AP151A

AP151A - Nobel metal recovery

Info

Publication number: AP151A
Application number: APAP/P/1990/000167A
Authority: AP
Inventors: Dean Robert Butler
Original assignee: Dean Robert Butler
Priority date: 1989-03-07
Filing date: 1990-03-07
Publication date: 1991-10-02
Also published as: GB2248631B; CN1036283C; PH27611A; BR9007193A; YU46290A; IE62041B1; AP9000167A0; WO1990010721A1; MA22480A1; ZA901703B; OA09217A; GR900100161A; CU22322A3; AR246558A1; AU5260690A; IE900781L; CA2045551A1; NZ232793A; IL93664A; MY105658A

Abstract

Recovery of gold is enhenced to levels higher than indicated by conventional fire assay technique by dilution of the leaching liquor in the cyanide of gold ores at any time prior to the recovery of gold from solution. Depending on ore type pulp densities are reduced to 0.1 to 10 percent and held for a period of time, followed by conventional carbon in pulp recovery processes. In some cases solids are separated from the dilute solution and gold is extracted from the solution using carbon or iron exchange resins.

Description

This invention relates to improvements in recovery of noble metals from ores and tailings. Throughout this specification noble metals are intended to Include gold, silver and the platinum group.

Thia invention is partly predicated on the diacovery that noble metals in extremely fine form are often present in higher concentrations than le revealed by normal assay techniques in common use.

For example platinum or gold oree can contain more metal than that recovered in conventional wet chemical or fire-assay methods, it is thought that where there are metal absorbing materials such as clay, carbon, or sulfides in the ore or other mettle being analysed, tome of the metal taken into aolutlon become· adiorbed onto these materials and ie not detected, in the caae of gold leached into solution by aqua regia in wet aaeaye or by cyanide in the cyanide extraction process, the gold complex becomes absorbed by clay, carbon, sulfides, or other material and ie thus undetected by solution essay. In the fire assay technique, because clays convert to a boro silicate glass under fire assay conditions, gold is also lost to detection in that technique. Conventionally, gold leached by the cyanide process, usually at pulp densities of 35 to 50%, may be recovered from the leach solution in a subsequent stage by contacting the solution or pulp with activated carbon, usually in a concentration range of 10 to 20 grama of carbon per litre of solution (carbon in pulp (Cl>) process], but on occasions up to 40 grams per litre have been used. In some instances the carbon has been added to the leaching circuit as well (CIL process), in the same concentration ranges in order to improve gold leach rates so that the gold recorded equated with the assayed grade of the ore.

Particular treatments for clay or sulfide ores have been proposed. Australian Patent 569175 treats sulfide ores in a pressure oxidation step prior to cyanide leaching. After leaching the pulp is diluted by washing to

AP 0 0 0 1 5 1

- 3 improve the flocculation in tha subsequent thickening stage, following which the liquor is separated from the pulp.

Gold is then extracted from solution and tha concentrated pulp ia then subjected to a carbon in leach circuit at 35 to 401 solids to extract further gold.

The conventional assay technique for gold ie either by the wet q^thod, which is leaching with aqua regia followed by Measurement of the dissolved gold by atomic absorption spectroscopy or similar techniques, or by fire ; assay. In some instances when the recovery of gold by the CIP process was not up to the assayed grade, adoption of the CIL process, with addition of carbon to tha leach circuit resulted in Increased recovery. The amount of carbon waa increased, in some cases to 40 grams per litre, until the head grade recovery was achieved, in other Instances the Carbon in leach (CIL) process wee adopted to improve gold leach rates and gold recovery rates and thereby decrease the required number of carbon contacting tanks, thus decreasing the capital coat of construction of the gold recovery plant. In some circumstances, however, increasing the carbon loadings to the leach vessel, or the subsequent contacting stagea, waa found to be undeeirable because of the formation of fine carbon particles caused by attrition during pulp agitation. The consequent loss of carbon with its attached gold reduced the effectiveness of the process. However prior to this invention it was not suspected that there were also undetected values of metal in some ore samples. These comments also apply to ore contentrate and tailings.

It is an object of this Invention to improve recovery rates of noble metals including gold or other z valuable metals from ores particularly clay containing ores.

In another aspect of the invention there is provided a method of recovering metal values by the leaching method in which the pulp density of the slurry is adjusted to below 151 either, prior to, during, or at the end of the

AP 0 0 0 1 5 1

IS

- 4 Pulp 1* defined a* a mixture of one or more Bolide with one or more liquors. Pulp density ia defined by Arthur Γ Taggart in hie book Handbook of Mineral breaaing aa the decimal fraction of solid· in pulp, by weight. This ie commonly referred to ee e percentage figure, calculated ea the decimal fraction of solid· in pulp, by weight, multiplied by one hundred.

By lowering the pulp density as compared to conventional processes, such aa in th· cyanide leeching process for gold ores where pulp densities in the range of 35 to 55¾ are normally used, more noble metal, particularly gold, ie recovered than by conventional proceeeee and in tome case· gold recovery ie higher than the assayed value obtained by using conventional assay technique.

Processes according to thia Invention utilising collectors such as carbon or ion exchange resine generally have pulp densities in the collection stage lees than 15%, preferably 0.1 to 10¾ more preferably 0.5¾ to 2¾. The low pulp density may be used in the leeching stage or alternatively conventional pulp densities may be employed in the leeching stage or the pulp may then diluted to densities of less then 15¾ prior to the stage in which noble metal ie recovered from solution.

Once the dilution of the pulp has occurred it ie an important aspect of this invention to minimise the occurrence of localised sones in the slurry or in the recovery process in which pulp density is above 15¾ or higher than the preferred dilution.

