Process for extraction of metals from aqueous ammoniacal solutions with beta-diketone extractants while maintaining a voltage potential to control entrainment of the aqueous layer into the organic layer

Improvement in the process of extracting metals, particularly copper, from aqueous ammoniacal solutions with beta-diketone extractants dissolved in a water immiscible hydrocarbon diluent, such as a kerosene, in which the improvement comprises application of a voltage potential across the interface of the aqueous and organic layers of an extraction circuit with an electrostatic coalescer, and maintaining the voltage potential until the layers have been separated, to control entrainment of the aqueous layer into the organic layer, thereby significantly reducing metal production cost and providing improved metal quality by minimizing ammonia entrapment and carry over to the aqueous stripping solution from which the metal is recovered, typically by electrowinning of the aqueous acid strip solution.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to an improvement in the process of extracting 
metals, particularly copper, from aqueous ammoniacal solutions employing 
as the extractants beta-diketone compounds dissolved in a water immiscible 
hydrocarbon diluent. In particular, the improvement comprises the use of 
electrostatic coalescers to control entrainment of the aqueous phase in 
the organic extractant phase to significantly reduce metal production 
costs and to provide improved metal quality recovery by minimizing ammonia 
entrainment and carry over to the aqueous stripping solution from which 
the metal is recovered, typically by electrowinning of the aqueous strip 
solution in a tankhouse. 
Beta-diketones are typically employed to extract a metal, such as copper, 
from aqueous ammoniacal leach liquors. A typical circuit involves two 
stages of extraction, a wash stage and one to two stripping stages with an 
aqueous acid stripping solution. Entrainment of the ammoniacal aqueous 
phase in the organic, and carry over to the stripping solution from which 
the metal is eventually recovered, results in higher metal production 
costs and poorer quality of the metal recovered. 
Electrostatic coalescers have been employed in the past to break water in 
oil emulsions in the oilfields. Such coalescers have been employed in an 
attempt to reduce aqueous entrainment in a process of extracting copper 
from aqueous acidic leach solutions. Such systems from acid leach 
solutions, do not however pose the problems associated with ammoniacal 
solution entrainment, and accordingly do not solve all the entrainment 
problems posed in extraction of metals, particularly copper, from aqueous 
ammoniacal solutions. 
DESCRIPTION OF THE INVENTION 
In this description, except in the operating examples or where explicitly 
otherwise indicated, all numbers describing amounts of ingredients or 
reaction conditions are to be understood as modified by the word "about". 
While the present invention is particularly useful in applications where 
ammoniacal leach solutions are encountered in the treatment of copper 
containing sulfidic ores, the present invention is applicable or useful in 
the extraction of copper from any aqueous ammoniacal solution containing 
copper values regardless of its source. The invention may also be employed 
in relation to other metals to be extracted from aqueous ammoniacal 
solutions, such as nickel or zinc. 
In its general application accordingly the present invention is an 
improvement in the process of extraction and recovery of a metal, 
particularly copper, from aqueous ammoniacal solutions in a process 
comprising: 
(1) contacting a metal pregnant aqueous ammoniacal solution containing 
metal values with a water insoluble beta-diketone metal extractant 
dissolved in a water immiscible organic diluent in the absence of a 
voltage potential to extract the metal values from said aqueous ammoniacal 
solution into said organic solution to form two layers, (a) a metal 
pregnant organic layer and (b) a metal depleted aqueous layer; 
(2) applying a voltage potential across the interface between the metal 
pregnant organic layer and the metal depleted aqueous layer to coalesce 
entrained droplets of the aqueous phase present in the organic layer to 
substantially reduce entrainment of the aqueous phase in the organic 
layer; 
(3) separating the metal pregnant organic layer from the metal depleted 
aqueous layer while maintaining the voltage potential until the separation 
has been completed; 
(4) contacting the metal pregnant organic layer with an aqueous acidic 
stripping solution, whereby metal values are stripped from the organic 
layer into the aqueous acidic stripping solution; 
(5) separating the aqueous acidic stripping solution from the organic 
layer; and 
(6) recovering the metal from said aqueous acidic stripping solution. 
