WO1990010109A1 - Appareil et procede de lavage de pate a papier - Google Patents

Appareil et procede de lavage de pate a papier Download PDF

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Publication number
WO1990010109A1
WO1990010109A1 PCT/US1989/005436 US8905436W WO9010109A1 WO 1990010109 A1 WO1990010109 A1 WO 1990010109A1 US 8905436 W US8905436 W US 8905436W WO 9010109 A1 WO9010109 A1 WO 9010109A1
Authority
WO
WIPO (PCT)
Prior art keywords
drum
pulp
mat
washing
plenum
Prior art date
Application number
PCT/US1989/005436
Other languages
English (en)
Inventor
Frank Paskowski, Jr.
Fletcher B. Smith
George L. Shank
Steve Munro
Original Assignee
Beloit Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Beloit Corporation filed Critical Beloit Corporation
Publication of WO1990010109A1 publication Critical patent/WO1990010109A1/fr

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Classifications

    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21D—TREATMENT OF THE MATERIALS BEFORE PASSING TO THE PAPER-MAKING MACHINE
    • D21D1/00—Methods of beating or refining; Beaters of the Hollander type
    • D21D1/20—Methods of refining
    • D21D1/40—Washing the fibres
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
    • D21C9/02—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents
    • D21C9/06—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents in filters ; Washing of concentrated pulp, e.g. pulp mats, on filtering surfaces