Another alternative is to carry out the dilution in stages by separating the liquor from the solids as they leave the leaching circuit, sending the liquor to the recovery stage to recover some metal from solution and treating the separated solids with more leach liquors at a pulp density below 30¾ to recover further noble metal end again separating the solids from the liquor. The stages can be repeated until it is no longer economic to continue the process.-—----------

- 5 in this way the solid» are contacted with a large volume C|| liquor in total, even though most of the liquor may be a relatively email recirculating volume having multiple contacts with the solids. Separation of the liquor from the aolida may be accomplished by any of the conventional methods including centrifugation, filtration, decantation or aimilar. Leaching may also be performed at _:the higher pulp densities followed by dilution and recovery of noble metal by uaing a collector in an extended contaotor_xcircuit in which a tank/» containing carbon or other collector is followed by a desorption tank/e containing little or no carbon. Thue the sequence of metal desorption from the eolide and adsorption onto the collector ie maintained, but with a lesser Inventory and attrition of the collector than if each tank in the extended circuit contained normal levela of collector material. The collector material may be carbon, ion exchange resins, chelating resins or polymeric adsorbent resins hereinafter referred to as resins.

In one preferred aspect of this invention it has been discovered that a higher recovery of the noble metal can be obtained if a lixiviant or leaching aid in addition to the leaching agent 1· used. λ preferred lixiviant or leaching aid is chloride ions obtained, for example, from ea water. Chloride ion concentration ie preferably in the range of 10 to 100 grams per litre.

Methods of recovering the leached material from solution need not be restricted to using collectors such as carbon, reeine and other adsorbents, but may Include methods, such as zinc cementation, cementation, solvent extraction, ₃electrowinning and precipitation. These alternative methods may be necessary to recover the gold or other metal from solution in en alternative procedure in which a relatively short chain, polar, soluble organic material such ae methanol or acetone is added to the pulp. The organic material, in the case of gold leaching, will alter the dielectric constant of the solvent and allow more gold to be

APO 0015 1

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- i desorbed into the liquor than would normally ba desorbed without the organic material pretent. whan carbon or ratine era uaad aa collectors the organic material may interfere with the adsorption of gold by theaa materials and thus lower the recovery of gold from the liquor.

Teetwork carried out has shown that the loading of gold onto carbon, both at high and low pulp densities and at high and low cyanide concentrations, is a reversible reaction. Thera is a tendency for both the ore particles and the carbon to absorb gold from cyanide solution. The term ore” can also mean ora concentrates, tailings, or other noble metal containing solids, λ reversible equilibrium is formed between the ore particles, the liquor and the carbon. At high pulp densities I>30%) this equilibrium favours the ore particles. As the pulp density is lowered the equilibrium moves toward the liquor and carbon. At low pulp densities [<5%] the equilibrium is also affected by both the cyanide concentration and pH. as the cyanide concentration Is lowered the equilibrium moves back toward the ora particles. If the pH ie too high the equilibrium will favour the ore particles and liquor rather than the carbon. The distribution of gold between the three phases is time dependent, in that for leachee where the conditions do not favour the carbon tide of the equilibrium, the gold will unload from the carbon and load back onto the ore particles. It is for this reason that localised sones of high pulp density are to be avoided ee gold recovery will be reduced.

Thus the dilution of the pulp density according to the present invention ie for a different purpose to the dilution washing step disclosed in Patent 569175 where the dilution step assists in reducing the amount of flocculant required in the subsequent thickening and separation step.

In contrast to the present invention Patent 569175 ie not attempting to increase solution of gold and is not attempting to recover gold above the conventional assay of

- 7 In another aspect, the present invention, provide· a method of recovering noble metal value*, particularly platinum, silver or gold, in which a noble metal bearing ore is leached with a leach solution to dissolve the metal in the presence of at least 65 grams per litre of a carbon source material or a resin having metal absorption properties.

This process ensures increases in gold recovery.

When certain ore types such as those having clay, carbon, or_&sulfide materials in the ore are treated according to thia invention, metal values recovered are greater than the assay grade of the ore when measured by conventional assay. For clay and silica containing ores, dilution below 10* shows Immediate improvement in gold recoveriea, but for sulphldic orea dilution below 2t appears to be necessary for economically significant improvement in recovery.

Again these observations also apply to other noble metals. The recovery of the noble metal onto the collector may be achieved by adding the collector (e.g. carbon or realn) or the pulp on a recirculating tank or alternatively uaing a aeries of short columns packed with a collector through which the solution is passed a number of times so that the amount of collector exposed to each litre of solution is of the same order a* that required in the recirculating tank.

The gold complexes are removed from solution and taken up by solids by several possible mechanisms. They include (1) Ion exchange, (2) physical absorption, (3) chemical absorption (4) direct reduction, (5) method (1) or ..

(2) followed with partial or complete reduction depending on the solid and the environment, (6) partial or complete complexation. The mechanism employed on any particular solid particle is dependent on both the solids type and the chemical and physical conditions in place at that time.

