AU5416890A - Ion flotation with quaternary ammonium cationic surfactants - Google Patents

Description

ION FLOTATION WITH QUATERNARY AMMONIUM

CATIONIC SURFACTANTS

This invention relates to 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 physicochemical 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 ionizable, 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 chlorocomplexes 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 Charewicz and 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.

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 practise 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 quaternary ammonium bases as collectors for aurocyanide ions in alkaline solution in the absence of free cyanide or competing ions and also in mixed metal cyanide liquors.

We have now found that a class of quaternary

ammonium 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 cationic surfactant of formula (I): x- ( I ) wherein R¹ is a C₁₀ - C₁₈ alkyl group,

R³ is a lower alkyl group, or benzene ring optionally substituted with one or more lower alkyl groups, and

R² and R⁴ are lower alkyl groups;

or R¹, R² and R⁴ are methyl groups;

R³ is a benzene ring substituted with a

C₁₀ - C₁₈ alkoxy group; and X is a halogen atom.

Preferably the long chain (C₁₀ - C₁₈) alkyl or alkoxy group contains from 12 to 16 carbon atoms, most preferably 12 carbon atoms.

The term "lower alkyl", as used herein, refers 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 some preferred reagents for use in accordance with the invention are set out below. All of these compounds are known per se. Also shown for

comparison are formulae of some compounds (A,CTAB, and DTAB) which have already been proposed for use as

flotation reagents. Only A and CTAB have been used previously for the ion flotation of gold. Compounds B, D and R are also known per se, but have not been suggested previously for use as ion flotation reagents.

Some of the remaining compounds of formula (I) as defined above are new and the invention also includes these compounds per se. Methods for their synthesis of some of these compounds which are described in the literature, have proved unsatisfactory in our hands and the present specification therefore describes new methods for the preparation of these compounds as described hereinafter.

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

The compounds A, CTAB (cetyltrimethylammonium bromide) and DTAB (dodecyltrimethylammonium bromide) were obtained from commercial sources.

Example Cl Preparation of Compound B (a) Dlmethyldodecylphenylammonium bromide

Dimethyl aniline (24.2 g, 02 mol), dodecyl bromide (49.8 g, 0.2 mol), acetone (60 ml) water (60 ml) were refluxed together for 16 hours, with stirring. The mixture was then cooled and extracted with diethyl ether (3 x 50 ml). The aqueous layer was separated and

evaporated on the rotary evaporator under reduced

pressure (20 mm Hg). The brown gum was dried by adding ethanol

(50 ml) and azeotroping off the final traces of water.

The product which solidified on cooling to 5ºC was dissolved in ethanol, charcoaled and finally

recrystallised from ethanol/acetone (1:4 ratio), weight was 3.5 g, m.p. 97-98ºC.

The reaction had only proceeded to partial

completion and the organic layer from above was

evaporated down to remove the ether, treated with more water (100 ml) and refluxed with stirring for a further 48 hours. After the normal work-up procedure the weight of recrystallised product was 10.5 g. Total weight of recrystallised product was 14 g. The NMR and IR spectra were consistent with those predicted for the required product; m.p.

97-98°C.

Example C2 Preparation of Compound D (a) Preparation of 4-Dodecyloxyaniline To a mixture of 4-hydroxyacetanide (50.4g, 0.33 mol), dodecylbromide (83.1g, 0.33 mol) and absolute ethanol (100 ml), in a flask fitted with an overhead stirrer, reflux condenser and drying tube, was added a solution of sodium metal (7.8g 0.34 mol) in absolute ethanol (200 ml). The mixture was stirred and heated under reflux for 4 hours. The mixture was cooled and the precipitate of sodium bromide filtered. The ethanol was evaporated in vacuo and the crude product suspended in a mixture of water (1000 ml) containing cone. HCl (10 ml). The mixture was filtered and the product washed with water. TLC (EtOAc, silica) showed starting material at R_f = 0.38 and product at R_f = 0.52, thus indicating complete conversion, with no other products or starting material left. The 4-dodecyloxyacetamide was stirred and heated under reflux in a mixture of ethanol (500 ml) and 50% potassium hydroxide solution (200 ml) for 8 hours. The mixture was allowed to cool and extracted with ether (4 x 150 ml), dried (K₂CO₃) and evaporated in vacuo.

