AU6502090A - Ion flotation with anionic reagents - Google Patents

Description

ION FLOTATION WITH ANIONIC REAGENTS

This invention relates to an ion flotation reagents and to methods for their production and use. The invention is particularly, but not exclusively concerned with the extraction of gold using ion flotation

techniques.

Particulate flotation is a physiochemical method of concentrating valuable minerals from finely-ground ore. The process involves a selective treatment of the

valuable components to facilitate their attachment to air bubbles, which form a froth concentrate. Ideally, ion flotation is a procedure whereby valuable ions in a mixture of charged species are selectively removed by rising air bubbles. It resembles conventional froth flotation in that it employs a collector and similar equipment. It differs in that the substance to be separated is not usually present initially as a solid. The collectors are ionisable, surface-active organic compounds, cationic for the flotation of anions, anionic for the flotation of cations. These additives perform the dual function of complexing with the ions in solution and transporting these previously surface-inactive components to the foam phase. Such separation of ions is usually accomplished at low gas flow rates, producing a small volume of foam without tall columns or violent agitation of the liquid phase. Ion flotation is of enormous practical significance since ions are often successfully floated and concentrated from 10^-7 to 10^-4 M solutions. [NOTE: References are collected at the end of this

description].

The first of the low gas-flow rate foam separation techniques was introduced by Sebba in 1959. A surfactant ion of opposite charge to the ion to be removed was added in stoichiometric amounts. Sebba concluded that the collector must be introduced in such a way that it exists as simple ions and not micelles. The foam produced after subjecting this mixture to air bubbles then collapsed, thereby concentrating the inorganic ion. Rubin et al. (1966) investigated other variables associated with the technique, including the effect of metal ion

concentration, pH and temperature, using a soluble copper (II) ions recovered by a sodium lauryl sulphate (anionic) collector. Berg and Downey (1980) studied the use of quaternary ammonium surfactants of the type R₁N(R₂)₃Br as collectors in the flotation of anionic chloro-complexes of platinum group metals. The use of quaternary ammonium compounds as

collectors to remove precious metals from solution was further studied by Mikhailov et al. (1975) and Charewiczand Gendolla (1972). In both cases such compounds were used in the flotation of gold cyanide ions. The latter paper used both Au(CN)₂- and Ag(CN)₂- ions and various commercially available quaternary ammonium bases to determine the relative selectivity of the bases for one monovalent ion over the other. The former paper claims selective gold removal but the exact nature of the quaternary ammonium base used is unclear. Schmuckler (1969) studied gold removal from precious and base metal solutions by using a selective ion

exchange resin featuring the thiourea group. The paper claims selective recovery of noble metals from other materials due to the presence of this group. Other workers, for example Mooiman and Miller (1984) have used amine compounds as solvent extraction reagents for precious metals such as gold. Hodgkin (1981) has

developed an amine-based ion exchange resin to

selectively extract gold.

Because of the continuing interest in gold as a precious commodity, we have investigated the application of ion flotation to a current gold-extractive technology with a view to decreasing operational costs and delays and improving productivity. Prior to 1894, gold was commercially leached from ores by chlorine but modern-day practice involves cyanidation of ore material to produce the Au(CN)₂- ion. This procedure also results in the formation of cyanide complexes of iron, copper, lead, zinc, cadmium and silver. In particular we have

investigated the suitability of various surface active materials as collectors for aurpcyanide ions in the absence of free cyanide or competing ions and also in mixed metal cyanide liquors.

We have now found that a class of anionic surface active compounds which have particular characteristic features are especially suitable for use as ion flotation reagents and superior to the compounds used in the prior art. According to one aspect of the present invention, there is provided a method for ion flotation in which the flotation reagent employed is a compound of the general formula (I):

(I) wherein

Y⁺ is a metal cation or a hydrogen ion, and

either R¹ is an isothiouronium or amine group

m is 10 to 18,

n is O, and

R² is H, or R¹ is H

m is 10 to 18

n is 1 to 4, and

R² is an isothiouronium or amine group.

The isothiouronium group has the formula

The nitrogen atoms of the amine or isothiouronium groups (R¹, R²) may be substituted with one or two N- (lower)alkyl substituents.