Accordingly, the present invention ie able to achieve recoveries of noble metal valuta such as gold which are greater than conventional assay grades by simply --AP 0 0 0 1 5 1

- 8 contacting th· leach solution with gold adsorbent carbon source material or exchange resins, with conventional pulp densities of 35 to 55% ore by weight, carbon or reein concentrations greater than 65 grams per litre are preferred, with a more preferred renge being above 300 gram· per litre, where lower pulp densities are used as taught by this invention, an even higher gold recovery is possible and carbon concentrations of from 100 to 300 grama per litre are practicable.

zt is essential when considering Increase· in carbon or reeln loadings to Increase the carbon or resin concentrations in relation to the quantity of the ore in the pulp, zn order to achieve improved recovery of gold above the conventional assay grade, at least 60 kilograms of carbon source material per tonne of ore should be used. It ie preferred to use at least 90 kilograms per tonne and more preferably to use above 100 kilogram· of carbon per tonne of ore. This inert»»· in carbon loading per tonne of ore can be achieved either by adding more carbon, or by reducing the pulp density and maintaining the carbon concentration per litre, where a recovery medium other than carbon la used e.g. ion exchange reeln, the medium it to be used in concentrations that will produce the same gold adsorbing capacity as activated carbon at the concentrations recommended for thie Invention

Xn a further aspect of the leaching process, dispersant· are added to the pulp to aid in the separation of the solids particles. it is thought that in the absence of very low (around 1%) pulp densities, a gold complex which ie desorbed from a solids particle is likely to be adsorbed by another solids particle if thee· solids particles art in close proximity. Dilution increases the distance between these particles and the dispersant ensures that there is uniform separation. Any suitable dispersants may be used. Preferred dispersant» are sodium silicate, sodium carbonate, tri-sodium polyphosphate and sodium hexametaphosphate. By

- 9 the pulp eflida nay be naturally maintained in a condition of separation where by the Freundlich adaorption equation (C,-kC_A w^ere C, ia the concentration of gold per unit area of the solids, C_A is the concentration of gold per unit volume of the pulp liquor, k end n being conatanta) describes the gold distribution in the pulp between the solids and the liquor. At higher pulp densities the solids, particles proximity effect predominates end alters the gold distribution in the pulp in favour of the aolids particles, l This solids particles proximity effect may be lessened by the Introduction of one or more dispersants into the pulp.

By maintaining discrete solids particles rather than agglomerates of floes, the dispersant lessens the likelihood of a gold complex being desorbed from one solids particle and then immediately being adsorbed by an adjacent solids particles.

The addition of tha carbon material may require re-design of the subsequent carbon contacting circuit due to the Increased carbon concentrations. By selection of e harder grade of carbon and control of agitation, the attrition of the carbon particle· expected at such high loadings can be reduced.

For gold extraction tha preferred method of leaching, desorption and adaorption is as follows. The leaching is performed in a cyanide solution at s relatively high pulp density to minimise tankage requirements. At the end of the leach cycle the pulp is diluted to s low pulp density (less than 30%) and maintained for a minimum periods of time in a holding tank. The pulp density and time will z both vary according to the type and grade of the material ~ being leached, the ore type, the liquor type and tb· leachant. For most gold ores the economic optimum pulp density lies between 0.1 and 10% and the residence time in the holding tank is up to 30 minutes preferably 15 minutes. The pulp is then preferably centrifuged with a short residence time in the centrifuge to separate solids from the

- 10 unseparated pulp if then passed through a column containing an absorbent material such as carbon or resin to absorb the gold which is present in the liquor. The pulp so treated will then deaorb more gold into the liquor which it either barren or containing only low levels of gold, this desorption occurs to maintain the constant ratio of gold per unit area of solids/gold per unit volume of liquor. if the centrifuge separation step has been carried out the solids may be recombined with gold depleted liquor to extract

IO further gold from the solids. As the concentration of the gold per unit area of eollda is lowered, so the amount of desorption of gold from the aolids into the liquor is lowered. It has been observed that when the pH of the pulp la alkaline (>9) the collection efficiency of the carbon collector la inhibited, zf the pulp was to be in contact with the carbon throughout this desorption stage until auch time aa the gold had desorbed to the equilibrium level for that pulp density, an extremely long and economically , unvlable contact time would be required. Not only would 2q the operating coats be very high, but the capital costa would be immense.

This desorption effect, which occurs both In the presence and absence of carbon, may be utilised to aid the overall process. This is accomplished by using a short column of carbon to adsorb the majority of the gold from the liquor and then placing the pulp into an agitated tank containing little or no carbon. This allows the pulp enough residence time to desorb gold into the liquor to either the maximum level attainable in relation to the gold

2Q grade on the solids, or to such lower levels ss may be dealred by the operator. The pulp is then passed .through a second carbon column and into a second desorption tank.

This cycle may be repeated until the value of the gold recovered per cycle is not greater than the cost of setting up and operating for that cycle. At any stage in these cycles the pulp density may be lowered one or more times to increase the gold recovery._

- 11 In «11 stages of th« process, it is important that zonae of high pulp denaity sra not allowed to contact the pulp or partially clarified liquor in tha intarval between pulp dilution and tha completion of adaorption of tha matal onto tha adsorbent. Zf a sone of high pulp density is allowed to form in this interval, than tha metal values will ba absorbed onto the solids in this high pulp density sone and will thua not ba available for recovery by the proceaa adsorbent. Zf the process adsorbent has surfact irregularities or internal pores where solids in tha pulp may collect to form sonea of localised high pulp density, than thasa solids will absorb the metal values in preference to tha process adsorbent. With clay or fine ora particles present in the pulp, a slime layer may coat tha surface of tha carbon. Thus with process adsorbents such as carbon or ion exchange resins which have aurface structures capable of hosting these localised high pulp density sones, the preferred method of recovering the noble metal values ia by contacting tha procaas adsorbent with a clarified liquor.

The method of production of thia clarified liquor must be euch that either no sonee of high pulp density are formed, or if they are, that thair area of contact with the pulp or resultant clarified liquor ia minimal and such contact occurs for a minimum period of tlma.

If a solids removal step ia not used, tha collectors used such as sine or ion exchange fibres must not cause pulp solids to form localised high pulp density tones.

