WO2006004503A1

WO2006004503A1 - Method and system for manufacturing precipitated calcium carbonate

Info

Publication number: WO2006004503A1
Application number: PCT/SE2005/001017
Authority: WO
Inventors: Anders Gidner; Erik Mattsson; Fredrik LUNDSTRÖM; Gert Wass; Jörgen DAHLIN; Kim Carlsson; Lars Stenmark; Mats Larsson
Original assignee: Stora Enso AB; Chematur Engineering AB
Current assignee: Stora Enso AB; Chematur Engineering AB
Priority date: 2004-07-02
Filing date: 2005-06-28
Publication date: 2006-01-12
Anticipated expiration: 2007-01-02
Also published as: SE528693C2; SE0401719L; SE0401719D0

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

METHOD AND SYSTEM FOR MANUFACTURING PRECIPITATED CALCIUM CARBONATE

FIELD OF INVENTION

This invention relates to a method and a system for manufacturing precipitated calcium carbonate (PCC). The method and the system is preferably, but not exclusively, intended for improving the brightness of a paper filler recovered from e.g. waste produced from the deinking of recycled paper.

DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION

Deinking of recycled paper is performed as part of the paper recycling process. The waste or deinking sludge produced from the deinking includes organic material, e.g. fibre and ink, and inorganic material, e.g. paper filler. Today this waste is typically incinerated on site; land filled; or sent off site for external treatment.

The ash remaining after incineration of deinking sludge is not suitable for reuse in the paper manufacturing process since the inorganic material has been transformed by the heat to a more abrasive material and since the ash is gray in color.

However, a method has been developed that encloses the incineration ash in PCC, see U.S. Pat. No. 5,759,258 (MINERALS TECHNOLOGIES INC.). Ash containing mixed mineral phases such as gehlenite, anorthite, and perovskite from the combustion of waste produced from the deinking of recycled paper is added to a reactor in which carbon dioxide gas is bubbled through an aqueous slurry of calcium hydroxide so that PCC is produced. Because the mineral phases in the ash contain calcium as part of their crystal structure, the PCC will precipitate and grow on the ash particles. This results in a "recycled" PCC pigment containing an ash "core." The properties of the recycled PCC are similar to virgin PCC, i.e. non-abrasive and bright in color.

Different general methods of manufacturing PCC, are disclosed in e.g. Manufacture of precipitated calcium carbonate, J. Laine, Paperi ja Puu - Papper och tra, No. 11, pp. 725, 1980, and in references therein.

SUMMARY OF THE INVENTION

A limitation of the disclosed methods for manufacturing PCC is that they are all time-consuming due to the slowness of the chemical reactions involved.

Other problems with the known methods for manufacturing PCC are that they may be inefficient and uneconomic; they need systems for increasing the pressure if the rate of the chemical reactions involved shall be increased; they need supply of large amounts of energy, and they need supply systems for, and supply of, carbon dioxide and water of high quality. Further, size, color, shape, and morphology of the PCC may not be optimum using the known methods for manufacturing PCC.

A drawback of incineration of deinking sludge is that the above-identified patented method for enclosing the incineration ash in PPC is needed in order to at all reuse the ash in the paper manufacturing process.

Further, large amounts of PCC have to be formed to improve the brightness of the incineration ash. The U.S. Pat. No. 5,759,258 mentions that the incineration ash portion can comprise up to about 50 percent of the weight of the product. In a typical product, the ash portion will be in the range of 5 to 30 percent, and preferably between 10 and 25 percent. Thus, large amounts of PCC have to be produced in order to recycle the ash remaining after incineration of deinking sludge.

Still further, since the incineration ash is abrasive and grey, it may not be suitable for reuse a large number of times. The PCC precipitated on the ash may be worn off by time.

In accordance with a first aspect of the present invention, precipitated calcium carbonate is manufactured in a high- pressure part of a system for supercritical water oxidation (SCWO) . The pressure and temperature of a stream containing oxidizable material and water are increased to reach a condition supercritical to water (i.e. a pressure of at least 22 MPa and a temperature of at least 374 °C) or close thereto. The stream is caused to flow in a first reaction chamber system of the SCWO system, while oxidant, preferably oxygen, is added to the first reaction chamber system so that the oxidizable material of the stream is oxidized through SCWO to form carbon dioxide, and water. The oxidized stream is then caused to leave the first reaction chamber system.