Definitions

  • This invention relates generally to the field of papermaking, and more particularly to pulp production for papermaking. Specifically, this invention pertains to an apparatus and process for washing pulp.
  • the goal of pulp washing is to obtain pulp essentially free from soluable impurities.
  • a preferred washing system would completely remove waste liquor and other impurities, while using only a minimal amount of wash liquid.
  • any wash fluids added during the washing stage must also be treated, either by evaporation or other means. Therefore, it is desirable to minimize the amount of wash fluid added during the washing process.
  • the papermaking industry has adopted the term "dilution factor" in assessing the efficiency of wash systems with respect to the amount of wash fluid used.
  • the dilution factor can be described as the amount of water or other wash liquid put into the system and not taken out of the system with the washed pulp as the pulp is removed from the system. If the quantity of wash fluid added is equal to the quantity of washed fluid passing from the system with the pulp, the dilution factor is zero. Low dilution factors are, therefore, desirable.
  • displacement washing A second, more efficient type of washing was pursued, generally characterized as displacement washing.
  • displacement washing techniques diffusion of the wash liquid through the pulp material is controlled to avoid mixing, and the wash liquid merely replaces the liquor in the pulp.
  • Early forms of displacement washing occurred as batch processes in large tanks separate from the digester known as diffusers. However, because of the characteristics of pulp fibers, channelling, or tunnelling, through a large pulp column often occurred, and wash liquor would pass through a pulp column while bypassing entrapped liquor volumes held by fiber networks.
  • the interior of the drum was partially evacuated by the use of a hydraulic drop leg to create a partial vacuum, while the area above the mat remained at atmospheric pressure.
  • the increased pressure differential between the atmospheric area outside the drum and the low pressure area inside the drum improved drainage through the mat and allowed showers to be used. Wash liquid sprayed through nozzles displaced the liquid in the mat.
  • vacuum washers of this type relate to flashing of hot fluids.
  • drainage of liquor through a pulp mat improves with elevated temperatures, and higher temperatures therefore improve washing efficiency.
  • vacuum washers which operate with up to a minus five psi (-5 psi) in the drum created lower equilibrium temperature conditions. Therefore, it was not possible to significantly raise the operating temperature of vacuum washers to further improve the drainage characteristics.
  • Gravity deckers and vacuum washers included outer housings and hoods to contain liquor and odors within a confined area.
  • the hood around the washer could be provided with a seal, and a blower used to increase the air pressure above the mat within the sealed hood area.
  • the pressure washer operated with a positive pressure (i.e. above atmospheric pressure) in the hood, and a slightly higher than atmospheric pressure in the drum. Mat formation occurred as in previous washers in which the rotating drum picked up a fiber mat from a vat of slurry.
  • the pressure washers of this type also utilized showers in the wash zone, for displacing the liquor from the cellulose fibers.
  • feed consistencies were limited on pressure washers, as in gravity deckers and vacuum washers, to about one to one and one-half percent (1 - 1 %). At higher feed consistencies clumpy mat formation occurred, and drainage characteristics varied significantly throughout the pulp mat, resulting in inconsistent washing.
  • a subsequent development in the art of pressure washers was the utilization of a headbox, or similar device, for distributing the stock on a foraminous drum, to replace the vat pick-up techniques used in gravity deckers and early vacuum and pressure washers.
  • Headbox-like devices through the input of energy maintain the stock in a fluidized state, allowing the use of higher stock consistencies with improved mat formation.
  • a distinct separation could be created between the formation zone and the washing zone of such washers, allowing new washing techniques to be utilized.
  • the mat forms the base of a pond of wash fluid thereabove, and the wash fluid drains through the pulp mat as the foraminous roll rotates.
  • the air or other gas supplied to the hood to pressurize the hood permeates the mat and drum to cause an overpressure in the interior of the drum.
  • both the hood and the drum operate at above-atmospheric pressure, with the hood being at a higher pressure, generally, than the drum interior.
  • the fiber mat passes to a repulper, screw conveyor or other apparatus, which generally operate at atmospheric pressure.
  • a seal is provided between the pressurized zones and the non-pressurized mat take-off area. Since the interior of the drum is at overpressure, a pressure differential exists between the drum interior and the take-off area, with the drum interior being at higher pressure. Thus, pressure from the drum attempts to escape from the drum, blowing the mat from the drum surface and into the repulper or screw conveyor. This was generally considered to be advantageous for pulp removal, and, in fact, in some pressure washers bypass conduits have been provided from the washer hood to the drum interior to even further increase the overpressurization of the drum interior, to aid in blowing the mat from the drum.
  • the escaping air from the drum has undesirable effects. Because of the nature of washing, the interior surfaces of the washer become coated with a thin layer of black liquor. Air escaping from the drum and impinging on the liquor covered surfaces in the take-off area, generates a foam with the liquor, which entraps air bubbles. The foam becomes mixed with the pulp in the repulper and is passed with the pulp to subsequent washing stages or other treatment steps. Air in the pulp is generally undesirable, in that drainage is reduced and subsequent wash stages are, therefore, less efficient. In the past, this has been compensated for by the use of defoaming agents, or by the use of larger downstream washers to achieve the desired degree of washing in spite of the reduced drainage characteristics of the pulp. However, with certain types of pulp, foaming becomes an even more significant problem, and the additional equipment cost for subsequent wash stages is economically undesirable. Additionally, large volumes of defoaming agents can be expensive, and their use may not be totally without hazard.