Throughout this specification the preferred method is described in relation to gold recovery using cyanide leaching and carbon recovery. The process is equally applicable to other noble metal valuta using the appropriate leaching solution and the appropriate recovery adsorbent such as a rasin.

in another aspect, the present Invention provides an Improved wet aaaay technique for metal valuta, particularly gold. If an aqua regia leach, which is being performed on an ora aampla, has carbon material added to the ——

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- 12 liquor, some gold will report to the liquor and some gold will report to the carbon. The leached ore ia then subjected to repeated cycles of releaching with freah aqua regia and carbon until no significant gold ia detected in either the carbon or the liquor for that cycle. This allows previously undetectable gold to be measured. The gold ia measured in both the carbon and the leach solution.

Xn all examples in this patent the cyanide, chlorine and thiourea leaches were performed at ambient temperatures and pressures. Although results are not shown 1 in 5 examples were run as blanks. Abbreviations and symbols (shown in brackets) used in this patent include; pulp density (PD), chlorine (Cl), cyanide (CN), thiourea (Tu), grams per tonne (g/t), parta per million (ppm), micrometres (urn), micrograms (ug), grams (g), grams per litre (g/1), high volume aqua regia (HVA*), chloride ion (cl*), greater than (>), less than (<) carbon in pulp proceaa (CIP), carbon in leach process (CIL), Di-iso-Butyl-Ketone (DIBK), Atomic Absorption Spectroscopy (AA8) and percentage (»). ih millivolt· are quoted with respect to the Standard Hydrogen electrode, ixemple 1

Test work with clay type ores indicates that more gold is maintained in cyanide solution if the pulp density of the slurry is lowered, all other conditions remaining conetant. This is shown by the following results on <38 micron fractions, chosen to avoid any nugget effect.

Xn this example, three (3) ore samples (<38 ppm) were leached for 6 hours at pH 10.5 and a cyanide concentration of 0.1¾ W/W. The pulp density varied from 5¾ to 30». The pulps were then filtered and the leach liquor analysed for soluble gold by atomic absorption spectroscopy. This result clearly indicates that at the end of the leach more gold is present in the leach liquor when lower pulp densities are used. -. ........

- 13 Pulp density of leach solution

Apparent head grade gtdd detected in liquid

Ore (1) 30¾ 10¾

Ore (2) 30¾ 10¾

0.27g/t

0.60g/t

0.67g/t

1.15g/t

Ore (3) 30¾ 1.90g/t

10¾ 3.90g/t

5¾ 6.90g/t

Low pulp density on its own, however, does not release all the gold in solution, when carbon additives of 200 grant/litre were made at the start of the leech, considerably more gold wee obtained.

Pulp Carbon Apparent head density concentrate grade gold

Ore (1) of leech solution

10¾

10¾ (g/D

0.0

200 detected in liquor or c*rfeon 0.60g/t l.63g/t

AP 0 0 0 1 5 1

Ore (2) 10¾ 0.0 1.15g/t

10¾ 200 2.74g/t

Bxample 2

An example of the unloading of gold from carbon over time when the pH of the leach liquor ie too high ie ae follows:

Pulp Density 1.0¾ pH 10.Θ Sample Weight 20g. Carbon concentration 200g/litre, cyanide concentration 1 ppm. -BAD ORIGINAL

- 14 Leach Total gold in

Tima total ug gold on Liquor tig carbon

Omin	00	<0.3
15min	19	<0.3
30mln	10.4	<0.3
eomln	5.3	<0.3
360min	00	<0.3

The above ore waa assayed at 0.25 grams/tonne using fire assay and aqua regia and cyanide leach methods. The maxlmu^ extraction of gold onto carbon occurred at about 15 minutes ¹when a recovery of 0.95 grams/tonne was obtained. This represents an increase of 0.7 grams/tonne over the gold assayed by the traditional methods, i.e. with no carbon in the leach solution. A· the ore sample used in all of these teste waa a cyclone overflow product with a else range of 851 minus 10pm, 1001 minus 18//m, there was no chance of a nugget effect influencing the results.

Thus there is at least 0.95 grams/tonne gold in this ore which ie leachable but 0.7 grams/tonne of which dost not report to the liquor in a standard leach test. if an equilibrium ls set up between the ore, liquor and carbon, then a change in pulp density from 50% to 1¾ would be expected to release gold cyanide complexes from the ore and make them available for adeorbtion by the carbon. The following tests were carried out to demonstrate this effect.

Initial leach Dilution stage

Pulp density 50% 1.0% pH 10.1 10.1

Time 15 min 30 min

CN ppm 100 1.0

Carbon

Concentration 00 200 g/1

Results: gold on carbon 21.5//g*l. 07g/t gold from a 20graa sample gold in liquor - 30^g/litce for 2 litres 60pg 3.0g/t gold in ore

- 15 This leach ie directly comparable with that in the first part of this example in that the final pulp density, cyanide concentration and carbon concentration were the same.

,. Thus, for the first leach, a maximum quantity of

0.95g/t gold was recovered from the carbon with a barren liquor contributing aero gold, for a total of 0.95g/t head gold.

For the second leach, a grade of 1.07g/t gold was ¢. recovered from the carbon with the liquor contributing another 3.0g/t gold, for a total of 4.07g/t gold.

Thus, in this case, it is possible to either leach as CIL at low pulp density, or to leach at high pulp density without carbon and than to dilute into an adsorption circuit at low pulp danalty with high carbon levels, the second option being preferable in this case.