Next, the oxidized stream is caused to flow in a second reaction chamber system, while lime is added to the second reaction chamber system so that the carbon dioxide of the oxidized stream and the lime react to form precipitated calcium carbonate. The pressure within the second reaction chamber system is kept above atmospheric pressure, e.g. kept above the pressure needed to obtain supercritical water, whereas the temperature may be lower. Finally, the pressure of the stream may be reduced, and the various constituents of the stream can be separated. SCWO is an efficient method for destruction of organic material to carbon dioxide and water. The process can be used for treatment of various wastewaters 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 milk-of-lime, slaked lime or simply slake. The calcium hydroxide slurry is reacted with the carbon dioxide in the stream to form PCC.

Optionally, if calcium hydroxide is added in stoichiometric surplus, additional carbon dioxide can be supplied to the stream with the calcium hydroxide to produce larger quantities of PCC.

Alternatively, the lime may be added as dry, unslaked lime, CaO, which react 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 tests have shown that injected calcium hydroxide can be converted to PCC rapidly and efficiently.

The presence of water and carbon dioxide at high pressure gives excellent opportunities to relatively easily incorporate a system for the manufacturing of PPC into the system for SCWO. In principle, only a lime supply source and a reaction chamber system have to be added to the SCWO system in order to obtain a combined system for SCWO and manufacturing of PCC.

The cleanliness and gentle nature of the SCWO process provides for the possibilities of incorporating PCC production in the SCWO system. Also, by producing PCC, the carbon dioxide is bound in solid phase and thus, it does not contribute to the green-house effect.

The high pressure in the reaction chamber system considerably shortens the carbonation reaction time, compared to common low-pressure processes. If a high- pressure reaction process according to the prior art has to be obtained, separate expensive and highly energy consuming high pressure system has to be provided. The combined system of the invention gives obvious synergy effects.

The stream containing oxidizable material and water may or may not contain paper filler.

In accordance with a second aspect of the present invention, the above method for manufacturing precipitated calcium carbonate is applied on a stream of sludge containing paper filler. The organic part of the sludge is oxidized through SCWO, while the paper filler is transported in the stream through the first reaction chamber system while being left essentially unconverted with respect to chemical composition. Then, the paper filler is transported in the stream through the second reaction chamber system while the formed precipitated calcium carbonate is at least partly precipitated onto the essentially unconverted paper filler. Finally, the paper filler is recovered.

When sludge, such as e.g. deinking sludge, is treated by SCWO, very bright filler can be recovered, in some instances almost as bright as virgin filler. However, the brightness of the recovered filler is more or less determined by the raw material, i.e. quality of the filler in the recycled paper and amount of inorganic impurities in the raw material or obtained from the deinking process. The above method increases the brightness of filler 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 released to ambient pressure. The method of the present invention uses the high carbon dioxide content of high quality and the high pressure in the effluent from the SCWO process, before pressure let down, to rapidly and efficiently convert calcium hydroxide to PCC.

If a high brightness calcium hydroxide is used the formed PCC is brighter than the recovered filler. This means that the PCC will increase the overall brightness of the mix of the formed PCC and the recovered filler compared to the brightness of solely recovered filler. The increased brightness is believed to be the result of a combination of mixing effect and precipitation of the manufactured calcium carbonate onto the less bright recovered filler.

In accordance with further aspects of the present invention, there are provided systems for producing PCC in accordance with the above-identified methods.

Other features and advantages of the invention will become more readily understood from the following detailed description taken in connection with the appended claims and attached drawing.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 is a schematic representation of a system for manufacturing precipitated calcium carbonate according to an embodiment of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS

With reference to Fig. 1, a system for manufacturing 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 stirrer. The feed tank 11 is connected to a booster pump 12 and a high pressure feed pump 13. The booster pump 12 supplies the high pressure feed pump 13 with a constant suction pressure. The high pressure feed pump 13 raises the feed pressure to a pressure needed for water to be supercritical. In the present embodiment the high pressure feed pump 13 raises the pressure of the stream of wastewater to about 250 bar and pumps the wastewater through the system.