In the above process it is critical in step (3) to maintain the voltage 
potential until separation of the layers has been completed, since it has 
been discovered, surprisingly, that when the voltage potential is removed 
prior to separation, entrainment of the aqueous phase into the organic 
phase quickly rises to unacceptable levels. 
The voltage potential in steps (2) and (3) can conveniently be obtained by 
the use of electrostatic coalescers. 
With the use of the electrostatic coalescers the entrained coalesced 
aqueous phase is separated from the organic layer and returned to the 
aqueous layer. With application of a voltage potential across the 
interface of the aqueous and the organic layers, significant reduction in 
the entrainment of the aqueous layer in the organic layer is achieved. 
Typically a weakly acidic wash of the separated organic layer is carried 
out prior to the strong acid stripping of the metal values from the 
organic layer in step (4). Entrainment of the aqueous layer in the organic 
layer results in transfer of ammonia to the weakly acidic wash stage. 
Increased entrainment results in increased acid demand in the w ash stage 
to neutralize the ammonia. Similarly, entrainment of the aqueous wash 
liquor in the washed loaded organic layer results in the transfer of 
ammonium ions to the electrowinning tankhouse and higher ammonium levels 
in the tankhouse will result in poorer quality of the metal recovery 
cathodes, which do not command a premium price in the marketplace. Also 
entrainment of the strip solution in the stripped organic results in 
transfer of acid to extraction where it neutralizes free ammonia upsetting 
the balance in the leaching of the metal into the ammoniacal aqueous leach 
solution. Commercial plant operations employing beta-diketones as the 
extractant in the extraction reagent comprised of a solution of the water 
insoluble diketone extractant in a water immiscible hydrocarbon diluent, 
such as a kerosene, suffer from all of the above problems. Attempts to 
reduce the entrainment by passing the organic layer through the usual 
(non-electrostatic) coalescing media have proven to be ineffective. It has 
now been discovered that applying a voltage potential across the interface 
of the organic and aqueous layers and maintaining the voltage potential 
until the layers have been separated successfully and significantly 
reduces aqueous entrainments in the beta-diketone containing organic 
layer, thereby minimizing, if not eliminating, all the operational 
problems discussed above. 
As discussed above, typically a weakly acidic washing stage at a pH of 
about 6-7 is carried out prior to the stripping step (4) in which a strong 
acid stripping solution is employed and controlled and carefully 
maintained to ensure a pH electrolyte of about 3-4, so that the acid in 
the tankhouse spent must balance, or be less than the acid required to 
strip the metal from the organic phase. Typically the circuit incorporates 
sulfuric acid as a pH control, and sulfuric acid is the preferred acid 
stripping solution. 
The beta-diketone extractants, particularly useful for extraction of metals 
such as copper, zinc or nickel, are more specifically defined as having 
the formula 
##STR1## 
where R is phenyl or alkyl substituted phenyl, R' is alkyl, 
alkylsubstituted phenyl or chloro substituted phenyl and R" is H or CN 
with the provisos that (1) when R is phenyl, R' is a branched chain alkyl 
group of at least 7 carbon atoms and (2) when R is alkyl substituted 
phenyl, the number of carbon atoms in the alkyl substituent is at least 7 
and at least one such substituent is a branched chain. R is desirably a 
monoalkyl substituent and preferably contains 9 or more carbon atoms. The 
various alkyl groups are preferably free from substitution and contain 
less than 20 carbon atoms. Accordingly, the alkyl groups will contain at 
least 7, preferably 9 or more carbon atoms, up to about 20 carbon atoms. 