This gave 90 g of crude 4-dodecyloxyaniline. Tic (EtOAc, silica) showed a product of R_f = 0.63. A trace of starting material was present along with a red impurity. The product was recrystallised from ethanol using

decolourising charcoal and again several times from petroleum-ether bp 40-70º. Almost colourless plates were obtained m.p. 55-58° (Lit. 57-60º). Yield 44.7 g (49% overall). (b) Preparation of 4-dodecyloxytrimethylanilinium iodide

4-Dodecyloxyaniline (27.7 g, 100 mmol) was dissolved in dry DMF (65 ml) and cooled in an ice-bath. In a flask fitted with an overhead stirrer, reflux condenser, drying tube and pressure equalising addition funnel,

2,6-Lutadene (purified by refluxing and distilling from KOH, 21.4 g, 23.3 ml, 200 ml) was added with stirring, followed by slow addition of methyl iodide (70.1 g,

31 ml, 500 mmol). After the addition, the reaction mixture was allowed to stir at 0° for 1 hour, allowed to warm up to room temperature and left overnight. Dry acetone (refluxed and distilled from CaCl₂, 100 ml) was added to the solid mass and the mixture filtered. The product was refluxed in dry acetone (500 ml), cooled and filtered, then recrystallised twice from methanol, washed with acetone and dried in vacuo. Colourless plates, first crop 28 g, m.p. 163-5° (with frothing), remelt mp 160°. Second crop 6.3 g, same m.p. 77% yield total.

(c) Anion exchange to bromide salt

The iodonium salt (10 g, 22.4 mmol) was dissolved in chloroform (75 ml) [C] = 0.299 M and extracted with an aqueous potassium bromide solution containing 204.4 g KBr in H₂O (750 ml, [C] = 2.99 M), using a fast overhead stirrer for hour. Methanol (10 ml) was added and the layers allowed to separate. The lower milky organic layer was separated and a further 10 ml of methanol was added. Further standing gave clear layers. Tested for iodide in the organic layer by adding a small aliquot to a warm mixture of aqueous starch containing a few drops of dilute HCl and 30% hydrogen peroxide solution. (A blue-black colour indicated the presence of I, an

orange-yellow colour Br. If a blue-black colour was detected in the organic layer, the extraction had to be repeated using fresh solution). The organic layer was dried over silica gel,

filtered and evaporated in vacuo. A further starch test was applied to the solid to check if complete exchange had occurred. The bromide salt was recrystallised from methanol. Yield 7 g (78%) colourless plates m.p. 123-5° (froths). ¹H and ¹³C NMR indicated the correct product.

This procedure was repeated three times. Example C3 Preparation of Compound R (a) N - Dodecyl-3. 5-Dimethylaniline

A mixture of water (100 ml), 3,5-dimethylaniline (100 g) and sodium bicarbonate (70 g) was heated to

90-95°C. Dodecylbromide (150 g) was added over 1 hours, and the mixture heated with stirring for a further 8 hours. The mixture was cooled and the organic layer separated to yield the product as a viscous oil (130 g).

¹H and ¹³C NMR. were in agreement with the desired product.

(b) Methylation of N-Dodecyl-3, 5-Dimethylaniline

(Compound R) N-dodecyl-3, 5-dimethylaniline (30 g) was dissolved in DMF. (60 ml) containing a suspension of anhydrous

K₂CO₃ ( 14 g ) . The mixture was cooled in ice and

methyliodide (60 g) added. The mixture was stirred at 0° for 1 hour during which an effervescence occurred, followed by 48 hours at room temperature. The resultant solid was taken up in hot chloroform and filtered to remove insoluble KI. Chloroform was removed under reduced pressure and the residue was washed with ether. The crude iodide salt of compound R was collected by filtration, dissolved in chloroform (200 ml) and stirred with KBr (400 g) in water (1000 ml) for 3 hours. The chloroform layer was separated and stirred with further KBr (400 g) in water (1000 ml). Separation of the chloroform layer and removal of the solvent under reduced pressure gave the bromide salt of R as colourless

crystals (20 g). The reaction was repeated and the combined product recrystallised from ethyl acetate to yield compound R as colourless crystals (28 g). ¹H and

¹³ C NMR were in agreement with the desired product.