Preferably m is from 12 to 16, most preferably 12. The terms "lower alkyl" and "lower alkoxy" as used herein refer to groups which contain from 1 to 6 carbon atoms, preferably 1 to 3 carbons.

The invention in a further aspect also provides the use, as an ion flotation reagent, of a compound of formula (I) as defined above.

Formulae of preferred reagents for use in accordance with the invention are set out below.

The compounds of formula (I) as defined above are new and the invention also includes these compounds per se, and the methods for their synthesis described hereinafter.

The compounds of formula (I) as stated above wherein R¹ is an isothiouronium group

m is 10 to 18,

n is 0, and

R² is H,

may be prepared by reacting a compound of formula (XII)

(XII) wherein X is a halogen atom and m is 10 to 18, with thiourea to give a compound of formula (XIII)

(XIII)

wherein X and m are as defined above, and treating the compound (XIII) with an alkali to give the desired salt.

Compounds of formula (XII) may be prepared by reacting a dialkyl malonate with a compound of the formula X-(CH₂)_m-X, wherein X is a halogen atom, m is 10 to 18, and R is an alkyl group, to give a compound of formula (XI)

(XI) wherein X, m and R are as defined above, and hydrolysing the compound (XI) to give the desired product.

Compounds of formula (I) wherein

R¹ is H

m is 10 to 18

n is 1 to 4, and

R² is an isothiouronium or amine group, may be prepared by hydrolysing a compound of formula ( IX) o

(IX)

or compound of formula (XX) o o o "

_{2 2 5 2}

(XX) wherein m is 10 to 18, n is 1 to 4, R¹ and R are alkyl groups and X is a halogen atom, to give the dicarboxylic acid (formula (IX) or (XX); R = H) and, if required, converting the acid to the desired metal salt. Compounds of formula (IX) may be prepared by reacting a compound of formula (VIII)

(VIII) wherein m, n, R and X are as defined in Claim 10, with an amine of formula HN(R¹)₂, wherein R¹ is an alkyl group.

Compounds of formula (XX) may be prepared by reacting a compound of formula (VIII)

(VIII) wherein m, n, R and X are as defined in Claim 10, with thiourea.

Compounds of formula (VIII) may be prepared by sequentially alkylating a dialkyl malonate with compounds of the formulae

H-(CH₂)_m-X

and X-(CH₂)_n-X

respectively, wherein m, n and X are as defined above .

The invention, in its various aspects, is further described and illustrated by the following non-limiting Examples. (All temperatures are stated in degrees

Celsius.)

PREPARATION OF FLOTATION REAGENTS

Example 1 Preparation of Compound H3 The preparation of Compound H3 is outlined in

Reaction Scheme 1. Diethylmalonate (VI) was sequentially alkylated with dodecylbromide followed by

1,3-dibromopropane to yield the diester (VIII). The diester (VIII) was contaminated with the intermediate diethyl dodecylmalonate (VII) which could not be removed by distillation, since the intermediate bromide (VIII) also cyclized on heating.

The crude bromide (VIII) was treated with excess dimethylamine in methanol/water by stirring at room temperature for 24 hours. Methanol, water and excess dimethylamine were removed on a rotary evaporator and finally ethanol was used for azeotropic removal of water. The residue was dried in vacuo.

Amine salts of the type (IX) were found to be partially ether soluble, particularly in the presence of diethyl dodecylmalonate (VII). They were finally

purified by precipitation from petroleum ether as the hydrochloride salt. The compound was found to be

extremely hygroscopic and difficult to handle.

The ester groups of (IX) were removed by hydrolysis using ethanolic potassium hydroxide. The reaction was monitored by infra-red spectroscopy, to observe the disappearance of the ester carbonyl band at 1740 cm^-1.

Normally ethanolic potassium hydroxide gives quantitative hydrolysis in a few hours at room temperature, but in this case very little reaction occurred after six days. The reaction mixture was finally heated at reflux to affect hydrolysis.

An ethanolic solution of the dicarboxylic acid was treated with concentrated hydrochloric acid to yield the hydrochloride salt Compound H3 on evaporation. The product was purified by extraction into a small quantity of absolute ethanol followed by decolourisation with charcoal. Evaporation of the solvent yielded the product (Compound H3) as a very hygroscopic white solid. The compound was stored in this form. Further dissolution of the product in dilute sodium hydroxide produced the sodium salt form of this compound.