Further, when R' is alkyl, the carbon alpha to the carbonyl group is 
desirably not tertiary. Preferably, R" is H, R is a branched 7, 8, 9, 12 
or 17 carbon chain or a chlorophenyl or a short chain (1-5 carbon) alkyl 
substituted phenyl and R is phenyl or a 7, 8, 9 or 12 carbon alkyl 
substituted phenyl group. One preferred beta-diketone is 
1-phenyl-3-isoheptyl-1,3-propanedione. 
Other preferred diketones are those which are sterically hindered. Such 
compounds are modifications of the beta-diketones defined in the formula 
above in which the substituents are such as to provide a sterically 
hindered beta-diketone. The sterically hindered diketones can be 
represented by the formula [II] below: 
##STR2## 
The hindered beta-diketones are those where R is phenyl or alkyl 
substituted phenyl, R' is alkyl, and R", R'" and R"" are the same or 
different and are chosen from the group consisting of H, alkyl having from 
1 to about 8 carbon atoms, and aralkyl having from 7 to about 14 carbon 
atoms, with the proviso that (a) no more than two of R", R'" and R"" are H 
and (b) the total number of carbons in all R groups is at least 13. 
Preferred compounds are those in which (i) R is phenyl, R" is methyl or 
benzyl, R' is branched hexyl, and R'" and R"" are H; (ii) R is phenyl, R" 
is H, R' is butyl, R'" is ethyl and R"" is H, and (iii) R is phenyl, R" is 
H, R' is a straight or branched chain alkyl group containing from 5-8 
carbon atoms and R'" and R"" are methyl. 
In the process of extraction a wide variety of water immiscible liquid 
hydrocarbon solvents can be used in the metal recovery process to form the 
organic layer in which the diketone extractant is dissolved. These include 
aliphatic and aromatic hydrocarbons such as kerosenes, benzene, toluene, 
xylene and the like. A choice of essentially water-immiscible hydrocarbon 
solvents or mixtures thereof will depend on factors, including the plant 
design of the solvent extraction plant, (mixer-settler units, extractors) 
and the like. The preferred solvents for use in the present invention are 
the aliphatic or aromatic hydrocarbons having flash points of 130.degree. 
Fahrenheit and higher, preferably at least 150.degree. and solubilities in 
water of less than 0.1% by weight. The solvents are essentially chemically 
inert. Representative commercially available solvents are CHEVRON.TM. ion 
exchange solvent (available from Standard Oil of California) having a 
flash point of 195.degree. Fahrenheit; ESCAID.TM. 100 and 110 (available 
from Exxon-Europe) having a flash point of 180.degree. Fahrenheit; 
NOR.TM. 12 (available from Exxon-USA) with a flash point of 160.degree. 
Fahrenheit; CONOCO.TM. C1214 (available from Conoco) with a flash point of 
160.degree. Fahrenheit; and Aromatic 150 (an aromatic kerosene available 
from Exxon-USA having a flash point of 150.degree. Fahrenheit), and other 
various kerosenes and petroleum fractions available from other oil 
companies, such as the ORFORM.TM. SX series of solvent extraction diluents 
(available from Phillips 66: SX 1, 7, 11, and 12 each having a Flash Point 
above 150.degree. F. varying up to 215.degree. F.); and ESCAID.TM. series 
of hydrocarbon diluents (available from Exxon: 100, 110, 115, 120, 200 and 
300, each having a Flash Point above 150.degree. F.; and EXXOL.TM. D80 
solvent (also available from Exxon and having a Flash Point above 
150.degree. F.). 
In the extraction process, the organic solvent solutions may contain the 
beta-diketone in an amount approaching 100% solids, but typically the 
diketone will be employed in an amount of about 20-30% by weight. 
