercritical.

In the present embodiment, the high pressure pump 13 for supply raises the pressure in the stream with wastewater to about 250 bar and pumps the wastewater through the system.

The wastewater stream then enters a preheater 14, where it is preheated by the outlet stream from the reactor. Then the stream of wastewater enters a heater 15, which raises the temperature of the stream of wastewater to a temperature suitable for starting the oxidation. In the present embodiment, the heater 15 raises the temperature in the stream to about 350-400 ° C, after which the stream of wastewater is allowed to flow from an inlet 16a towards an outlet 16b in a SCWO reactor 16. Typically, the heater 15 is used only when starting the oxidation. Once the exothermic reaction has started, sufficient heat is generated, and the heater 15 can be bypassed. 528 693 Oxygen from a supply tank 17 is fed to the SCWO reactor 16 via an oxygen pump 18 and an oxygen evaporator 19, the oxidizable material in the wastewater stream being oxidized by supercritical water oxidation to form carbon dioxide and water. The oxidation reaction generates heat, and as a result the reactor temperature increases, e.g. to about 600 ° C. If the content of organic material in the wastewater stream is too high for complete oxidation without exceeding the maximum design temperature of the SCWO reactor, a multi-stage SCWO reactor, optionally with cooling water supply, can be used.

The oxidized wastewater stream is made to leave the SCWO reactor 16 through its outlet 16b, and fed to the preheater 14 to preheat the incoming wastewater stream.

The heat from the reaction can be easily recovered in a steam generator comprising a steam boiler 20 and a boiling water pump 21 arranged to pump boiling water to the steam boiler 20, in which a specified steam quality can be generated. The temperature of the wastewater downstream of the steam boiler 20 can be 150-230 ° C.

The effluent is then cooled to its final effluent temperature in a waste water cooler 22. The final effluent temperature may be 10-80 ° C.

The cooled effluent is passed through an optional pressure reducing device 23, e.g. pressure reducing coils, to reduce the pressure. Pressure control water can be added before the pressure reducing device 23 to control the pressure upstream.

The wastewater is then allowed to flow from an inlet 24a towards an outlet 24b in a PCC reactor 24. A slurry of calcium hydroxide, also known as lime milk, is fed from a supply tank 25 to the PCC reactor 24 via a calcium hydroxide pump 25. wherein the carbon dioxide in the oxidized stream with wastewater and the lime reacts to form PCC. The temperature of the stream inside the PCC reactor 24 may be any suitable temperature, but is preferably 10-80 ° C. The pressure in the stream inside the PCC reactor 24 can be any suitable pressure above atmospheric pressure. The pressure can be a pressure that is overcritical for water, e.g. 230 bar, and in such a case the pressure reducing device 23 can be dispensed with.

The reacted, discharged stream from the PCC reactor 24 is passed through a further pressure reducing device 27, e.g. pressure reducing coils, to reduce the pressure.

Pressure control water can be added before the pressure reducing device 27 to control the pressure upstream.

The reacted stream then enters a gas-liquid separator 28, which separates any gases in the reacted stream, e.g. traces of N, and unreacted 0, and CO 2, from the liquid / solid phase comprising the water and the PCC.

It will be appreciated that in alternative variants of the embodiment as above, the PCC rectifier 24 is arranged between the steam boiler 20 and the outlet water cooler 22, between the preheater 14 and the steam boiler or between the SCWO reactor outlet 16b and the preheater 14.

The temperature inside the PCC reactor 24 will then obviously be higher.

Similarly, the lime can be added to the supply tank as dry, quicklime, which is reacted with the water therein to form calcium hydroxide, after which the calcium hydroxide is pumped to the PCC reactor 24. 528 693 Furthermore, if large amounts of PCC are to be produced, additional carbon dioxide can be added. to, e.g. bubbled through, PCC reactor 24! Preferably, calcium hydroxide is added in amounts up to stoichiometric amounts to form PCC according to the reaction C8. (OH) 2 + CO, -) CaCO 3 (S) + H 2 O.

Furthermore, the PCC reactor 24 may be a continuous process mixing reactor comprising two mixing reaction chambers in series described in the above-identified article Manufacture of precipitated calcium carbonate, J. Laine, Paperi and Puu - Paper and Wood, no. ll, pp. 725, 1980.

The contents of this article, as well as the contents of the above-identified U.S. pat. no. 5,759,258 (MINERALS TECHNOLOGIES INC.) Are incorporated herein.

The reaction parameters of the PCC reaction process in the PCC reactor 24 are controlled to create PCC with a desired crystal size and morphology.

It should be noted that the reaction to create PCC may continue after the current has left the PCC reactor 24.

The PCC reaction can, for example, be terminated inside the gas-liquid separator 28.

SCWO can be used to treat recycled paper sludge to recycle paper fillers. The recycled paper filler has proven to be of good quality, ie. high brightness, see e.g.