The stream of wastewater enters then an economizer 14, where it is preheated by reactor effluent. Next, the stream of wastewater enters a heater 15, which raises the temperature of the stream of wastewater to a temperature suitable to start the oxidation. In the present embodiment the heater 15 raises the temperature of the stream to about 350-400 ⁰C, after which the stream of wastewater is caused to flow from an inlet 16a toward an outlet 16b of a SCWO reactor 16. Typically, the heater 15 is used only at start-up of the oxidation. When the exothermic reaction has started, sufficient heat is generated, and the heater 15 can be by¬ passed.

Oxygen from a supply tank 17 is fed to the SCWO reactor 16 via an oxygen pump 18 and an oxygen vaporizer 19, wherein the oxidizable material of the stream of wastewater is oxidized through 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 ⁰C. If the content of oxidizable material in the stream is too high for complete oxidation without exceeding the SCWO reactor's maximum design temperature, a multi-stage SCWO reactor, optionally with quench water supply, may be used.

The oxidized stream or effluent is caused to leave the SCWO reactor 16 through its outlet 16b, and is fed to the economizer 14 for preheating the incoming stream of wastewater.

The heat of the reaction may easily be recovered in a steam generator comprising a steam boiler 20 and a boiler water pump 21 provided for pumping boiler water to the steam boiler 20, in which a specified quality of steam may be generated. The temperature of the effluent downstream of the steam boiler 20 may be 150-230 °C.

The effluent is then cooled to its final exit temperature in an effluent cooler 22. The final exit temperature may be 10- 80 ⁰C.

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

The effluent is then caused to flow from an inlet 24a toward an outlet 24b of a PCC reactor 24. A slurry of calcium hydroxide, also known as milk-of-lime, from a supply tank 25 is fed to the PCC reactor 24 via a calcium hydroxide pump 26, wherein the carbon dioxide of the oxidized stream of wastewater and the lime react to form PCC. The temperature of the stream within the PCC reactor 24 may be any suitable temperature, but is preferably 10-80 °C. The pressure of the stream within the PCC reactor 24 may be any suitable pressure above atmospheric pressure. The pressure may be a pressure supercritical to water, e.g. 230 bar, and in such an instance the pressure reduction device 23 may be dispensed with.

The reacted stream output from the PCC reactor 24 is passed through a further pressure reduction device 27, e.g. pressure reduction coils, to reduce the pressure. Choke water may be added before the pressure reduction device 27 to control the upstream pressure.

The reacted stream then enters a gas/liquid separator 28, which separates any gases in the reacted stream, e.g. traces of N₂ and non-reacted O₂ and CO₂, from the liquid/solid phase including the water and the PCC.

It shall be appreciated that in alternative variants of the embodiment described above, the PCC reactor 24 is arranged between the steam boiler 20 and the effluent cooler 22; between the economizer 14 and the steam boiler 20; or between the SCWO reactor outlet 16b and the economizer 14. Obviously, the temperature within the PCC reactor 24 will then be higher.

Similarly, the lime may be added to the supply tank 25 as dry, unslaked lime, which is caused to react with water therein to form calcium hydroxide, after which the calcium hydroxide is pumped to the PCC reactor 24.

Still further, if larger amounts of PCC are to be manufactured, additional carbon dioxide may be supplied to, e.g. bubbled through, the PCC reactor 24. Preferably, calcium hydroxide is added in amounts up to stoichiometric amounts to form PCC by the reaction

Ca(OH)₂ + CO₂ -» CaCO₃ (s) + H₂O.

Further, the PCC reactor 24 may be a continuous process stirred reactor comprising two stirred reaction chambers in series as being disclosed in the above-identified article

Manufacture of precipitated calcium carbonate, J. Laine,

Paperi ja Puu - Papper och tra, No. 11, 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 hereby incorporated by reference.

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

It shall be noted that the reaction to form PCC may be continued after that the stream has left the PCC reactor 24. For instance, the PCC reaction may be finished within the gas/liquid separator 28.

SCWO can be used for treating of deinking sludge to recover paper filler. The recovered paper filler has proven to be of good quality, i.e. high brightness, see e.g. U.S. Patent

Application Publication No. US 2004/0020616 Al (DAHLBLOM et

AL.), and the articles Oxidation of deinking sludge in supercritical 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. Dahlin, Workshop,

Managing pulp and paper process residues, 30-31 May 2002,

Barcelona, Spain, the contents of all of which being hereby incorporated by reference. The latter references state that while the recovered paper filler has 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 deinking sludge, containing paper filler, wherein the organic part is oxidized through SCWO while the paper filler is transported in the stream through the SCWO reactor 16 and is left essentially unconverted with respect to chemical composition. By the term "essentially unconverted" paper filler is in the present text meant that the paper filler is essentially not chemically reacted or converted to other chemical substance.