Example 3

If saline water forms part of the pulp and carbon is used as the absorbent medium, it is important for maximum

2q gold recovery that the pH of the pulp entering the column be above pH Θ. The carbon will tend to raise the pH of the pulp to an equilibrium pH value which it dependent on the type and conditioning of the carbon present. Passing the pulp through a packed bed column of 2 metres in length at a column velocity of 1 cm/second will enable this equilibrium pH to be attained. if saline water is used in the pulp a . lower equilibrium pH is attained for the same entrance conditions. Examples of gold recovery from saline and non-saline pulps passing through a carbon column at various entrance pHs are as follows:- .,,

The pulp ors size was <10 //m, cyanide concentration

0/017.-pulp density 501 diluted to 1.0% after 30 minutes leaching at initial pH 9. — —----35 bad

- 16 5

Chloride	Column Entrance	Exit	AU	ppm in head
concentration	P«	PH	ore	absorbed in
liquor (g/l)			on	carbon
00	5.0	9.3		3.80
00	6.0	9.5		3.83
00	8.0	9.7		3.84
00	9.0	9.5		3.63
00	10.0	10.5		3.60
19	5.0	8.5		1.15
19	6.0	8.4		1.30
19	8.0	8.6		1.16
19	8.5	8.7		4.01
19	9.0	8.9		5.54
repeat
19	9.0	8.9		5.15
19	10.0	9.7		3.28
Example 4

l20

I|| the count of the testwork it was discovered that the presence of chloride ion· in th· liquor enhanced the recovery of gold par cycle onto carbon. The following tabla ahowa the affect of varying the chloride ion concentration in the leach liquor for a series of teats in which all other conditions wart the aan· as in example 3 with the column entrance p»9.

cl* concentration in liquor Gold recovered on carbon as g/l calculated as ppm in head ore

0.0 3.63 10 4.49 19 (average sea water level) 5.54 50 5.58

100 3.79 150 2.46

- 17 it is expected that the presence of other geld complexing agents would give similar results. It is also anticipated that other noble metals would respond similarly to the presence of agents with which they could complex. Example 5

The following are example's of the effect of the desorption time between carbon column passes. The pulp was pumped upwards through three carbon columns in aeriea. In between the carbon contacts, lmpellor agitated tanks were * used to allow desorption residence time for the pulp. The carbon columns were all 2 metres long and the carbon was subsampled in 25cm lengths along the column. in each case

carbon sample A is from the pulp entry end of the sample B is from the pulp exit end of the column.	column a
INITIAL	COLUMN 1	COLUMN	2	COLUMN	3
carbon	//gAu on	carbon	Au	carbon	//g Au on
sample	carbon	sample	on carbon	sample	carbon
λ	27.7	A	9.3	A	4.8
fi	22.0	B	6.2	B	3.6
C	12.2	C	4.5	C	2.9
D	5.9	D	3.2	D	1.4
B	3.1	B	2.0	I	0.6
F	3.2	F	1.2	F	0.3
G	2.0	G	0.5	0	0.4
H	1.5 Desorption	H 0.7 H 0.3 time between columns wee 15 minutes in

both cases. In each column the bulk of the gold was loaded on to the first 75cm of carbon, being carbons subsamples λ to c. For columns 2 and 3 the gold loaded onto the λ to c carbons subsamples was substantially higher than for the r > to H carbons eubsamples immediately preceeding them.

Calculations based on this data indicate that if desorption stages were not included between the carbon contacting stages, extremely long carbon columns would be required to obtain similar recoveries. BAD ORIGINAL

- ie -

μ% Au detected on carbon carbon aeh residue welght(g)

Ixaaple 6

The following ie an example showing the lessened recovery of gold from a pulp, when tonee of high pulp density are allowed to form on or in the absorbent material where the recovery it by adsorption onto carbon, The ore used is a <10μα fraction, chosen to avoid any nugget effej After leaching and dilution to llfb, the agitated pulp one portion of 2 litres passed through a carbon column another portion of 10 litres passed through an identical column containing froth carbon. The carbons were ashed «nA then subjected to an aqua regia digest followed by DXBX extraction. The disk was analysed for gold by AA8.

litre contact 10 litre contact 74 107

3.7 11.3

Thue the carbon from the 10 litre contact adsorbed only 1.5 times as much gold at the carbon from the 2 litre contact, The only difference between the two contacts was the much greater quantity of solids entrapped in the carbon during the 10 litre contact.

Thus for recovery of metal values from a pulp or partially clarified liquor, a preferred process adsorbent Fwould be one with minimal surface irregularities and ’ internal pores and fissures and having the metal bonding sites on the outer eurfece. Such en adsorbent la ion exchange fibre, but other adsorbents which minimise the formation of areas of high pulp density in, on or around the adsorbent materiel may also be used,

Example 7

The following ie an example of the leaching of gold ores using complexing agents other than cyanide. .The ore

Head	grade	(fire assay)	0.26	PP®	Au
Head	grade	(aqua regia)	0.21	ppm	Au
Head	grade	(standard CN
		bottle roll)	0.20	ppm	AU
Head	grade	(modified CN
		leech It P.D.)	1.01	ppm	Au
Head	grade	(modified Tu
		leach It P.D.)	2.81	ppa	Au

The modified CN leach conditions were - 0.1% CN,

P.D. 1%, pH 10.3, Tims 30 mins.

The modified Tu leach conditions were - 0.11 Tu,

P.D. 1%, pH 10, Time 30 mine.

After leaching, both the pulps were separately passed through 1 metre long carbon column at a flow velocity of lcm/sec. The gold adsorbed onto the carbon waa analysed by a standard athing of the carbon followed by boiling of the ash with aqua regia. The gold was extracted from the aqua regia into DISK which was analysed for its gold content by AA8. Thus In the above example both modified cyanide

2q and thiourea leachee extracted more gold than traditional methods of extraction and analysis, with the thiourea leach giving a greater gold value than the cyanide leach.

