EP0265736A2 - Procédé de lixiviation de métaux nobles à partir de minerais ou de leurs concentrés par des solutions cyanurées additionnées d'eau oxygénée - Google Patents

Procédé de lixiviation de métaux nobles à partir de minerais ou de leurs concentrés par des solutions cyanurées additionnées d'eau oxygénée Download PDF

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Publication number
EP0265736A2
EP0265736A2 EP87114714A EP87114714A EP0265736A2 EP 0265736 A2 EP0265736 A2 EP 0265736A2 EP 87114714 A EP87114714 A EP 87114714A EP 87114714 A EP87114714 A EP 87114714A EP 0265736 A2 EP0265736 A2 EP 0265736A2
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Prior art keywords
leaching
solution
concentration
aqueous
oxygen
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Granted
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EP87114714A
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German (de)
English (en)
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EP0265736B1 (fr
EP0265736A3 (en
Inventor
Helmut Dr. Knorre
Andrew Dr. Griffiths
Jürgen Dr. Lorösch
Joachim Fischer
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Evonik Operations GmbH
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Degussa GmbH
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B11/00—Obtaining noble metals
    • C22B11/08—Obtaining noble metals by cyaniding

Definitions

  • the invention relates to a method for leaching noble metals, essentially gold and / or silver, from ores or ore concentrates with an aqueous alkaline cyanide solution with the addition of hydrogen peroxide as an oxidizing agent.
  • the operating costs of the compressors are a function of the tank height due to the hydrostatic pressure. Leach tanks with a height of up to 20 m used today therefore entail increased costs.
  • heap leaching is also used to leach precious metals with a cyanide solution.
  • large ore heaps generally 3 to 10 m high
  • an aqueous cyanide lye solution from pH 8 to 13.
  • the ore-free leaching solution which emerges at the bottom is circulated, part of the lye being continuously removed to separate the noble metal and replaced by fresh lye.
  • a serious disadvantage of this heap leaching is that the atmospheric oxygen required in the ore heap for the oxidation of the precious metal has to be introduced into the ore heap through the leaching solution. Since the concentration of dissolved oxygen in the leach solution in the ore heap decreases sharply from top to bottom, there is only incomplete leaching, especially in the lower regions. This fact is responsible for the usually very low gold yield - 40 to 60% of the gold in the ore - in heap leaching.
  • H2O2 hydrogen peroxide
  • H2O2 can also oxidize the cyanide under certain conditions, resulting in excessive consumption of both H2O2 and cyanide.
  • Another problem was that H2O2 can inhibit the dissolution process by passivating the gold surface.
  • the US-PS 3,826,723 teaches a method for cyanide leaching of gold and / or silver with the addition of hydrogen peroxide, this being added as a stabilized H2O2, preferably as a 50% solution, in such an amount that 0.3 to 15 g H2O2 per Liters of leaching solution corresponds to - calculated from the information in lines 51 to 55 on column 2 of the cited US patent and 1.2 g / ml for the density of the 50% by weight H2O2.
  • the present invention was therefore based on the object of improving the known process for leaching noble metals with the addition of hydrogen peroxide so that this can be carried out more economically.
  • H2O2 With the lowest possible consumption of H2O2, no higher sodium cyanide consumption should be caused compared to conventional leaching technology with air fumigation.
  • the improved process should also be able to be safely controlled on an industrial scale and avoid the disadvantages of conventional technology.
  • the object is achieved by a process for leaching gold and / or silver from ores or ore concentrates using an aqueous cyanide leaching solution with a pH of 8 to 13 with the addition of an aqueous H2O2 solution, which is characterized in that the addition the aqueous H2O2 solution is regulated via the concentration of the oxygen dissolved in the leaching solution and an oxygen content of 2 to 20 mg / l is set in the leaching solution.
  • the sub-claims 2 to 9 are training of this method.
  • the O2 concentration should be 2 to 20 mg O2 and preferably 7 to 13 mg O2 per liter of leaching solution.
  • the dissolved oxygen contained in the leaching solution results from the decomposition of the added H2O2, but can also partially get into the leaching solution by absorbing atmospheric oxygen, for example in the production of an ore pulp, in the intensive circulation of the same during leaching, or in the case of heap leaching during Spray the leach solution.