The paper filler is transported in the stream through the PCC reactor 24 while the PCC formed therein may be partly precipitated onto 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 residue products.

Alternatively, the water containing the recovered paper filler and the formed PCC is pumped to a paper making machine located near the SCWO system of the invention. The overpressure in the SCWO system as caused by the high pressure pump 13 may be used for transporting the recovered paper filler and the formed PCC to the paper making machine.

Optionally, before separating the paper filler and the formed PCC from the water or before pumping the water containing the recovered paper filler and the formed PCC to a paper making machine, the water containing the recovered paper filler and the formed PCC may be pumped through a cleaning device which separates dark particles formed in the SCWO system or contained in the original sludge, e.g. by means of different settling rates of the dark particles as compared to those of the recovered paper filler and the formed PCC. The cleaned water containing the recovered paper filler and the formed PCC may be pumped to a further reactor, to which carbon dioxide, e.g. as separated in the gas/liquid separator 28, and lime may be added to perform a second carbonation reaction to produce further PCC and obtain an even brighter end product.

The paper filler, which is an inorganic material, contains talc, clay, and/or calcium carbonate. Preferably, the formed PCC is caused to be at least partly precipitated onto the paper filler. The amount of generated calcium hydroxide is selected depending on the sludge used, and the wanted properties of the recovered mix of paper filler and formed PCC.

The formed PCC will increase the overall brightness of the mix of the formed PCC and the recovered filler compared to the brightness of solely recovered filler. The increased brightness is believed to be the result of a combination of mixing effect and precipitation of the manufactured calcium carbonate onto the less bright recovered filler.

Tests have been conducted to verify the result of the present invention.

Paper filler that was recovered from deinking sludge through SCWO had an ISO brightness of 76.6% without there being produced any PCC in the SCWO system. While producing PCC at supercritical pressure and at a temperature of about 20 ⁰C the brightness of the end product was increased essentially linearly from an ISO brightness of 76.6% to an ISO brightness of about 80% when PCC was produced in amounts up to an amount at which it stood for 34% of the total amount of the end product (i.e. recovered filler and produced PCC). When the temperature was increased to 65 °C, the increase in brightness still existed, but it was not as strong as at the lower temperature.

In another experiment PCC was produced in a SCWO system, which was fed by a stream containing water and diesel. PCC was produced at supercritical pressure and at temperatures ranging from about 20 ⁰C to about 380 ⁰C. At lower temperatures PCC was produced having an ISO brightness of above 90%. At higher temperatures (>300 ⁰C), the brightness was decreased as the temperature increased.

Although the invention has been described with particular reference to specific embodiments thereof, the forms of the invention shown and described in detail are to be taken as preferred embodiments of same. It is to be understood, therefore, that various changes and modifications may be resorted to without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A method for manufacturing precipitated calcium carbonate in a high pressure system, characterized by the steps of:

- increasing the pressure and temperature of a stream containing oxidizable material and water;

- causing said stream to flow from an inlet (16a) toward an outlet (16b) in a first reaction chamber system (16) of said high pressure system;

- adding oxidant to said first reaction chamber system and oxidizing the oxidizable material of said stream through supercritical water oxidation to form carbon dioxide and water in said first reaction chamber system;

- causing said oxidized stream to leave said first reaction chamber system through said outlet;

- causing said oxidized stream to flow from an inlet (24a) toward an outlet (24b) in a second reaction chamber system (24) of said high pressure system; and

- adding lime to said second reaction chamber system, and having the carbon dioxide of said oxidized stream to react with said lime to form precipitated calcium carbonate in said second reaction chamber system, while the pressure in said second reaction chamber system is kept above atmospheric pressure.

2. The method as set forth in claim 1 wherein the pressure of said oxidized stream as flowed from the inlet toward the outlet in the second reaction chamber system is a pressure above the pressure needed for obtaining supercritical conditions for water.

3. The method as set forth in claim 1 wherein the pressure of said oxidized stream as flowed from the inlet toward the outlet in the second reaction chamber system is a pressure below the pressure needed for obtaining supercritical conditions for water.