Approximately 2g of the product was obtained from 20 gram of the crude bromide (VIII).

Example 2 Preparation of Compound G

Compound "G" is a thiourea adduct which can be made by the procedure shown in Reaction Scheme 2.

Sodium metal (7.0 g) was added to dry absolute ethanol (200 ml) in a 500 ml round-bottom flask fitted with a reflux condenser. Anhydrous powdered potassium iodide (4.0 g) was added down the condenser and the mixture heated on a water bath to dissolve the solid. Heating was continued whilst dry diethyl malonate (70.0 g) was added and the flask shaken to effect mixing. The mixture was heated for a further 30 minutes and then a cold solution of dibromododecane (105.0 g) in

tetrahydrofuran (75 ml) was added. After heating at reflux for 6 hours on the water bath, the mixture was cooled and the ethanol and tetrahydrofuran removed by evaporation under reduced pressure. Dilute hydrochloric acid was added to dissolve the solid and the solution was extracted with diethyl ether (2 x 50 ml). The combined organic phase was washed with water (50 ml), dried

(anhydrous sodium sulphate) and the diethyl ether removed by flash distillation. The residue was distilled under reduced pressure (230-238ºC/20 mm to obtain bromododecyl malonate (XI) (65.4 g).

Bromododecyl malonate (XI) (20.0 g) was heated at 75°C with 50% potassium hydroxide (20.0 g) for 10 hours in a round-bottom flask. The resultant solution was neutralised with concentrated hydrochloric acid until the pH was less than 5. A white precipitate formed and the solid was collected and washed with distilled water (2 x 50 ml). Bromododecyl malonic acid (XII) (16.2 g) so prepared was dried at 70°C.

Bromododecyl malonic acid (XII) (22.0 g) was

dissolved in dimethylformamide (420 ml) and to the solution was added thiourea (5.8 g). The reaction mixture was heated at 104°C for 6 hours and the dimethyl formamide then removed by evaporation under reduced pressure. The mixture was then cooled and a white precipitate formed. The solid was washed with distilled water and dried at 65°C to give compound (XIII) which melts at 76-79°C.

Sodium hydroxide (0.35 g) was dissolved in distilled water (35 ml) and heated to 80ºC. Compound (XIII) was added and the solution stirred for 2 hours to give a solution of Compound G.

REACTION SCHEME 2

Example 3 Preparation of Compound H

The preparation of Compound H is outlined in Reaction Scheme 3.

Diethyl 2-(3-bromopropyl)-2-dodecylmalonate (1g, 22mmole) (VIII) was dissolved in ethanol (2mL) and thiourea

(0.17g, 2.2 mmole) was added. The mixture was stirred and refluxed for 16 hours. A white crystalline solid separated. Ethanol was stripped off leaving a residue that was a mixture of white solid and a colourless viscous oil presumed to be unreacted bromo compound.

Ethanol (lmL) 'was added and the mixture refluxed a further 2 days. The ethanol was again stripped off leaving a residue that was again a mixture of white crystals and a viscous oil. The crystals (0.12g) (i) were filtered off, washing with dichloromethane. White lustrous plates separated from the filtrate on standing and evaporation of most of the dichloromethane. These were triturated with hexane and filtered, washing with more hexane, (0.30g) (ii).

IR and H¹NMR identified (i) as 1,3-disothiouroniumpropyl dibromide, formed from 1,3-dibromopropane impurity in the diethyl 2-(3-bromopropyl)-2-dodecylmalonate.

IR and C¹³ NMR of (ii) indicated this was the expected isothiouronium bromide. Yield 25%. 4.4 (Diethylcarboxy)hexadecylisothiouronium bromide

(0.25g, 0.476 mmole) was dissolved in 90% formic acid (lmL) and an equivalent amount of methane sulphonic acid added (0.92g, 0.062mL, 0.952 mmole). The mixture was stirred and refluxed for 5 hours. A brown gas was evolved during reflux and the solution turned a deep brown colour. Formic acid was stripped off on a rotary evaporator and the residue treated with water ( lmL). A cloudy solution was obtained but no solid separated. Water was stripped off and the residue triturated with ether, a whitish oil separating. Ether was evaporated off and the residue triturated with hexane, a small amount of white solid separating, (compound H).