  • Another object of the present invention is to provide a pulp-washing apparatus and process which minimizes foam generation while washing pulps with high foam generation characteristics.
  • Yet another object of the present invention is to provide a pulp-washing apparatus and process which operates as a continuous, flow-through pulp washing process.
  • Still another object of the present invention is to provide a pulp washer drum pressure relieving system which can be added easily, quickly and inexpensively as a retrofit to existing pressure washers.
  • a pressure washer apparatus for removing the overpressure from the inside of the washer drum.
  • the intake for a blower supplying air to pressurize the washer hood is connected to the drum interior, thereby securing its fluid supply, at least in part, from the drum interior.
  • the inlets to the pressurizing blower can be controlled in such a manner as to provide various levels of negative pressurization within the drum, thereby increasing the pressure differential between the washer hood and drum interior, to maximize fluid drainage through the pulp mat.
  • the pressure/vacuum washer of the present invention does not suffer significantly from the problems of flashing experienced in other vacuum washers. High wash temperatures can be maintained to still further improve draining.
  • Relief of pressure in the drum can be achieved independently from the blower system used to pressurize the hood; however, economies of operation and installation costs can be achieved by combining the pressure relief system for the drum with the pressurizing system for the hood.
  • a pipe with flow control means is provided between the drum interior and a plenum above the repulper in a pressure washer.
  • the blower intake is connected to the plenum, and a common wall between the plenum and the repulper also includes a valve apparatus for controlling the flow of air therethrough.
  • the air provided to the plenum can be controlled between the supply from the repulper area, which is atmospheric, and the drum area, which is sealed and, but for relief through the present invention, would be at a greater than atmospheric pressure.
  • the pressure in the drum can be maintained at atmospheric pressure, or at various levels of subatmospheric pressure.
  • the pressure relief system for the drum interior can be independently connected to the blower inlet apart from the repulper plenum; or, as mentioned previously.
  • a separate evacuation system with scrubbers discharging to atmosphere can be used to relieve pressure in the drum.
  • Any of a number of apparatus may be utilized for removing the pulp mat from the drum at the outlet side of the washing zone, including steam doctors, water doctors, or the like.
  • Figure 1 is a side-elevational view of a pulp washing apparatus employing drum pressure relief embodying the present invention.
  • Figure 2 is an enlarged view of the mat formation zone of the pressure washer shown in Figure 1.
  • Figure 3 is an enlarged view of the mat removal zone in common pressure washers of the prior art.
  • Figure 4 is an end-elevational view of a typical pressure washer with the end wall removed, exposing the internal regions of the washer.
  • Figure 5 is a side-elevational view of a suitable arrangement for the pressure relief apparatus of the present invention.
  • Figure 6 is an end view of the pressure relief apparatus shown in Figure 5.
  • number 10 designates a pressure pulp washer embodying the present invention, which employs the use of a drum pressure relieving apparatus 12.
  • Pulp washer 10 includes generally, as viewed externally, a housing 14 consisting of a vat 16 and a hood 18.
  • Hood 18 includes a generally semi-cylindrical sidewall 20, and end walls 22 and 24.
  • the vat and hood enclose the washer to allow pressurization and to contain the liquids and fumes within the washer.
  • a variety of viewing ports 26 and manhole-type access ports 28 are provided at appropriate locations in the vat or hood for observation of the washing process and/or for performing minor adjustments or repairs.
  • a washing drum 40 is disposed within the vat and hood, and is suitably journaled at its ends for rotation therein.
  • Drum 40 includes a honeycomb-like support structure 42, and a foraminous surface material 44 disposed on the honeycomb support structure.
  • a drive means 46 is provided for rotating the drum 40 within the vat and hood, and rotates the drum in a clockwise direction, as viewed in Figure 2 and indicated by the arrow identified by number 47.
  • Drum 40 receives and supports a mat of pulp fibers, carrying the fibers between a mat formation zone 50 and a mat removal zone 52, while passing through first a washing zone 54 and, second, a dewatering zone 56.
  • the mat formation zone 50 includes a formation plate 60 and suitable adjustment mechanisms such as a pivotal support 62 and a plurality of position control apparatus 64 for controlling the position and attitude of the formation plate with respect to drum 40.
  • the position control apparatus 64 may be pneumatic or electrically operated cylinders, mechanical screw devices, or other suitable apparatus for controllably moving formation plate 60.
  • a headbox 66 receives a flow of pulp slurry, and deposits the pulp in a generally uniform distribution along the drum surface at the inlet edge of formation plate 60.
  • formation plate 60 is controlled in position such that a region of convergence towards the drum 40 is defined between the drum and the formation plate, so that, as the stock mat rotates from the formation end of the screen plate toward the washing zone 54, the mat is slightly compacted.