4. The method as set forth in any of claims 1-3 comprising the step of reducing the pressure of said stream subsequent to the formation of said precipitated calcium carbonate.

5. The method as set forth in claim 4 wherein the pressure of said stream is reduced to atmospheric pressure, or to a pressure slightly above atmospheric pressure, subsequent to the formation of said precipitated calcium carbonate.

6. The method as set forth in any of claims 1-5 wherein the temperature of said oxidized stream as flowed from the inlet toward the outlet in the second reaction chamber system is between 5 °C and 600 °C, more preferably between 10 ⁰C and 80 °C.

7. The method as set forth in any of claims 1-6 wherein said lime is added to the second reaction chamber system as a slurry of calcium hydroxide.

8. The method as set forth in any of claims 1-7 wherein additional carbon dioxide is added to the second reaction chamber system.

9. The method as set forth in any of claims 1-8 wherein precipitated calcium carbonate is formed also downstream of said second reaction chamber system.

10. The method as set forth in any of claims 1-9 wherein calcium hydroxide is added to the second reaction chamber system in amounts up to stoichiometric amounts relative to the amounts of carbon dioxide to form precipitated calcium carbonate by the reaction

Ca(OH)₂ + CO₂ -> CaCO₃ (s) + H₂O

11. The method as set forth in any of claims 1-10 wherein said oxidized stream is caused to flow through at least two reactor chambers of said second reaction chamber system, and said lime is added to said at least two reactor chambers of said second reaction chamber system so that the carbon dioxide of said oxidized stream and said lime react to form said precipitated calcium carbonate in said at least two reactor chambers of said second reaction chamber system.

12. The method as set forth in any of claims 1-11 wherein reaction parameters of said second reaction chamber system are controlled in order to form said precipitated calcium carbonate of a desired crystal size and morphology.

13. The method as set forth in any of claims 1-12 wherein said stream containing oxidizable material and water is a sludge containing a paper filler.

14. The method as set forth in claim 13 wherein

- said paper filler is transported in said stream through said first reaction chamber system while being left essentially unconverted;

- said paper filler is transported in said stream through said second reaction chamber system while said precipitated calcium carbonate is at least partly precipitated onto said essentially unconverted paper filler; and

- said paper filler is recovered.

15. The method as set forth in claim 13 or 14 wherein said precipitated calcium carbonate is at least partly caused to be precipitated onto said essentially unconverted paper filler.

16. The method as set forth in any of claims 13-15 wherein said formed precipitated calcium carbonate and said essentially unconverted paper filler are transported to a paper making machine by means of said pressure kept above atmospheric pressure.

17. The method as set forth in any of claims 13-16 wherein the weight of said formed precipitated calcium carbonate is less than the weight of said essentially unconverted paper filler, preferably less than half the weight of said essentially unconverted paper filler, and most preferably less than a third of weight of said essentially unconverted paper filler.

18. The method as set forth in any of claims 13-17 wherein calcium hydroxide is added to the second reaction chamber system in amounts less than stoichiometric amounts relative to the amounts of carbon dioxide to form precipitated calcium carbonate by the reaction

Ca(OH)₂ + CO₂ → CaCO₃ (S) + H₂O

19. A high pressure system for manufacturing precipitated calcium carbonate characterized in:

- a pump (12, 13) and a heater device (14, 15) provided for increasing the pressure and temperature of a stream containing oxidizable material and water; - a first reaction chamber system (16) having an inlet (16a) and an outlet (16b), said inlet being provided for receiving said stream containing oxidizable material and water;

- an oxidant supply system (17-19) provided for supplying oxidant to said first reaction chamber system to oxidize the oxidizable material of said stream through supercritical water oxidation while flowing in said first reaction chamber system to form carbon dioxide and water, wherein the outlet of said first reaction chamber system is provided for outputting said oxidized stream;

- a second reaction chamber system (24) having an inlet (24a) and an outlet (24b), said inlet (24a) of the second reaction chamber system (24) being provided for receiving said oxidized stream; and

- a lime supply system (25-26) provided for supplying lime to said second reaction chamber system to have the carbon dioxide of said oxidized stream to react with said lime to form precipitated calcium carbonate while flowing in said second reaction chamber system at a pressure above atmospheric pressure, wherein the outlet of said second reaction chamber system is provided for outputting said precipitated calcium carbonate.