  • the leaching solution means the aqueous phase of the ore pulp.
  • the O2 concentration to be measured to regulate the H2O2 addition depends on how quickly the added H2O2 decomposes and how quickly the dissolved oxygen formed from H2O2 and possibly introduced otherwise is consumed.
  • aqueous H2O2 solutions can be used to adjust and maintain the desired O2 concentration. Surprisingly, however, it was found that the leaching can be carried out particularly economically, ie with low consumption of H2O2 and NaCN, if the H2O2 is metered in in the form of a dilute aqueous solution. H2O2 concentrations of 0.5 to 5 wt .-%, preferably 1 to 2 wt .-%, are particularly suitable.
  • the stabilizers usually used are also diluted, the decomposition of H2O2 is surprisingly not accelerated in the presence of the ore to be leached, but even slowed down. As a result, a good distribution of the H2O2 in the pulp is possible with stirring leaching without there being any loss of active oxygen.
  • the H2O2 consumption drops to a tenth with practically unchanged gold yield if, instead of an H2O2 solution with 35% by weight, one is metered in with 1% by weight.
  • H2O2 concentration in the leaching solution of 0.05% by weight should generally not be exceeded; the H2O2 concentration is preferably kept below 0.03% by weight during the leaching, particularly preferably below 0.02% by weight.
  • the H2O2 concentration in the leaching solution will be below 0.03% by weight if the H2O2 addition is controlled via the O2 concentration - 2 - 20 mg O2 / l - and none within the leaching solution or the ore pulp significant H2O2 concentration gradient is present.
  • the expert will determine the H2O2 concentration in the leaching solution in preliminary tests and choose the H2O2 concentration for leaching, which results in the most economical use in a specific application.
  • the gold dissolution rate when using dilute H2O2 solutions exceeds that of conventional leaching with air gassing, at least in the initial phase of leaching.
  • approximately the same maximum gold yield is achieved in both processes, the leaching time required for this being possibly considerably shorter in the process according to the invention - cf. Examples 1b and 3a with 2 and 3b.
  • the O2 concentration should be in the range from 2 to 20 mg, preferably from 5 to 15 mg and very particularly preferably from 7 to 13 mg O2 per liter of the aqueous phase.
  • the control-related fluctuations can be, for example, a consequence of the inertia of the O2 concentration measurement using an O2 electrode; Process-related fluctuations are, among other things, the result of concentration differences occurring in large agitator tanks.
  • the addition of the H2O2 solution is controlled depending on the O2 concentration. In terms of measurement and control technology, this can advantageously be accomplished using an oxygen electrode chain.
  • the leaching can be carried out before, during or after the leaching with the addition of H2O2 in the presence of atmospheric oxygen dissolved in the leaching solution.
  • the method according to the invention with the conventional leaching methods with gassing with air, however, it is preferred to cover the increased need for active oxygen required during the first phase of the leaching by dosing with hydrogen peroxide according to the invention and only to be gassed with air in the second phase.
  • H2O2 solution as a function of the concentration of oxygen dissolved in the aqueous phase of the ore pulp can no longer be regulated continuously with satisfactory accuracy if the O2 concentration measurement is carried out in the, usually very large, leachate tank.
  • Reasons for this include the too long mixing time of the H2O2 with the ore pulp and the resulting differences in concentration, as well as the long response time when using an oxygen electrode chain; A too slow addition of H2O2 may extend the leaching time, an excess of H2O2 affects however, because of the possible oxidation of the cyanide, the economics of the process.
  • An economical metering of the H2O2 solution which can be used in continuous and discontinuous leaching processes, according to the invention consists in branching off from the main stream of the ore-free or leaching ores containing leaching solution a measuring current which is very small compared to the main stream, this measuring stream controlled by the desired one Value of the O2 concentration measured in the measuring stream, while maintaining a constant pH between 8 and 13 aqueous H2O2 solution metered in and at the same time adding an aqueous H2O2 solution to the main stream in proportion to the quantity.