  • Washing zone 54 extends from the formation plate 60, and includes a pond of wash liquid 70 to displace the liquor, or other liquid, in the pulp mat, which displaced liquid flows through the mat, the foraminous drum surface and the honeycomb support structure, and is collected in the interior of drum 40 by collection means not shown.
  • the collected liquid which may include some wash liquid, is then suitably collected and conducted from the washer.
  • the pond extends generally from a sealed area at, or near, the elevation of headbox 66, upwardly through the upper . left quadrant of the washer, as shown in Figure 2. Wash fluid supply is provided through an inlet 74.
  • wash showers may be included in the washing zone 54, and includes shower pipes 76 and supply conduits 78, with appropriate valves 80.
  • the wash showers spray additional displacing liquid on to the pulp mat as the mat leaves the pond 70.
  • the dewatering zone 56 extends generally along the upper half of the upper right quadrant of the washer, as seen in Figure 2, or generally from the last of the washing means, which may be the pond or last shower pipe 76, to a seal mechanism 82.
  • the stock is dewatered from a consistency of approximately eight percent (8%) upon leaving the pond, or last shower, to a consistency of from twelve to fourteen percent (12% - 14%) as the mat passes beneath the seal mechanism 82.
  • the pulp mat is separated from the drum and discharged into a repulper, screw conveyor or other such device for further treatment or handling.
  • a repulper may be used to break-up the pulp mat and mix it with liquid to lower its consistency, if the pulp is to enter a second stage washer.
  • a screw conveyor may be used if the pulp will be further processed or stored at the higher consistency present following dewatering.
  • Drainage through the mat in the washing zone 54, and in the dewatering zone 56 is enhanced in that the area of hood 18 from the mat formation zone 50 up to the region of seal mechanism 82 is pressurized.
  • a blower 84 having an inlet 86 and an outlet 88, is provided for pressurizing the hood. Outlet 88 discharges into the hood in the washing zone 54, and, in the embodiment shown in Figure 1, inlet 86 is connected to a plenum 90 shown in Figures 4 and 5 above the mat removal zone 52.
  • the mat removal zone 52 following seal mechanisms 82 is at atmospheric pressure, and air escaping from the above atmospheric pressure area of drum 40 through the foraminous surface material 44 to the atmospheric pressure area following seal 82 would blow the mat from the drum.
  • This mat removal technique utilizing air escaping from the drum, is used in prior art pressure washers and is illustrated for explanation purposes in Figure 3.
  • Seal mechanism 82 is provided to isolate the above-atmospheric pressure area within hood 40, along the washing and dewatering zones, from the atmospheric pressure are of mat removal zone 52.
  • Seal mechanism 82 includes a support plate 100 which holds an inflatable bag 102 having a surface lining 104 of wear-resistant material. Inflatable bag 102 is inflated supplied and is forced against the surface material 104, to hold the material against the mat of pulp. Water and air are provided to the inflatable bag through inlets 108, and is removed through a drain 110.
  • pulp washer 10 is a conventional pressure washer.
  • the drum interior designated by numeral 112 is pressurized by the air, or other pressurizing gas which permeates the mat 113 and foraminous drum surface 44 from the pressurized hood region 114.
  • the mat 113 passes beneath seal mechanism 82, the mat 113 enters a region 116 of atmospheric pressure.
  • a receiving plate 118 may be provided for directing the mat toward the repulper, or other subsequent process apparatus.
  • Drum 40 is sealed at its ends to the end walls 22 and 24 by conventional rotating and nonrot ting apparatus known to those knowledgeable in the art.
  • a conduit 120 for evacuating the drum extends through end wall 22 in a region within the sealed interior area 112 of drum 40.
  • conduit 120 is connected to plenum 90 above the mat removal zone 52.
  • a valve mechanism 124 such as a sliding plate, is provided to adjustably control the flow of fluid through conduit 120.
  • a wall 126 includes a sliding gate 128 for controllably admitting air through an opening 130 from unsealed atmospheric regions of washer 10.
  • a positive removal apparatus Since insufficient pressure remains in drum interior 112 to blow the sheet from the drum in the mat removal zone 52, a positive removal apparatus must be provided. Any of a number of such apparatus could be used, including a rotating roll for picking the sheet from drum 40, water doctors, or steam doctors to separate the sheet from the drum.
  • a mat of pulp material is deposited on the foraminous surface material 44 of drum 40 in the mat formation zone 50.
  • the mat is slightly compacted and enters the washing zone 54. Wash liquid from the pond 70 displaces the fluid in the mat, forcing the fluid through the foraminous surface material and into the drum 40. Additional displacement type washing is provided from the shower pipes 76, and dewatering of the mat occurs in known fashion throughout dewatering zone 56.
  • blower 84 draws air through its inlet 86 from the plenum 90.
  • a conduit with or without positive pumping apparatus can be provided, connected to the drum interior 110, and merely discharging to atmosphere.
  • the plenum in the mat removal zone 52 can be eliminated, and conduit 120 connected directly to inlet 86 with an alternative atmospheric inlet available for proper inlet mixing.
  • the drum pressure relieving system of the present invention can be added quickly and easily to existing pressure washers by adding a conduit which places the drum interior in flow communication with the blower inlet region, as then used. Additionally, a valve in the conduit and one controlling flow from the original air source should be added. Normally, the blower inlet receives air from the escaping washer air, to minimize the escape of fumes by recirculating the fumes. Typically, this source is at or near the mat removal zone as described herein, making retrofit a simple undertaking.