In the process, the volume ratios of organic to aqueous (O:A) phase will 
vary widely since the contacting of any quantity of the oxime organic 
solution with the metal containing aqueous solution will result in the 
extraction of metal values into the organic phase. For commercial 
practicality however, the organic(O) to aqueous(A) phase ratios for 
extraction are preferably in the range of about 50:1 to 1:50. It is 
desirable to maintain an effective O:A ratio of about 1:1 in the mixer 
unit by recycle of one of the streams. In the stripping step, the 
organic:aqueous stripping medium phase will preferably be in the range of 
about 1:4 to 20:1. For practical purposes, the extracting and stripping 
are normally conducted at ambient temperatures and pressure although 
higher and lower temperatures and pressures are entirely operable. It is 
preferable to strip at elevated temperatures. While the entire operation 
can be carried out as a batch operation, most advantageously the process 
is carried out continuously with the various streams or solutions being 
recycled to the various operations in the process for recovery of the 
metal, including the leaching, extraction and the stripping steps. 
In the extraction process the extractant should be soluble in the organic 
water-immiscible solvent. In general the diketones of the present 
invention will be soluble to such an extent and amount described above. If 
necessary or desirable to promote specific desired properties of 
extraction, solubility modifiers generally known in the art may be 
employed. Such modifiers include long chain (6-30) carbon aliphatic 
alcohols or esters such as n-hexanol, n-2-ethylhexanol, isodecanol, 
isohexadecanol, 2-(1,3,3 trimethylbutyl)-5,7,7-trimethyl octanol and 
2,2,4-trimethyl-1,3-pentanediol mono-or di- isobutyrate, long chain 
phenols, such as heptylphenol, octylphenol, nonylphenol and dodecylphenol; 
and organo phosphorous compounds such as tri-lower alkyl (4-8 carbon atom) 
phosphates especially tributyl phosphate and tri-(2-ethylhexyl) phosphate. 
Where indicated to be desirable, kinetic additives may also be employed. 
In the stripping step (4), a sulfuric acid solution containing greater than 
about 5 to about 200 g/l sulfuric acid is the preferred stripping agent as 
it permits the subsequent recovery of the copper by conventional recovery 
steps either in the form of copper sulphate crystals or by electrowinning 
to cathode copper. Electrowinning is typically the preferred means of 
recovery of the metals from solutions suitable for electrowinning. Other 
mineral acids may be used such as hydrochloric or nitric; however, such 
may require other recovery methods or specialized handling equipment.

The invention can be further illustrated by the following example. 
EXAMPLE 
In this example, the extraction circuit employed one extraction stage, one 
wash stage and one stripping stage to illustrate the invention. The 
aqueous ammoniacal feed solution was a standard commercial feed containing 
30 gpl Cu, 1.5 gpl Zn, 38 gpl ammonia, 55 gpl sulfate. The organic phase 
was a commercial organic phase of 1-phenyl-3-isoheptyl-1,3-propanedione 
dissolved in kerosene. Aqueous entrainment, without any coalescer 
treatment, in the loaded organic layer averaged 9500 ppm. When a 5000 VAC 
(volts alternating current) potential was applied across the interface 
between the organic layer and the aqueous layer, the aqueous entrainment 
was reduced to 1500 ppm. When the voltage was turned off and the two 
layers allowed to remain in contact, the aqueous entrainment level 
returned to 9500 ppm within a short period of time. 
Based on the foregoing example, the use of electrostatic coalescers, 
applying a voltage potential across the interface of the aqueous and 
organic layers in an extraction circuit employing a beta-diketone 
extractant and using the process of the invention will successfully and 
significantly reduce the aqueous entrainment in the organic extractant 
phase. 
As indicated earlier, the present invention is particularly useful in 
regard to recovery of copper from aqueous ammoniacal solutions. Such 
solutions are found from a variety of sources such as those encountered in 
leaching of chalcocite concentrates. In such applications the copper 
pregnant leach solutions from which the copper is to be recovered by 
extraction will contain on the order of about 15-170 g/l copper and 
typically about 30-40 g/l copper at a pH of about 8.5 to 11. In solutions 
encountered from other applications, the solutions may contain copper at 
higher levels, on the order of 125-170 g/l, such as solutions encountered 
in ammoniacal copper chloride printed board etchants.