  • an H2O2 solution of the same concentration will be metered into the measuring stream and the main stream.
  • a dilute aqueous manganese (II) salt solution - for example manganese sulfate - is metered into the measuring stream in such an amount that the O2- Concentration measurement is carried out in the presence of 0.1-50 mg, preferably 0.5-10 mg, of manganese ions per liter of the aqueous phase of the measuring current.
  • This addition of Mn ions accelerates the decay of the H2O2 and thus enables rapid detection of the O2 concentration available from the H2O2 addition, which is necessary for safe control.
  • Mn ions other metal compounds catalyzing the H2O2 decomposition can also be used.
  • the O2 concentration in the measuring stream need not be identical to that in the main stream. Rather, the O2 concentration resulting from catalytically accelerated H2O2 decomposition indicates the available amount of active oxygen.
  • the method according to the invention can be used, for example, in heap leaching and stirring leaching.
  • heap leaching the O2 concentration in the lye is adjusted before the ore heap dribbles through the addition of H2O2 to lye according to the invention, this also being possible in a measuring stream. Due to the slow decomposition of the H2O2 contained in a very low concentration in the leaching solution, it is possible to keep the O2 content of the leaching solution at an effective O2 concentration level in the lower regions of the ore heap during heap leaching and thus to accelerate the leaching process and the To increase precious metal yield.
  • FIG. 1 A particularly advantageous embodiment of the invention is explained in more detail using the example of agitation leaching on the basis of the schematic illustration (FIG. 1):
  • the system contains the ore pulp-absorbing leach tank (1), a measuring cell (2) which is very small in comparison to this, a mixing device (3) for the production and absorption of the dilute, for example 1-2% by weight, H2O2 solution from, for example 70% H2O2 (16) and water (17), one each Storage container for the Mn (II) salt solution (4) and lye (18) (eg sodium hydroxide solution), a pH measuring device (5), a metering pump (6) or (7) for the addition of an aqueous lye Measuring cell or to the leach tank, a control unit (8) for regulating the lye dosage, an O2 electrode with measuring device (9), each a metering pump (10) or (11) for the addition of H2O2 to the measuring cell or to the leach tank, one Control unit (12) for controlling the H2O2 dosage, lines for supplying (13) the fresh and discharging (14) the leached ore pulp, the measuring current feed (15) and lead (15a).
  • H2O2 solution
  • a measuring current is continuously branched off from the leach tank (1) and fed to the measuring cell (2) via line 15.
  • An ore pulp measuring stream flows continuously through the measuring cell, which is designed, for example, as a stirring vessel, for example 100 l / h. - And via line (15a) back into the tank (1).
  • the pH measuring device (5) controls a diaphragm metering pump (6) for lye dosing via the unit (8); the pH is kept at a constant value between 9 and 12; If necessary, the pulse frequency of the metering pump (6) is converted into a signal current by means of a frequency converter and used for quantity-proportional lye dosing by means of the metering pump (7).
  • Mn (II) salt solution is metered in continuously from container (4) - preferably with a concentration of 50-100 mg Mn2+ / l solution with a throughput of about 1 l / hour. - In the measuring current, the metering point being best arranged at the input of the measuring cell.
  • the dosage of the dilute H2O2 solution is controlled via an oxygen electrode chain (9) which controls a diaphragm pump (10) via the control unit with a transducer (12).
  • the target value of the O2 concentration it is preferably in the range 7 to 13 mg O2 / l of the aqueous phase, is stored in the transmitter (in 12).
  • the pulse frequency of the metering pump (10) is converted via a frequency converter (in 12) into a signal stream which, using the quantities of ore pulp flowing through (1) or (2), the metering pump (11) for adding the H2O2 solution to the leach tank (1 ) controls.
  • the H2O2 dosing point in (1) is located at a point that ensures rapid mixing (high turbulence).
  • Fig. 1 can also be used in a slightly modified form for heap leaching, in which case (1) is a container containing the continuously pumped leaching solution and (13) the entry of the low-oxygen and (14) the exit of the H2O2 represent enriched leaching solution with which the ore pile is sprayed.
  • the measuring current is free of ore.