REACTION SCHEME 3

ION FLOTATION

Reference will be made to the accompanying drawings in which:

Figure 1 is a diagram of the experimental apparatus used;

Figures 2 to 5 are graphs showing the results obtained.

Equipment

The flotation equipment used in the bench-scale laboratory experiments is illustrated in Figure 1 and consisted of a modified Hallimond tube cell or column 1 of volume approximately 1 litre. A sintered glass frit 2 in the base of the column allows air to pass through the cell from inlet 3, metered by appropriate flowmeters and regulators (not shown). Side ports 4,5 fitted to the column allow continuous monitoring of pH and/or

temperature (4) and removal (5) of small sub-samples of the liquid contents of the cell. The liquid feed to column enters through port 6 and the exit air stream flows out through port 7. The froth formed during flotation is discharged from the overflow lip 8 at the top of the cell and collected in another container (not shown). The column may be completely drained at the end of a batch experiment by using the tailings outlet port 9.

Procedure

A solution containing a known concentration of gold (as the auroσyanide ion) and a known molar ratio of surfactant to gold was prepared and mixed thoroughly. After adjustment of the pH to the desired level, the feed liquid was injected into the flotation cell through port 6 and the air supply connected to inlet 3. Air was then immediately bubbled into the cell and froth began to form at the top of the column. When the first drop of froth spilled over the upper lip of the cell, a timer was started and at known intervals after this point,

sub-samples of the liquid contents of the cell were removed via the side port and analysed for their gold content by atomic absorption spectrophotometry. At the completion of the experiment (when either the surfactant is exhausted or the elapsed time reaches a certain value) the air supply was disconnected and the collected froth and a sub-sample of the final cell contents were analysed for gold. During the test, pH was maintained at a constant level by adding appropriate quantities of acid or base, and the level Of water in the cell was also regulated to a constant depth by the addition of water through port 6. Handling of Results

Gold recovery (material reporting to froth) as a function of time is calculated by the formula: R% = (1 - C_t/C_o x 100 where C_t is the liquid sub-sample gold concentration at time t, and C₀ is the concentration in the initial feed. The ratio C_t/C_o represents the fraction of gold from the feed left in the cell at time t.

Another important parameter in ion flotation studies is the upgrade ratio, calculated by: UR = C_f/C_o where C_f is the concentration of gold in the product froth and C_o is the initial feed gold concentration. Varying the molar ratio of surfactant to gold affects both the recovery and the upgrade ratio in any experiment. For example, Figures 2 and 3 show the results obtained using a feed solution containing 50 ppm of gold and CTAB (cetyl trimethyl ammonium bromide) as the surfactant in various ratios.

When treating mixed solutions, containing both gold and silver, the upgrade ratio for silver is also

determined. The ratio of the upgrade ratios of gold to silver [UR_Au/UR_Ag] at the peak value of the upgrade ratio for gold is called the "peak magnitude" and is a measure of the selectivity of the reagent.

Example 4

The surfactant was Compound H3. The feed solution was a waste gold and silver cyanide mixture (Au_i=0.2ppm, Ag_i=0.6ppm). Copper of concentration 200 ppm was also found in this residue solution.

Figure 4 shows the upgrade ratios obtained for an air flow of 52.5 cm³/min.l at a PH value of 10.0.

The peak magnitude is 3.8 ± 0.2. Example 5

The surfactant was Compound H3. The feed solution was a waste gold and silver cyanide mixture (AUi=0.2 ppm, Ag_i=0.6 ppm). Copper of concentration 200 ppm was also found in this residue solution. The flotation was performed in a 34 litre ion flotation column of similar design to the apparatus illustrated in Figure 1.

Figure 5 shows the upgrade ratios obtained for an air flow of 35 cm³/min.l at a pH value of 9.5.

The peak magnitude is 29.9 ± 0.6.

REFERENCES

Berg, E.W. & Downey, M.D.,

Analvtica Chimica Acta. 120, 237 (1980);

Charewicz, W. & Gondolla, T.,

Applied Chemistry. 15383 (1972);

Hodgkin, J.H., 1981, Selective Extraction of Gold,

Canadian Patent 1174861.

