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 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 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 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 R ₁ N(R ₂ ) ₃ Br as collectors. iφ 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 Mij ailov et al. (1975) and Charewicz

- 3 - and Gendolla ⁽ 1972 ⁾ . m both cases such compounds were used ⁱ n the flotation of gold cyanide ions. _T he latter paper used both Au(CN) ₂ - and Ag(CN) ₂ - ions and various commercially available quaternary ammonium _b ases 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.

Schmuc ler ⁽ 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.

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 _d elays and improving productivity. Prior to 1894, gold was commercially leached from ores by chlorine but mo _d ern- _d ay practice involves cyanidation of ore material to produce the ^A u ⁽ C ^N) 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 aurocyanide ions in the absence of free cyanide or competing ions an _d also in mixed metal cyanide liquors.

We have now found that a class of non-ionic surface active compounds which have particular characteristic features are especially suitable for use as ion flotation reagents an ^d superior to the compounds use _d in the prior

art. The npn-ionic surfactant type includes chemicals which exist as protonated amines when dissolved in acidic solutions and which revert to insoluble long-chain species in alkaline solution.

According to one aspect of the present invention, there is p gvided a method for ion flotation in which the flotation reagent employed is a compound of the general formula (I):

NHP

/

R ¹ Y - C •HX (I) Q

wherein R ¹ is a. C ₁₀ - C ₁₈ alkyl group,

Y is O, S, NH, (CH ₂ ) _n or (CHR) _n where R is a lower alkyl group and n is 1 to 6. X is a halogen atom, and . P ^* an ' Q are the same or different groups and each is hydrogen or a lower alkyl, lower alkenyl, aryl or substituted aryl group.

' Preferably R ¹ contains from 12 to 16 carbon atoms, most preferably 12 carbon atoms.

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 carbon atoms.

When used as ion flotation reagents, the compounds of formula (I) require acidic conditions, if maximum selectivity (for gold) is to be achieved. Selectivity is highly dependent on pH. For example, compounds L and M (as described hereinafter) are only suitable for use as gold cyanide ion flotation reagents in media where the pH is less than 6.

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

The names and formulae of preferred reagents L and M for use in accordance with the invention are set out below. These compounds are known per se, but neither has been suggested previously for use as a gold ion flotation reagent. (Compound L has been used for iron cyanide precipitation and flotation but not for ion flotation and not for the recovery of gold or silver).

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 are described hereinafter, and in the literature.

(L) S-dodθcylisoihiourβa hydrobromidβ

(M) O-dodβcylisourea hydroc loridθ

SUBSTlTUf E SHEET

The invention, in its various aspects, is further descri-bed arid illustrated by the following non-limiting Examples- fAll temperatures are stated in degrees Celsius. )

PREPARATION OF FLOTATION REAGENTS

Example 1 Pr paration of Compound L

_

_ 16 g Thiourea (A.R. grade), 15 cc absolute ethanol and 51.5 g dodecyl rbromide were refluxed for 5 hours.

The clear solution was treated with 400 cc acetone

(redistilled), filtered and recrystallised from redistilled acetone to yield 54.5 g of product, m.p. 109-llloC.

Thin layer chromatography showed that the product contained a small amount of impurity.

Further recrystallation from acetone gave 52.2 g of crystals, m.p. 111<>C. The product (carbamimidothioic acid dodecyl ester) was shown to be pure by thin layer chromatography.

Example 2 Preparation of Compound M

18.63 g Dodecanol, 2.19 g cyanamide and 5.75 g cyanamide-dl'-hydrochloride were combined. The mixture was gradually heated to 80°C and well stirred for a period of 3 hours. The mixture was cooled, triturated with diethyl ether and filtered.

The product on drying yielded 12.2 g, m.p. 92-93°C.

Two repeat experiments were carried out on twice the above scale, giving products with m.p. 92-93°C.

All batches of the product (carbamimidic acid dodecyl ester) were pure by thin layer chromatography.

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 11 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 IP. 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 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 ubbled 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. Λur±fc_g 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 Besults

Gold recovery (material reporting to froth) as a function of time is calculated by the formula:

R% = (1 - C _t /C.) x 100

where C _£ is the liquid sub-sample gold concentration at time t, and C ₀ is the concentration in the initial feed. The ratio C _t /C ₀ 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.

where C _f is the concentration of gold in the product froth and C ₀ 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 trimethylammonium 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 ] is given the term selectivity. The peak value of selectivity usually occurs near the lower limit of surfactan frothability.

All feed solutions contained lOppm each of gold and silver cyanide ions. In all cases airflow was maintained at 224 cm ³ /min. The pH of the solutions was either 3 or 5 and was adjusted by the addition of sodium hydroxide or hydrochloric acid.

Example 3 (Figure 4) The surfactant used was Compound L (invention compound). Recovery (%) of silver is higher at pH=5 than for pH=3. Recovery of gold is much the same for either pH.

Example 4 { igure 5)

The surfactant used was Compound M (invention compound). Recovery (%) of silver is higher at pH=5 than for pH=3. Recovery of gold is much the same for either pH.

Example 5 (Figure 6)

The surfactant uεfed was Compound L. Upgrade Ratio (UR ₆₀ ) of gold is higher at pH=5 than at pH=3 for the same dose of surfactant. Ultimately, however, the upgrade ratio of gold does rise as high at pH=3 as for pH=5. Rejection of silver i-s best at pH=3.

Example 6 (Figure 7)

The surfactant used was Compound M.Upgrade ratio (UR ₆₀ ) of gold is highest at pH=3 and rejection of silver is superior at this pH.

Example 7 (Figure-8)

Selectivity Index (URj_ _u /UR _Ag ) plotted as a function of surfactant dosage indicates that Compound L is of higher * selectivity when used at pH=3 than at pH=5.

Example 8 (Figure 9)

Selectivity Index (UR _Au /UR _Ag ) plotted as a function of surfactant dosage indicates that Compound is of higher selectivity when used at pH=3 than at pH=5.

Example 9 (Figure 10)

Over a range of surfactant doses, Compound L is clearly more selective at a pH=3 than at pH=5.

Example 10 (Figure 11)

Over a range of surfactant doses, Compound M is clearly more selective at a pH=3 than at pH=5.

REFERENCES

Berg, E.W. & Downey, M.D., Analytica Chimica Acta, 120, 237 (1980);

Charewicz, W. & Gondolla, T., Applied Chemistry, 15, 383 (1972);

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

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).