  • the method according to the invention can be carried out much more economically than the previously known method with the addition of H2O2, it is flexible in use and can be controlled safely. Compared to conventional leaching technology with air fumigation, it is advantageous that no NaCN losses occur, the operating costs for air fumigation can be omitted and the leaching time is sometimes considerably shorter with the same gold yield, which can lead to better plant utilization. Compared to conventional heap leaching, the use of H2O2 according to the invention makes it possible to increase the gold yield and shorten the leaching time.
  • the process according to the invention can be used particularly advantageously in the continuous leaching processes used in the mining industry.
  • the porphyry gold ore came from Southeast Asia and contained 6.2 ppm Au, 8 ppm Ag, 840 ppm Cu and 17300 ppm Fe.
  • the ore had a fineness of less than 400 ⁇ m.
  • An ore pulp with a solids content of 40% was produced in a 2 l beaker equipped with a stirrer; the NaCN content based on the aqueous phase was 0.033% by weight, the pH adjusted with CaO was 11.0.
  • H2O2 was added as a 35 wt .-% solution with good stirring of the pulp, (300 rev / min), but without mixing in air, once in such an amount that corresponded to an H2O2 concentration in the aqueous phase of the pulp of 0.023. After 24 hours, the gold yield was 46% of theory.
  • the leaching was carried out in a 2 l beaker provided with a propeller stirrer (300 rpm).
  • the pH was measured using a pH single-rod electrode, and a membrane metering pump was controlled by means of a controller (Dulcometer CFG type PHS 014), the sodium hydroxide solution (20 g NaOH / l) was metered in and the pH Value kept constant at 11.0.
  • the concentration of dissolved oxygen was monitored via an oxygen electrode chain with associated measuring device (O2 electrode WTW EO 190-1.5, O2 measuring device WTW OX 191), and a membrane (Dulcometer CFG type RHS 2000) was used to monitor a Dosing pump controlled, which dosed H2O2.
  • a setpoint of 12 mg O2 / l of the aqueous phase was specified. Due to the slow response time of the electrode, the O2 concentration fluctuated between 10.5 and 13.5 mg O2 / l, although the lowest possible slope was used when approaching the setpoint.
  • Example 2 The same ore was leached as in Example 1, the NaCN concentration of the alkali was again 0.033% by weight at the beginning, and the pH was kept at a constant value of 11.0 using sodium hydroxide solution. The leaching time was again 24 hours. Hydrogen peroxide was metered in at various concentrations, namely 1% by weight, 3.5% by weight and 35% by weight, during the leachings, the concentration of dissolved O 2 being regulated to 12 mg / l. The results follow from the table:
  • the gold yield is 90% at all examined H2O2 concentrations.
  • the gold was dissolved more quickly than with the conventional technique with air gassing. Due to the control, the O2 concentration fluctuated periodically, with the decreasing H2O2 concentration the dosing period longer, but the fluctuations around the target value became smaller, so that better regulation was possible.
  • Example 2 In the apparatus described in Example 2 was leached with the addition of 0.5 to 5 wt .-% hydrogen peroxide solution according to the invention; the NaCN content was 0.06% by weight, based on the aqueous phase; the pH was adjusted to 11.2 with CaO and kept constant by the controlled addition of 0.5N NaOH; Solids content approx. 60%; O2 concentration (setpoint) 12 mg O2 / l of the aqueous phase; Leaching time 24 hours.
  • the target value of the oxygen concentration was reached after 14 minutes when using 1 wt .-% H2O2; the maximum gold yield of 90 to 92% was obtained after 4 hours, regardless of the H2O2 concentration used.
  • Example 3 clearly shows the advantages of the method according to the invention: It is possible to set a higher O2 level in the pulp and to achieve this more quickly; the H2O2 dosage can be controlled safely, which results in a minimal and therefore economical consumption of oxidants; the gold yield and NaCN consumption are comparable in both processes, possibly slightly higher gold yield when oxidized with H2O2; Reduced leaching times enable greater ore throughput and better utilization of the plant capacity.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Manufacture And Refinement Of Metals (AREA)
EP87114714A 1986-10-31 1987-10-08 Procédé de lixiviation de métaux nobles à partir de minerais ou de leurs concentrés par des solutions cyanurées additionnées d'eau oxygénée Expired - Lifetime EP0265736B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE3637082A DE3637082C1 (de) 1986-10-31 1986-10-31 Verfahren zur Laugung von Edelmetallen aus Erzen oder Erzkonzentraten mit cyanidischen Loesungen unter Zusatz von Wasserstoffperoxid
DE3637082 1986-10-31