Abstract

Sont décrits un cylindre laveur sous pression perfectionné (10) et un procédé pour laver de la pâte de cellulose, dans lesquels un matelas de pâte (113) est créé sur un tambour rotatif (40) et se déplace à travers une zone de lavage (54) et une zone d'égouttage (56) entre la zone de formation du matelas (50) et une zone d'élimination du matelas (52). Un capot (20) est prévu au-dessus du tambour (40), avec des joints appropriés, et une atmosphère pressurisée est créée dans les zones de lavage et d'égouttage (54, 56). Pour réduire la pression qui normalement s'accumule à l'intérieur du tambour de lavage (40), une canalisation (120) est reliée entre l'intérieur du tambour (112) et l'admission (86) de la soufflante (84) utilisée pour pressuriser le capot. Ainsi, la production de mousse est réduite au minimum puisque l'intérieur du tambour (112) est maintenu à une pression égale ou inférieure à la pression atmosphérique. Un rouleau décolleur ou un racloir est prévu pour enlever le matelas du tambour (40).
PCT/US1989/005436 1989-02-23 1989-12-07 Appareil et procede de lavage de pate a papier WO1990010109A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US31453389A 1989-02-23 1989-02-23
US314,533 1989-02-23

Publications (1)

Publication Number Publication Date
WO1990010109A1 true WO1990010109A1 (fr) 1990-09-07

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Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/US1989/005436 WO1990010109A1 (fr) 1989-02-23 1989-12-07 Appareil et procede de lavage de pate a papier

Country Status (6)

Country Link
CN (1) CN1046576A (fr)
AU (1) AU4848890A (fr)
CA (1) CA2009328A1 (fr)
ES (1) ES2020636A6 (fr)
WO (1) WO1990010109A1 (fr)
ZA (1) ZA901338B (fr)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2656345A1 (fr) * 1988-11-11 1991-06-28 Sunds Defibrator Rauma Oy Procede pour l'epaississement d'une bouillie et pour son traitement a l'aide d'un liquide et filtre a tambour pour la mise en óoeuvre de ce procede.
WO1997016591A1 (fr) * 1995-10-31 1997-05-09 Tristar Industries Ltd. Tambour laveur a pression
WO1998054401A1 (fr) * 1997-05-26 1998-12-03 Andritz-Patentverwaltungs-Gesellschaft Mbh Dispositif de deshydratation et de lavage de suspensions de matiere fibreuse
US6017416A (en) * 1997-12-04 2000-01-25 Bonded Fiberglass Co. FRP hood for paper processing plant
EP1885944A1 (fr) * 2005-06-03 2008-02-13 Metso Paper, Inc. Dispositif de traitement de la pate cellulosique dans une machine de lavage comportant des elements d'obturation de soupape peripherique decales
EP1885946A1 (fr) * 2005-06-03 2008-02-13 Metso Paper, Inc. Dispositif de traitement de la pate a papier pour appareil de lavage, visant a faciliter l'entretien et le controle dudit appareil