U.S. Patent Application Publication no. US 2004/0020616 A1 (DAHLBLOM et al.), And the articles Oxidation of deinking sludge in sqpercritical water, A. Gidner et al., Presented at Workshop, Managing pulp and paper process residues, 30-31 May 2002, Barcelona, Spain, and Oxidation of deinking sludge in supercritical Water, In practice, J. 528 695 Dahlin, Workshop, Managing pulp and paper PIOCGSS Iesíduesf -31 May 2002, Barcelona, Spain, the contents of which are hereby incorporated by reference.

The latter references state that while the recycled paper filler is of sufficient quality for reuse in newsprint, the filler is not bright enough for general use in higher quality paper.

Thus, in a particular embodiment of the present invention, the stream containing oxidizable material and water is a sludge, e.g. a waste paper sludge containing paper filler, the organic moiety being oxidized by SCWO, while the paper filler is transported in the stream through the SCWO reactor 16 and left substantially unconverted in chemical composition. The term "substantially unconverted" paper filler in the present text means that the paper filler has not substantially chemically reacted or been converted to another chemical substance.

The paper filler is transported in the stream through the PCC reactor 24 while the PCC formed therein can be partially precipitated on the paper filler. The paper filler is separated together with the formed PCC and the water in the gas / liquid separator 28.

Finally, the paper filler and the formed PCC can be separated from the water and optionally other residues.

Alternatively, the water containing the recycled paper filler and the formed PCC is pumped to a papermaking machine located in the vicinity of the SCWO system of the invention. The overpressure in the SCWO system caused by the high pressure pump 13 can be used to transport the recycled paper filler and the formed PCC to the papermaking machine. 523 e9s «11 Optionally, before the paper filler and the formed PCC are separated from the water or before the water containing the paper filler and the PCC is pumped to a papermaking machine, the water containing the paper filler and the formed PCC may be pumped through a cleaning device which separates dark particles, formed in the SWCO system or present in the original sludge, e.g. using different sedimentation rates for the dark particles compared to those for the recycled paper filler and the PCC formed.

The purified water containing the recycled paper filler and the formed PCC can be pumped to an additional reactor to which carbon dioxide, e.g. separated in the gas / liquid separator 28, and lime can be added to effect a second carbonation reaction to produce additional PCC and obtain an even lighter end product.

The paper filler, which is an inorganic material, contains talc, clay and / or calcium carbonate. The formed PCC is preferably caused to at least partially precipitate on the paper filler. The amount of calcium hydroxide generated is selected depending on the sludge used and the desired properties of the recycled mixture of paper filler and PCC formed.

The formed PCC will increase the overall brightness of the mixture of the formed PCC and the recycled paper filler compared to the brightness of only recycled filler. The increased brightness is believed to be the result of a combination of mixing effect and precipitation of the created calcium carbonate on the less light recycled filler.

Tests have been performed to verify the results of the present invention. 528 693 12 Paper fillers recovered from recycled paper sludge by SCWO had an ISO brightness of 76.6%, without any PCC being created in the SCWO system. While PCC was manufactured at supercritical pressure and at a temperature of about 20 ° C, the brightness of the final product increased substantially linearly from an ISO brightness of 76.6% to an ISO brightness of about 80% when PCC was created in amounts up to an amount where it accounted for 34% of the total amount of the final product (ie recycled filler and created PCC). When the temperature was raised to 65 ° C, the increase in brightness still existed, but was not as strong as at the lower temperature.

In another experiment, PCC was manufactured in a SCWO system, which was fed with a stream containing water and diesel. PCC was manufactured at supercritical pressure and at temperatures ranging from about 20 ° C to about 380 ° C. At lower temperatures, PCCs were manufactured that had an ISO brightness of over 90%. At higher temperatures (> 300 ° C) the brightness decreased as the temperature increased. Although the invention has been described with particular reference to specific embodiments thereof, the embodiments of the invention shown and described in detail are to be regarded as preferred embodiments thereof. It is therefore to be understood that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (19)