Publications (3)

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EP0265736A2 true EP0265736A2 (fr) 1988-05-04
EP0265736A3 EP0265736A3 (en) 1990-01-24
EP0265736B1 EP0265736B1 (fr) 1992-01-15

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EP87114714A Expired - Lifetime EP0265736B1 (fr) 1986-10-31 1987-10-08 Procédé de lixiviation de métaux nobles à partir de minerais ou de leurs concentrés par des solutions cyanurées additionnées d'eau oxygénée

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US (1) US5275791A (fr)
EP (1) EP0265736B1 (fr)
AR (1) AR240177A1 (fr)
BR (1) BR8705756A (fr)
CA (1) CA1331518C (fr)
DE (1) DE3637082C1 (fr)
DO (1) DOP1987004572A (fr)
ES (1) ES2027674T3 (fr)
MX (1) MX169904B (fr)
NZ (1) NZ222354A (fr)
PH (1) PH24130A (fr)
PT (1) PT86035B (fr)
ZA (1) ZA876329B (fr)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2631043A1 (fr) * 1988-05-06 1989-11-10 Interox Chemicals Ltd Procede de lixiviation de l'or
EP0358004A2 (fr) * 1988-09-09 1990-03-14 Degussa Aktiengesellschaft Procédé de lixiviation de métaux nobles par des solutions cyanurées additionnées d'eau oxygénée
FR2645176A1 (fr) * 1989-04-04 1990-10-05 Interox Chemicals Ltd Fabrication de peroxydes
US5250272A (en) * 1988-09-09 1993-10-05 Degussa Aktiengesellschaft Process for leaching precious metals with hydrogen-peroxide and a cyanide leaching solution
US5262136A (en) * 1989-04-28 1993-11-16 Cra Services Limited Recovery of gold and silver from complex refractory sulphide ores by cyanidisation and oxidation with peroxides
CN102505079A (zh) * 2011-12-30 2012-06-20 中矿金业股份有限公司 氰化浸出前的金精矿预处理方法
RU2458160C1 (ru) * 2011-04-26 2012-08-10 Открытое акционерное общество "Ведущий научно-исследовательский институт химической технологии" Способ извлечения золота из цианидных растворов, содержащих ртуть
RU2460814C1 (ru) * 2011-04-13 2012-09-10 Открытое акционерное общество "Ведущий научно-исследовательский институт химической технологии" Способ извлечения золота из цианидных растворов c присутствующей в них растворенной ртутью
RU2557024C2 (ru) * 2013-12-02 2015-07-20 Федеральное государственное бюджетное образовательное учреждение высшего профессионального образования "Забайкальский государственный университет" (ФГБОУ ВПО "ЗабГУ") Способ кучного выщелачивания золота из руд

Families Citing this family (10)

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Publication number Priority date Publication date Assignee Title
DE3801741C1 (fr) * 1988-01-22 1989-06-15 Degussa Ag, 6000 Frankfurt, De
DE4017899C1 (en) * 1990-06-02 1991-11-14 Degussa Ag, 6000 Frankfurt, De Extn. of silver and gold from ores - by contacting ore with aq. leaching soln. contg. cyanide in presence of peroxo:borate, and sepg. cyano complexes formed
DE4323251A1 (de) * 1993-07-12 1995-01-19 Degussa Verfahren und Vorrichtung zur kontinuierlichen Bestimmung der Konzentration von Wasserstoffperoxid und Oxidationsmitteln hierfür
CA2367651C (fr) * 2001-03-13 2009-05-26 Her Majesty The Queen In Right Of Canada, As Represented By The Minist Of Natural Resources Canada Regulation de l'ajout de nitrate de plomb dans un procede de recuperation d'or
WO2013110420A1 (fr) * 2012-01-27 2013-08-01 Evonik Degussa Gmbh Enrichissement de minerais de sulfure métallique par flottation par moussage assistée par un oxydant
CN103243222B (zh) * 2013-04-24 2014-04-09 中南大学 一种改性石硫合剂及其在浸金工艺中的应用
CN104232908B (zh) * 2014-09-17 2017-04-12 河南省岩石矿物测试中心 一种从含金炼汞尾渣中回收黄金的方法
CN104911371B (zh) * 2015-04-30 2017-08-25 上海圣的新材料有限公司 一种提金剂及其制备方法
RU2624751C1 (ru) * 2016-04-11 2017-07-06 Федеральное государственное автономное образовательное учреждение высшего профессионального образования "Уральский федеральный университет имени первого Президента России Б.Н. Ельцина" Способ цианистого выщелачивания золота и серебра
CN110546129A (zh) 2017-04-26 2019-12-06 索尔维公司 降低发烟硫酸、焦硫酸或浓硫酸中的羧酸及其衍生物的含量