Mikhailov, V.N. Glazkov, E.N. and Larionov, E.V. Sb, Nauchn. Tr. Sredneaziat.

Naucho-Issled. Proektn. Inst. Tsvetn. Metall., (II), 1975, 103-107;

Mooiman, M.B. and Miller, J.D.,

Selectivity Considerations in the Amine Extraction of Gold From Alkaline Cyanide Solution, Minerals & Metallurgical

Processing, p. 153, August, 1984.

Rubin, A.J., Johnson, J.D., & Lamb, J.C.,

I.& E.C. Process Design & Development. 5, 368 (1966);

Schmuckler, G., U.S. Patent 3,473,921 (1969);

Sebba, F., Nature. 184, 1062 (1959).

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

A method for ion flotation characterised in that the flotation reagent employed is a compound of the general formula (I):

(I) wherein

Y⁺ is a metal cation or a hydrogen ion, and

either R¹ is an isothiouronium or amine group

m is 10 to 18,

n is 0, and

R² is H, or R¹ is H

m is 10 to 18

n is 1 to 4, and

R² is an isothiouronium or amine group.

2. A method as claimed in Claim 1, characterised in that R¹ contains from 12 to 16 carbon atoms.

3. A method as claimed in Claim 2, characterised in that R¹ contains 12 carbon atoms.

4. A method as claimed in Claim 1, characterised in that the compound of formula (I) is one of the compounds G, H or H3, having the following formulae:

o

5. A method for the extraction of gold using ion flotation, characterised in that the flotation reagent employed is an anionic surfactant as defined in any one of claims 1 to 4.

6. The use, as an ion flotation reagent, of a compound of formula (I), as defined in any one of the preceding claims. 7. The use as an ion flotation reagent in the ion flotation of gold cyanide, of an anionic surfactant of formula (I), as defined in any one of the preceding claims. 8. A method for preparing a compound of formula (I) as stated in Claim 1, wherein

R¹ is an isothiouronium group

m is 10 to 18,

n is 0, and

R² is H,

characterised in that a compound of formula (XII)

(XII) wherein X is a halogen atom and m is 10 to 18, reacted with thiourea to give a compound of formula (XIII)

(XIII) wherein X and m are as defined above, and treating the compound (XIII) with an alkali to give the desired salt.

9. A method as claimed in Claim 8, characterised in that the compound of formula (XII) is prepared by reacting a dialkyl malonate with a compound of the formula

X-(CH₂)_m-X, wherein X is a halogen atom, m is 10 to 18, and R is an alkyl group, to give a compound of formula (XI)

(XI) wherein X, m and R are as defined above, and hydrolysing the compound (XI ) to give the desired product.

10. A method for preparing a compound of formula (I) as stated in Claim 1, wherein

R¹ is H

m is 10 to 18

n is 1 to 4, and

R² is an isothiouronium or amine group, characterised in that a compound of formula (IX)

(IX)

or compound of formula (XX)

(XX)

wherein m is 10 to 18, n is 1 to 4, R¹ and R are alkyl groups and X is a halogen atom, is hydrolysed to give the dicarboxylic acid (formula (IX) or (XX); R = H) and, if required, the acid is converted to the desired metal salt.

11. A method as claimed in Claim 10, characterised in that the compound of formula (IX) is prepared by reacting a compound of formula (VIII)

(VIII) wherein m, n, R and X are as defined in Claim 10, with an amine of formula

HN(R¹)₂, wherein R¹ is an alkyl group.

12. A method as claimed in Claim 11, characterised in that the compound of formula (VIII) is prepared by sequentially alkylating a dialkyl malonate with compounds of the formulae H-(CH₂)_m-X

and X-(CH₂)_n-X

respectively, wherein m, n and X are as defined in

Claim 10.

13. A method as claimed in Claim 10, characterised in that the compound of formula (XX) is prepared by reacting a compound of formula (VIII)

(VIII) wherein m, n, R and X are as defined in Claim 10, with thiourea.

14. Compounds of the formula (I) as stated and defined in Claim 1.

15. Compounds of the formulae (G), (H) and (H3) as stated in Claim 4.