Example 8

This example ie a refractory sulphide ore milled to <10//m

Head grade (fire assay)

Head grade (aqua regia)

Head grade (standard CN bottle roll)

Head grade

AP 0 0 0 1 5 1 'modified CN leach) ^modified Cl leach)

20.82	ppa	Au
10.21	ppa	Au
5.62	ppm	AU
21.39	PP®	Au.
24.58	PP®	Au

The modified CN leach conditions were - 0.1% CN, P.D, 1%, pH 10.5, Time 6 Hr.

The modified Cl leach conditions ware - Cl pulp eB 1000 millivolts, P.D. It, pH 6.3, Tima 6 Hr.

BAO

- 20 Both the chlorine end cyanide leeches were run aa ClL, with carbon quantities of 200g/l. The gold adsorbed on the carbon waa analysed as described in the example 7.

The results show that both the modified cyanide and chlorine $ leaches extracted more gold than traditional methods of extraction and analysis, with the chlorine leach giving a greater gold value than the cyanide leach.

ft ^g***Pl· 5

The following ie an example of an ore leached by cyanide both with and without dispersants added to the pulp at the dispersant addition rate of 1 g/l.

Leach Pulp Carbon Dispersant Apparent head

	Density 1 solids by weight	Concentration (0/1)	(all 1 g/l)grade from gold detected in the
15				liquor or on the cerbon (ppm)
	5	00	Nil	1.59
	5	200	Nil	1.85
20	5	200	Sodium silicate	2.34
5	200	Sodium carbonate	1.81
25	5	200	tri-sodium polyphos- phate	2.05
	5	200	Sodium hexemeta- phosphate	3.02
30	5	200	Cyquest 3223	1.91
	10	00	Nil	1.40 .
I	10	200	Nil	1.53
35	10	200	Sodium hexameta- phosphate	1.94
10	200	Cyquest 3223	1.85 --

1.

Standard bottle roll cyanide hvar assay

- 21 rrom the above results it appear· that the dispersants are moat effective for this ore at pulp deneitiea of <101 solids by weight.

Example 10

This is an example of the use of ion exchange resin instead of carbon as a gold adsorbent. Gold recovery ie fro* an ore sited et <10 μη. This ore was assayed by fire assay at 0.15 ppm Au.

Ths or· was leached for 20 minutes at 50% PD with 0.01% CN added at a pH of 9.0. The pulp was diluted to 1% PD by the addition of a liquor containing 20 g/1 of Cl ions as sodium chloride. After dilution end adjustment of the pH to 9.0, the pulp was passed through a column containing a packed bed of a weak bate ion exchange reein. The resin used was PAZ4, developed by SELA INC. New York, U.S.A. The pulp flow velocity through the column wee 1 cm/eec. After being contacted by the pulp, the resin wae removed from the column, washed, dried and ashed at 600*c. The ashed residue wae digested in aqua regia and the aqua regia wae contacted with DISK which wae analysed for gold by AAS.

The gold detected represented a head grade of 1.36 ppm Au in the original ore, a duplicate had a grade of 1.24 ppm au in the original ore. Thus the uee of reein as an adsorbent allowed the detection of 1.21 ppm gold In the heed ore over and above that detected by fire assay.

Example 11

The following example indicates the desorbing effect of methanol at high and low pulp densities and the inability of carbon to recover gold from a pulp containing methanol.

Ore type - alluvial fines <38pm standard aqua regia assay 0.03ppm Au calculated ae gold in head

0.02 ppm 1.40 ppm

AP 0 0 0 1 5 1 bad original

Modified CM leach Cold from	Gold from carbon ppm 0.0	Total gold ppm 0.12
	at P.D. 301 liquor ppm (no carbon) 0.12
a)	R	CN leach at P.D. 30% (no carbon)
b).	R	+ 1 g/1 Methanol 0.20	0.00	0.20
c)	H	CN leach 4* 200 g/1 carbon
d)	It	P.D. 30% 0.06	0.28	0.34
·)	n	CN leach + 200 g/1 carbon P.D. 30% + Methanol 1 g/1 0.20	0.13	0.33
f)	R	CN leach at P.D. 5% (no carbon) 0.92	0.00	0.92
9)	R	CN leach at P.D. 5» + 1 g/1 (no carbon) Methanol 4.30	0.00	4.30
h)	R	cn leach * 200 g/1 carbon P.D. 5% 0.04	4.55	4.61
i)	R	CN leach + 200 g/1 carbon P.D. 5% + Methanol 1 g/1 4.06	0.79	4.85

Sxample 12

In another aspect of the patent, the recovery of metal values was accomplished by cementation onto zinc shavings. The ore used was sized at <10 μι& and had a head grade by fire assay of 0.18 ppm Au. After leaching and dilution the pulp had a condition of pH 8.5, CN 1 ppm and pulp density of 1%. The pulp wae split into two parte, each of 2 litres volume. The pH of Part 1 was adjusted with sodium hydroxide to 9.0 and the pH of the other part wae adjusted with sodium hydroxide to 10.0. Bach pulp was passed through a separate column containing sine shavings. The sine shavings ware dissolved in aqua regia and the gold values wsrs extracted into DIBK and analysed by AA8, The results were as follows:

Pulp pH

9.0

10.0

Blank

Ag Au analysed ss cemented onto sine

16.4

45.6

0.0

Por the 20 grams of ore present in each 2 litres of pulp, the recovered gold represented an apparent head grade of 0.92 ppm Au for the pH 9.0 pulp and an apparent head grade of 2.23 ppm Au for the pH 10.0 pulp. Both of these grades are in excess of the 0.18 ppm Au analysed by fire assay. Thus the use of zinc will allow the recovery of gold values from the process, the ssid values being greater than the values detected by traditional analysis. Deoxygenation waa not applied to the pulp, this represents a significant difference from standard zinc cementation procedurea which claim that the presence of CN levels in the pulp of 100 ppm or greater and deoxygenation are eaaential for succeasful gold recovery by this method.