10 15 20 25 528 693 13 PATENT REQUIREMENTS Process for the production of precipitated calcium carbonate in a high pressure system, characterized in that the steps are: - the pressure and temperature of a stream containing oxidizable material and water are raised, - said stream is made to flow from an inlet (16a) towards an outlet (16b) in a first reaction chamber system (16) of said high pressure system, - oxidizing agent is added to said first reaction chamber system and the oxidizable material in said stream is oxidized by supercritical water oxidation so that carbon dioxide and water are formed in said first reaction chamber system, - said oxidized stream is allowed to leave said first reaction chamber system through said outlet, - said oxidized stream is caused to flow from an inlet (24a) towards an outlet (24b) in a second reaction chamber system (24) of said high pressure system, and - lime is added to said second reaction chamber system, and the carbon dioxide in said oxidized current is caused to react with said lime so that precipitated calcium carbonate forms as in said second reaction chamber system, while the pressure in said second reaction chamber system is maintained above atmospheric pressure. The method of claim 1, wherein the pressure in said oxidized stream that is caused to flow from the inlet to the outlet of the second reaction chamber system is a pressure that is above the pressure required to obtain supercritical conditions for water. 10 15 20 25 528 693 14 The method of claim 1, wherein the pressure in said oxidized stream that is caused to flow from the inlet to the outlet of the second reaction chamber system is a pressure that is below the pressure required to obtain supercritical conditions for water. A method according to any one of claims 1-3 comprising the step of reducing the pressure in said stream, after the formation of said precipitated calcium carbonate. A method according to claim 4, wherein the pressure in said stream, after the formation of said precipitated calcium carbonate, is reduced to atmospheric pressure, or to a pressure slightly above atmospheric pressure. A method according to any one of claims 1-5, wherein the temperature of said oxidized stream which is caused to flow from the inlet to the outlet of the second reaction chamber system is between 5 ° C and 600 ° C, and more preferably between 10 ° C and 80 ° C. A method according to any one of claims 1-6, wherein said lime is added to the second reaction chamber system as a slurry of calcium hydroxide. A process according to any one of claims 1-7, wherein additional carbon dioxide is added to the second reaction chamber system. A process according to any one of claims 1-8, wherein precipitated calcium carbonate is also formed downstream of said second reaction chamber system. A process according to any one of claims 1-9, wherein calcium hydroxide is added to the second reaction chamber system in amounts up to stoichiometric amounts in relation to the amounts of carbon dioxide, to form precipitated calcium carbonate by the reaction Ca (OH) 2 + CO, - + CaCO 3 (s) + H 2 O. 11. ll. A method according to any one of claims 1-10, wherein said oxidized stream is caused to flow through at least two reactor chambers in said second reaction chamber system, and said lime is added to at least two reactor chambers in said second reaction chamber system so that the carbon dioxide in said oxidized stream and said lime reacts said precipitated calcium carbonate is formed in said at least two reactor chambers in said second reaction chamber system. A process according to any one of claims 1 to 11, wherein the reaction parameters of said second reaction chamber system are controlled to form said precipitated calcium carbonate having a desired crystal size and morphology. A method according to any one of claims 1-12, wherein said stream containing oxidizable material and water is a sludge containing a paper filler. The method of claim 13, wherein - said paper filler is transported in said stream through said first reaction chamber system while it remains substantially unconverted, - said paper filler is transported in said stream through said second reaction chamber system while said precipitated calcium carbonate at least on said substantially unconverted paper fillers, and - said paper filler is recovered. 10 15 20 25 528 693 16 A method according to claim 13 or 14, wherein said precipitated calcium carbonate is at least partially caused to precipitate on said substantially unconverted paper filler. A method according to claim 14 or 15, wherein said formed precipitated calcium carbonate and said substantially unconverted paper filler are transported to a papermaking machine by maintaining said pressure above atmospheric pressure. A method according to any one of claims 14-16, wherein the weight of said precipitated calcium carbonate formed is lower than the weight of said substantially unconverted paper filler, more preferably less than half the weight of said substantially unconverted paper filler, and most preferably less than one third of the weight of said substantially unconverted paper fillers. A process according to any one of claims 13-17, wherein calcium hydroxide is added to the second reaction chamber system in amounts less than stoichiometric amounts relative to the amount of carbon dioxide, to form precipitated calcium carbonate by the reaction Ca (OH) 2 + CO 2 -> CaCOB (s) + H20 19. High pressure system for the production of precipitated calcium carbonate, characterized by - a pump (12, 13) and a heating device (14, 15) designed to raise the pressure and temperature of a stream containing oxidizable material and water, - a first reaction chamber system (16 ) comprising an inlet (16a) and an outlet (16b), the inlet being arranged to receive said stream containing oxidizable material and water, an oxidizing agent supply system (17-19) adapted to add oxidizing agent. to said first reaction chamber system for oxidizing the oxidizable material in said stream by supercritical water oxidation while the stream flows into said first reaction chamber system so that carbon dioxide and water are formed and the outlet is arranged to discharge said oxidized stream, - a second reaction chamber system (24) comprising an inlet (24a) and an outlet (24b), wherein the inlet (24a) of said second reaction chamber system (24) is arranged receiving - said oxidized stream, and - a lime supply system (25-26) arranged to add lime to said second reaction chamber system to cause the carbon dioxide in said oxidized stream to react with said lime so that precipitated calcium carbonate is formed while the stream flows in said second reaction chamber. at a pressure above atmospheric pressure, the outlet of said second reaction chamber system being arranged to discharge said precipitated calcium carbonate.