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 10 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 1L. 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 subsamples 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 aurocyanide 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 analyzed 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 analyzed 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 C_o 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 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] has a peak value called the "peak magnitude" and is a measure of the selectivity of the reagent. The peak magnitude usually occurs near the lower limit of surfactant frothability.

All feed solutions contained 10ppm each of gold and silver ion. The respective Figure number, air flow rates, peak upgrade ratios and peak magnitudes are given in Table 1. Also shown is the surfactant operating concentration range (moles surfactant/moles of gold present).

From the above results, it is clear that DTAB is a superior reagent in terms of upgrade ratio and peak magnitude compared to CTAB. However it takes a good deal more DTAB than CTAB to form a stable ion flotation foam.

The effect of reducing airflow (illustrated using DTAB) is to increase upgrade ratio and peak magnitude significantly. The surfactant dose required also

increases. This example is illustrative for all

surfactants.

Compound A is a superior reagent in all respects to CTAB at the same airflow. Compound A is not as efficient as DTAB in producing high upgrade ratios and peak

magnitudes. However the dosage of chemical required for ion flotation with Compound A is less.

Invention compounds B and D are superior to compound A at the same airflow in terms of upgrade ratio and peak magnitude. Also, lower doses of these chemicals are required compared to compound A.

Compounds B and D are superior to compound CTAB at the same airflow, in terms of upgrade ratio, peak

magnitude and dosage.

Compounds B and D are superior to compound DTAB at the same airflow, in terms of upgrade ratio and dosage. It is possible for compound DTAB to achieve slightly higher selectivity than the new reagents B and D, however this is not the major consideration in their use.

Compound R is a superior reagent to compounds DTAB, CTAB and A at the same airflow, in terms of upgrade ratio and dosage. However, compound R is not superior to the prior art in terms of peak magnitude. Compound R is superior to new compounds B and D in terms of upgrade ratio only.

REFERENCES

Sebba, F., Nature, 184, 1062 (1959) Rubin, A.J., Johnson, J.D., & Lamb, J.C.,

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

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

Analvtica Chimica Acta, 120, 237 (1980)

Mikhailov, V.N., Glazkov, E.N. and Larionov, E.V. Sb, Nauchn. Tr. Sredneaziat. Nauchno-Issled. Proektn. Inst. Tsvetn. Metall., (II), 1975, 103-107.

Charewicz, W. 6. Gondolla, T.,

Applied Chemistry. 15, 383 (1972)

Claims (8)

1. A method for ion flotation, characterised in that the flotation reagent employed is a cationic surfactant of formula (I):

X- (I) wherein R¹ is a C₁₀ - C₁₈ alkyl group,

R³ is a lower alkyl group, or benzene ring optionally substituted with one or more lower alkyl groups, and

R² and R⁴ are lower alkyl groups;

or R¹, R² and R⁴ are methyl groups;

R³ is a benzene ring substituted with a

C₁₀ - C₁₈ alkoxy group; and X is a halogen atom.

2. A method as claimed in Claim 1, characterised in that the long chain alkyl or alkoxy group contains from

12 to 16 carbon atoms.

3. A method as claimed in Claim 2, characterised in that the long chain alkyl or alkoxy group contains 12 carbon atoms.

4. A method as claimed in any one of Claims 1 to 3, characterised in that the lower alkyl group(s) contain from 1 to 3 carbons.

5. A method as claimed in Claim 1, characterised in that the compound of formula (I) is selected from:

Benzyldimethyldodecylammonium bromide,

Dimetbyldodecylphenylammonium bromide,

Trimethyl-p-dodecyloxyphenylammonium bromide,

N,N-dimethyl-N-dodecyl-3,5-dimethylanilinium bromide,

Cetyltrimethylammonium bromide, and

Dodecyltrimethylammonium bromide.

6. A method for the extraction of gold using ion flotation, characterised in that the flotation reagent employed is a cationic surfactant as defined in any one of Claims 1 to 5.

7. The use, as an ion flotation reagent, of a compound of formula (I), as defined in any one of the preceding claims.

8. The use as an ion flotation reagent in the ion flotation of gold cyanide, of a cationic surfactant of formula (I), as defined in any one of the preceding claims.