Bxample 13 - Analytical technique

AP 0 0 0 1 5 1

The analysis of the carbon must be modified on account of the fine clay which enters into and is trapped within the carbon granules. When the carbon is washed, dried and ashed according to the standard carbon analysis method, this entrapped clay ie Inseparable from the ash. The standard procedure is to contact the carbon residue with aqua regia and to calculate the gold content of the ash from the analysis of the aqua regia aolution. -

- 24 However, it has been found that substantial amounts of the gold art adsorbed by the clay or other fine materiel remaining with the carbon ash. The gold may be removed from this materiel by recontacting the filtered residue from the _ init.ini aqua regia digest with fresh aqua tvglft gpn^ininq 20 g of carbon. This carbon is then ashed and digested with aqua regia as before.

However, tome of the clay ie entrapped in the carbon uaed in this second digestion, necessitating e repeat of the

IO cycle on this carbon aeh. This cycle is repeated until the gold assayed per cycle becomes statistically insignificant. Not all of the gold is extracted from the original ashed residue by this method, and repeated contacts of aqua regie end carbon are made until the gold extracted per cycle ie

IS again statistically insignificant. Thia method of aqua regie analysis ueing e low pulp density slurry and carbon may also be used on the head ore as well as carbon residues. In this case ae well es in cyanide leaching, it is found that the lower the pulp density the higher the quantity of recovered ₂0 gold. There le a practical limitation for the handling of aqua regia in this method, it ie found that the leeching of 20-25 g of ore in 500 millilitres aqua regie with 20 g carbon is a good working volume of aqua regia with an acceptable sample else. An example of the high volume aqua regia technique on a head ore is shown. The ore wee a calcine tailing from Cripple Creek in Colorado, U.S.A. of size less than 38 μ».

Head aeeay by fire aseaay is 1.43, (repeat 1.45 ppm)

Head assay by aqua regia/AAS wass 1.10 ppm, (repeat 1.13 ^ppm)

Conditions of the high volume aqua regia (HVAR) were g solids, 500 millilitres agua regia, 20 g carbon, 2 hr contact time.

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The analysis need not limited to these cycles, but may be extended

- 26 Th· following table show* the gold recovered at each •tag·.

mlcrograme of gold extracted per stage (1) 27.6 (2) 8.5 (3) 1.9 (4) 8.6 (5) 1.1 (6) 0.4 (7) 3.9 (8) 0.5 (9) ,, <0.3

Total: 52.5 j/g

Thli represent· a head grade of 2.1 g/t compared with the conventional aqua regia assayed grade of 1.10 g/t and the conventional grade by fire asiay of 1.43 g/t.

Example 14

An example of the HVAR technique used on a carbon from a modified cyanide leach le described in the following

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- 28 The result* showing the gold recovered at each stage 1* set out below:

Microgram* of gold extracted per stage (1) 81 (2) 93.5 (3) 11.9 (4) 60.0 (5) 7.7

The 81 vq of stage (1} represents the gold extracted by a standard aqua regia digest of an ashed carbon. Stages (2) to (5) lepresent the extra 173.1 χ/g of gold extracted by HVAR. It is obvious that a third cycle of HVJ\R on the solids marked (λ) would extract more gold, as wo^ld further cycle* of HVAR after analyses (3) and (5).

A shortcoming of the HVAR cycling technique ie that, because it ie done with hot aqua regia, an equilibrium is set up between the ore solids, the carbon and the liquor. With tome ore types the equilibrium favours the ore solids and so very little gold reports to the carbon or to the liqfcor.

Thie effect may be lessened by using a cold leaching process. This involve* the use of ambient temperature cyanide and aqua regia leaches with either oarbon or an organic extractant such aa DlBK In Lh« 1 vault. The organic extractant 1* preferred a* it may be read directly for gold content on an analyser such as an atomic absorption spectrophotometer.

By repeating ths cycle* of aqua regia and cyanide leaches on the solids residue, gold ie extracted at each cycle until such time as the gold extracted is statistically insignificant. Care must be taken to wash the solids residue so that it is free from liquor contamination from the previous cycle. This is to prevent an aqua regia-cyanide reaction, with the formation of toxic hydrogen cyanide gas.

- 29 An example illustrating the extra gold extracted by this method follows:

An ore (20 g sample) having a size less than 10 micron was assayed by repeated HVAR cycles.

A total of 42 micrograms of gold was obtained, with the last two HVAR cycles reporting <0.3 micrograma per cycle.

Thia represents a head grade of 2.1 g/t. The residue was then subjected to repeated alternating cycles of cyanide-DIBK and aqua regia-DIBK leaches.

The 1st cycle on the HVAR residue by cyanide - DIBK extracted 9.3 micrograms.

The 1st cycle on the cyanlde-DIBK residue by aqua regia-DIBK extracted 9.1 micrograme

The 2nd cycle by cyanlde-DIBK on the aqua regia-DIBK residue extracted 6.9 micrograme

The 2nd cycle by aqua regia-DIBK on the cyanide - DIBK residue extracted 7.6 micrograms

Obviously further cycles would extract further gold.

However a total of 32.9 micrograms extra has been extracted by the cyanide-DIBK and aqua regia-DIBK cycles. These 32.9 micrograms could not be extracted by HVAR cycles and represent a grade of 1.6 g/t over and above the 2.1 g/t analysed by HVAR techniques.

Bxample 15

A wet chemical analysis for silver was carried out on a 20g ore sample and the residue was retreated in 500 millilitres of acid with 20g of carbon added as per a high volume aqua regia cycle 1 (HVAR 1) analysis for gold.

