Electrochemical process for recovering metallic rhodium from aqueous solutions of spent catalysts

Metallic rhodium is recovered from aqueous solutions of spent catalysts by acidification and oxidation followed by electrolysis.

The present invention relates to an electrochemical process for recovering 
rhodium in metallic form from aqueous solutions of spent catalysts. 
Rhodium, generally in the form of a salt or complex, is employed as a 
catalyst for performing many syntheses in organic chemistry. 
Rhodium is a relatively rare metal and its cost is high. It is particularly 
important to be able to recover it from a catalyst which has lost its 
effectiveness, so that it can be recycled. 
It is known to recover rhodium from complexes which are soluble in an 
organic medium. Generally, the known processes consist in destroying the 
catalyst (e.g. by oxidation, acidic or alkaline attack, displacement by a 
stronger ligand, or combustion) to produce a recoverable form of the 
rhodium (e.g. a soluble or insoluble inorganic rhodium salt, or a complex 
of sufficient polarity) from which the rhodium can be recovered (e.g. by 
electrochemical reduction, filtration, combustion, liquid/liquid 
extraction, or fixation on a support). 
However, in the case of organic rhodium complexes, electrochemical 
reduction does not produce metallic rhodium but derivatives in which 
rhodium is in the oxidation state of zero (see, for example, E. Markrlik 
et al, J. Organomet. Chem., 1977, 142 (1), 95-103). It is possible, 
however, to obtain the deposition of metallic rhodium on an electrode by 
employing inorganic salts or inorganic complexes of rhodium in the 
presence of judiciously chosen electrolytes (see, for example, Chem. 
Abstr. 1984, 100, 147437q). 
It is known to employ water-soluble catalyst systems consisting of an 
inorganic or organometallic rhodium derivative and of a water-soluble 
phosphine (sodium salt of trisulphonated triphenylphosphine or NaTPPTS) to 
perform chemical reactions. For example, by employing a catalyst of this 
kind it is possible to perform the selective addition of a compound 
containing an active methylene group, such as a .beta.-ketoester, to a 
terminal conjugated diene (European Patent EP No. 44,771), the addition of 
a cyclic secondary amine to a terminal conjugated diene (European Patent 
EP No. 185,559), the selective alkylation of phenols (European Patent 
Application EP No. 161,132), or olefin hydroformylation (FR No. 
2,349,652). In general, when the reaction is finished, the aqueous phase, 
whose pH is close to neutrality, consists essentially of rhodium in the 
form of organometallic complexes, of sulphonated phosphines and of their 
degradation products (phosphine oxides), water, the alcohol (methanol) 
employed as cosolvent and of organic products (reactants and reaction 
products). Although the catalyst can be recycled, it must be regenerated 
after a determined number of operations. 
It has now been found, and it is this that forms the subject of the present 
invention, that it is possible to recover rhodium in metallic form by 
electrochemical reduction of an aqueous solution of a water-soluble 
catalyst containing essentially an inorganic or organometallic rhodium 
derivative, a water-soluble phosphine and its degradation products, which 
has previously been subjected to an appropriate oxidation and 
acidification treatment. 
The process of the present invention for recovering rhodium in metallic 
form from an aqueous solution containing an organic rhodium complex and in 
particular from a solution of a spent catalyst as described above 
comprises 
(a) treating the aqueous solution with an oxidizing agent and a strong 
acid, 
(b) electrolysing the solution thus obtained between an anode and a 
cathode, and 
(c) recovering metallic rhodium deposited on said cathode. 
The oxidizing treatment is intended to convert the residual trivalent 
phosphorus into pentavalent phosphorus and the monovalent rhodium to 
trivalent rhodium. The acidification is necessary for the rhodium to be in 
an electroreducible form. 
It is particularly advantageous to carry out the pretreatment on a 
preconcentrated aqueous solution of spent catalyst in order to have a 
solution containing a higher concentration of rhodium, from which most of 
the organic solvents, reaction products and the residual reactants have 
been removed. Generally, a concentrated solution is used whose volume 
represents approximately one third of the initial volume, so as to have a 
rhodium concentration of between 1 and 5 g/liter. The concentration is 
carried out by distillation, under reduced pressure if desired. 
The oxidation of the solution of spent catalyst, concentrated if desired, 
is carried out by means of an agent of high oxidative power, preferably an 
alkali metal hypochlorite or chlorate, such as aqueous bleach or sodium or 
potassium chlorate, but other oxidizing agents, e.g. hydrogen peroxide, 
can also be used. The oxidation can be effected at a temperature of 
between 20.degree. C. and the reflux temperature of the reaction mixture. 
The oxidation is preferably carried out at the reflux temperature and is 
complete after a time which can be from 1 to 4 hours. 
The acidification is carried out with the aid of a strong acid, so that the 
pH of the final solution is below 3 and, preferably between 0 and 1. It is 
particularly advantageous to employ a strong inorganic acid and especially 
hydrochloric, sulphuric, nitric or phosphoric acid. The acidification is 
preferably carried out at the reflux temperature of the reaction mixture, 
with the heating being continued for 1 to 4 hours. 
For the purpose of performing the pretreatment of the aqueous solution of 
the spent catalyst system it is immaterial whether the acidification 
follows or precedes the oxidation. 
The solution which is obtained after the oxidation and acidification 
treatments can be electrolysed directly. 
The electrolysis may be carried out continuously or noncontinuously. 
The electrolyser consists essentially of a cathode and an anode and, if 
desired, a diaphragm separating the cathode and anode compartments. The 
electrolyser may additionally comprise a reference electrode such as a 
saturated calomel electrode. 
The cathode generally consists of an electrically conductive material whose 
melting point must be sufficiently low to permit the recovery of rhodium 
and which must withstand the corrosiveness of the medium. It is 
particularly advantageous to employ a cathode made of stainless steel, 
mercury, copper or lead. A copper or lead cathode is preferably employed. 
The anode generally consists of an electrically conductive material which 
cannot be attacked under the conditions of the electrolysis. It is 
particularly advantageous to employ a graphite anode. 
When a separator diaphragm is employed, this advantageously consists of a 
porous material such as, for example, a sintered glass plate or an ion 
exchange, preferably cation exchange, membrane. When a separator diaphragm 
is employed, the anode compartment contains an electrolyte which is 
preferably the acid employed to perform the acidification during the 
pretreatment operation. 
When the electrolyser does not comprise a reference electrode, the current 
density at the beginning of the electrolysis is determined so that the 
reduction of rhodium is at its maximum while the reduction of the protons 
present in the strongly acidic medium is limited. The current density is 
generally between 0.5 and 2 A/dm.sup.2. Because of the simultaneous 
reduction of some of the protons present in the medium, the electrical 
yield is not quantitative. The electrolysis is generally stopped after the 
passage of 50 faradays per gram-atom of rhodium. 
When the electrolyser comprises a reference electrode (saturated calomel 
electrode), the potential applied to the cathode is chosen so that the 
reduction of rhodium is at its maximum while the reduction of the protons 
present in the strongly acidic medium is limited. The cathode potential is 
generally between -0.25 and -0.55 volt relative to the reference 
electrode. 
In order to perform the process according to the invention, an electrolyser 
is generally employed in which the anode, the cathode and the separator 
diaphragm are in parallel vertical planes. However, when a mercury cathode 
is employed, the anode, the cathode and the separator diaphragm are in 
parallel horizontal planes. 
An electrolyser comprising several anodes and cathodes arranged alternately 
and connected to the electrical supply by parallel circuits may be 
employed. 
Furthermore, it is possible to combine a number of elementary electrolysers 
in series. 
It is particularly advantageous to agitate the solution to be electrolysed 
either by means of a mechanical or magnetic stirrer or by circulation 
produced by a pump. 
It is also possible to employ electrolysers to which additional devices are 
attached, such as heat exchangers, expansion vessels or instruments for 
measuring the temperature or the pH. 
When the electrolysis is finished, the cathode(s) is (are) withdrawn from 
the solution and then rinsed with water and, if desired, with an organic 
solvent such as methanol or acetone and is (are) finally dried. 
Depending on the adhesiveness of the deposit on the cathode, this is 
recovered by mechanical scraping or else is detached under the effect of 
ultrasonics in an aqueous medium. 
The rhodium initially present in the solution is generally recovered in a 
yield greater than 80% in a solid form containing more than 50% (w/w) 
rhodium. 
Metallic rhodium may be recovered ether by melting the cathode or by 
melting the deposit obtained after scraping the cathode, it being possible 
in the latter case for the cathode to be used again. 
The rhodium in a metallic form which is obtained in this manner may be 
refined, according to known methods. 
In this way, it is not necessary to employ a specific reducing agent which 
is usually employed to precipitate rhodium, such as a borohydride, zinc or 
iron. 
The rhodium obtained according to the process of the present invention may, 
if desired, be converted into a salt (chloride, bromide, sulphate, 
nitrate) capable of being employed for preparing a complex which can be 
used as a catalyst in organic synthesis.