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US4578163A (en) * 1984-12-03 1986-03-25 Homestake Mining Company Gold recovery process

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CANADIAN MINING JOURNAL, Band 88, August 1967, Seiten 55-60, National Business Publications Ltd, Gardenvale, CA; E.L. DAY: "Some factors influencing the rate of dissolution of gold in sodium cyanide solutions" *
HYDROMETALLURGY, Band 16, Nr. 3, August 1986, Seiten 395-400, Elsevier Science Publishers B.V., Amsterdam, NL; R.M. GARCIA: "The recovery of silver from photographic film: a study of the leaching reaction with cyanide solution for industrial use" *

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2631043A1 (fr) * 1988-05-06 1989-11-10 Interox Chemicals Ltd Procede de lixiviation de l'or
EP0358004A2 (fr) * 1988-09-09 1990-03-14 Degussa Aktiengesellschaft Procédé de lixiviation de métaux nobles par des solutions cyanurées additionnées d'eau oxygénée
EP0358004A3 (en) * 1988-09-09 1990-04-25 Degussa Aktiengesellschaft Process for leaching noble metals using cyanidic solutions with hydrogen peroxide
US5250272A (en) * 1988-09-09 1993-10-05 Degussa Aktiengesellschaft Process for leaching precious metals with hydrogen-peroxide and a cyanide leaching solution
FR2645176A1 (fr) * 1989-04-04 1990-10-05 Interox Chemicals Ltd Fabrication de peroxydes
US5262136A (en) * 1989-04-28 1993-11-16 Cra Services Limited Recovery of gold and silver from complex refractory sulphide ores by cyanidisation and oxidation with peroxides
RU2460814C1 (ru) * 2011-04-13 2012-09-10 Открытое акционерное общество "Ведущий научно-исследовательский институт химической технологии" Способ извлечения золота из цианидных растворов c присутствующей в них растворенной ртутью
RU2458160C1 (ru) * 2011-04-26 2012-08-10 Открытое акционерное общество "Ведущий научно-исследовательский институт химической технологии" Способ извлечения золота из цианидных растворов, содержащих ртуть
CN102505079A (zh) * 2011-12-30 2012-06-20 中矿金业股份有限公司 氰化浸出前的金精矿预处理方法
RU2557024C2 (ru) * 2013-12-02 2015-07-20 Федеральное государственное бюджетное образовательное учреждение высшего профессионального образования "Забайкальский государственный университет" (ФГБОУ ВПО "ЗабГУ") Способ кучного выщелачивания золота из руд

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EP0265736B1 (fr) 1992-01-15
AU589818B2 (en) 1989-10-19
DOP1987004572A (es) 1998-12-05
AU8053387A (en) 1988-05-05
PT86035A (de) 1987-11-01
US5275791A (en) 1994-01-04
PH24130A (en) 1990-03-05
EP0265736A3 (en) 1990-01-24
NZ222354A (en) 1989-09-27
MX169904B (es) 1993-07-30
ES2027674T3 (es) 1992-06-16
AR240177A1 (es) 1990-02-28
BR8705756A (pt) 1988-05-31
PT86035B (pt) 1990-08-31
DE3637082C1 (de) 1988-05-19
CA1331518C (fr) 1994-08-23
ZA876329B (en) 1988-03-29

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