Further cycles corresponding to HVAR cycles 2, IA and 2A were carried out. The results were as follows.

Standard analysis 3.5ppm Ag

AP 0 0 0 1 5 1

HVAR cycle 1 HVAR cycle 2 HVAR cycle IA HVAR cycle 2A

Total

2.6ppm 1.2ppm 3.8ppm 0.8ppm 8.4ppm Ag

- 30 Thu· the method of analysis for silver of dilute leaching of an ore with a silver adsorbent in the leach results in higher levels of silver being analysed than by the standard method. it le anticipated that this effect would extend to other noble metala, with the analysis and recovery being almllar to that used for gold, but with leachanta and adsorbents suitable for that particular material.

Example 15

Apart from the effects of low pulp density and high carbon additions, there is a third factor which la noticeable in some ores. This is the sising and separate processing of various six· fractions of the ore. A convenient else for wet screening it 38 micrometres. The effect of sising and leaching the 2 fractions separately is shown.

High Clay Ore - sising + 38 μη fraction 70.1% by weight - 38 μη fraction 29.9% by weight leach conditions for the head and the two fraction!

Pulp Density 5% CN 0,1% pH 10.5

Carbon 200g/lltre, 6 hr/each time

The gold grades determined from the leaching process wer·:Head 1.10 g/t + 38 pm 1.65 g/t - 38 μη 2.73

9/t

Calculated head from fractions - 1.973 g/t Hard Rock Haematite Ore - + 38 μη fraction 63.2% by weight

- 38 μη fraction 36.8% by weight

The calculated fire assay head grade from analysis of the fractions was 4.52 g/t Au.

When leached under the same conditions as the above example the determined gold grade· were: * 38 μη fraction 9,76 g/t - 38 μη fraction 6.12 g/t

Calculated head from fractiona 8.420 g/t.

For any particular ore, a else in the range of 2 to 100 microns needs to be selected. In some cases milling the ore to below the critical else may be a practical alternative to the separate treatment of the two slse fractions. --—

- 31 From the above it can be seen that this invention provides a unique method of assaying and of recovering gold that was not previously detected. =-----------5

SAV ·:ΰ NOW PARTICULARLY DESCRIBED ANO UCiftfAlNL'U MY/OUU ' invention ano in what manner Tir. SAME I·-.

TO BE PERFORMED. t/WE DECLARE THAT IM CLAIM lS- 32 -

Claims

THE CLAIMS DEFINING THg INVENTION ARE AS FOLLOWS|

1. A method of recovering noble metal values from ores, ore concentrates or tailings by taking the noble metals into solution to form a leach elurry and subsequently recovering the noble metals from solution the improvement comprising dilution of the leach elurry so that the pulp density is below 15% solids by weight.
2. A method ae claimed in claim 1 in which the # dilution is carried out after completion of the leaching stage, and the dilute pulp is held for a time sufficient to increase the quantity of noble metal in solution prior to commencement of the recovery stage.
3. A method ae claimed in claim 1 or 2 wherein the leaching solution ie diluted to a pulp density of 0,1 to 10% iolids by weight.
4. A method ae claimed in Claim 1 or 2 in which a llxiviant, preferably chloride ion, ie present in the pulp in addition to the leach agent.
5. A method ee claimed in Claim 1 or 2 in which the noble metal is recovered by use of a collecting agent preferably carbon or an ion exchange resin or fibre.
6. A method as claimed in Claim 5 in which the collection agent is held in a series of short columns and the solution of the noble metal is passed through the eeries of columns.
7. A method as claimed in claim 5 in which at least 65 grams of the carbon or ion exchange reein are used per litre of the solution..__ —--==cv.v.-. .,0W PARTICULARLY DCSCR-SC'· % H_£RfOHMED^X 0^-T L ,VE CUM”

6. A method as claimed in Claim 2 in which the leaching stage is carried out at pulp deneitiee above 40% solids by weight, the eolide are separated from the solution, the solution is treated to extract heavy metals and the eolide are repeatedly treated with the recycled solution from the metal extraction stage or with fresh leaching solution until the eolide have effectively been treated with the volume of that solution equivalent to the desired dilution.
9. A method as claimed in Claim 1 or 2 wherein a solution agent is used to increase the quantity of metal in solution following the leeching stage.
10. A method ae claimed in Claim 9 wherein the solution agent is a short chain polar organic compound preferably methanol or acetone.
11. A method as claimed in Claim 1 or 2 wherein a dispersant is added prior to or after dilution of the pulp to increase the amount of noble metal in solution.
12. A method ae claimed in any one of claims 9, 10 or 11 in Milch the method of recovery of the heavy metal ie selected from cementation, electrowinning or precipitation.
13. A method as claimed in Claim 1 wherein gold is recovered from ores by leaching at pulp densities above 10% solids by weight and at the end of the leach stage diluting the pulp to a pulp density between 0.1% and 10% solide by weight and holding the diluted solution for up to 15 minutes prior to separation of the gold containing liquor from the pulp eolide passing the diluted liquor through a series of columns containing carbon or ion sxhange resin. —AP 0 0 0 1 5 1 bad original

SAVING NOW PARTICULARLY OESCRiF'D ANO M* 'AID INVENTION AND IN WH* f f’'*

K ^PERFORMED. l/WE DECLARE THAT bttfc '

- 34 14. A method a· claimed in Claim 13 wherein the pulp passes to a dilution tank after the leaching stage and after the paas of liquor through a carbon or ion exchange reeln column the separated pulp solids are diluted with gold depleted liquor and held In a recirculating tank for a period sufficient to allow further gold to enter into solution, followed by further separation of pulp solids and gold extraction from the liquor in the carbon or ion exchange resine.