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
SE531847E (sv) * 2007-12-13 2012-06-01 Metso Paper Inc Apparat för tvättning och avvattning av massa
CN102877345A (zh) * 2012-10-31 2013-01-16 江苏大唐机械有限公司 造纸原料洗涤脱水设备
CN104190598B (zh) * 2014-08-29 2017-02-08 山东天阳纸业有限公司 涂料吸集器
CN104528413B (zh) * 2014-11-18 2016-09-07 南宁侨虹新材料有限责任公司 无尘纸成形箱转鼓射流式均匀布料装置
CN106049155B (zh) * 2016-07-29 2017-10-20 重庆天春科技有限公司 一种自行去除杂质的纸浆打浆机

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1012815B (de) * 1951-10-12 1957-07-25 Rauma Repola Oy Waschvorrichtung fuer Faserstoffsuspensionen
US2999785A (en) * 1959-07-17 1961-09-12 Kamyr Ab Method and plant for washing of cellulosic pulp under overpressure
US3487941A (en) * 1967-05-10 1970-01-06 Pertti Olavi Haapamaki Pressure washer
US4808265A (en) * 1986-08-07 1989-02-28 Ingersoll-Rand Company Method of washing pulp in a pressure pulp washer adding two washing liquids

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1012815B (de) * 1951-10-12 1957-07-25 Rauma Repola Oy Waschvorrichtung fuer Faserstoffsuspensionen
US2999785A (en) * 1959-07-17 1961-09-12 Kamyr Ab Method and plant for washing of cellulosic pulp under overpressure
US3487941A (en) * 1967-05-10 1970-01-06 Pertti Olavi Haapamaki Pressure washer
US4808265A (en) * 1986-08-07 1989-02-28 Ingersoll-Rand Company Method of washing pulp in a pressure pulp washer adding two washing liquids

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2656345A1 (fr) * 1988-11-11 1991-06-28 Sunds Defibrator Rauma Oy Procede pour l'epaississement d'une bouillie et pour son traitement a l'aide d'un liquide et filtre a tambour pour la mise en óoeuvre de ce procede.
WO1997016591A1 (fr) * 1995-10-31 1997-05-09 Tristar Industries Ltd. Tambour laveur a pression
WO1998054401A1 (fr) * 1997-05-26 1998-12-03 Andritz-Patentverwaltungs-Gesellschaft Mbh Dispositif de deshydratation et de lavage de suspensions de matiere fibreuse
US6017416A (en) * 1997-12-04 2000-01-25 Bonded Fiberglass Co. FRP hood for paper processing plant
US6440265B1 (en) * 1997-12-04 2002-08-27 Bonded Fiberglass Company FRP hood for paper processing plant
EP1885944A1 (fr) * 2005-06-03 2008-02-13 Metso Paper, Inc. Dispositif de traitement de la pate cellulosique dans une machine de lavage comportant des elements d'obturation de soupape peripherique decales
EP1885946A1 (fr) * 2005-06-03 2008-02-13 Metso Paper, Inc. Dispositif de traitement de la pate a papier pour appareil de lavage, visant a faciliter l'entretien et le controle dudit appareil
EP1885944A4 (fr) * 2005-06-03 2011-11-16 Metso Paper Inc Dispositif de traitement de la pate cellulosique dans une machine de lavage comportant des elements d'obturation de soupape peripherique decales
EP1885946A4 (fr) * 2005-06-03 2011-11-30 Metso Paper Inc Dispositif de traitement de la pate a papier pour appareil de lavage, visant a faciliter l'entretien et le controle dudit appareil

Also Published As

Publication number Publication date
ZA901338B (en) 1991-01-30
ES2020636A6 (es) 1991-08-16
CN1046576A (zh) 1990-10-31
AU4848890A (en) 1990-09-26
CA2009328A1 (fr) 1990-08-23

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