The following Examples illustrate the present invention. 
EXAMPLE 1 
1. Pretreatment 
Into a 100-cc conical flask are introduced: 
a solution (10 cc) of spent catalyst containing: 
rhodium-sulphonated triphenylphosphine complex (estimated to contain 2.3 
g/liter of rhodium) 
trisulphonated triphenylphosphine, sodium salt and oxidation products 
(40.9% solids content) 
organic products (1.4%) 
a 15% aqueous solution of sodium hypochlorite (25 cc; d=1.22). 
The flask is heated on a water bath at 80.degree.-82.degree. C. for 1 hour 
20 minutes, with stirring. Hydrochloric acid (37%, d=1.19; 1.5 cc) is then 
added. Heating and stirring are continued for 1 hour 45 minutes. 
The precipitate which forms (0.152 g) is separated from the solution by 
filtration on sintered glass. The precipitate contains 0.064% (w/w) of 
rhodium, which is recovered by combustion according to known methods. 
2. Electrolysis 
The pretreated solution (25 cc) is introduced into the cathode compartment 
of an electrolysis cell whose two compartments are separated by a cationic 
membrane. 
A copper cathode in the form of a disc 2.4 cm in diameter is introduced 
into the cathode compartment. The anode compartment contains N 
hydrochloric acid (25 cc); a platinum grid of the same size is introduced 
therein. The cathode potential is set at -0.3 volt relative to a saturated 
calomel reference electrode. The solution is stirred. After the passage of 
359 coulombs the electrolysis is stopped. The copper cathode is covered 
with a grey deposit which is recovered by scraping, rinsed with water and 
with acetone and then dried. 
3. Results 
The initial solution contained 16.25 mg of rhodium and the final solution 
after electrolysis contains 2 mg thereof. The efficiency of recovery is 
88%. 
The deposit recovered contains 46% of rhodium and 39% of copper. 
EXAMPLE 2 
1. Pretreatment 
Into a 1000-cc conical flask are introduced: 
spent catalyst solution (100 cc) identical with that employed in Example 1, 
and 
15% strength sodium hypochlorite (300 cc). 
The flask is stirred and kept on a water bath at 90.degree.-94.degree. C. 
for 1 hour 35 minutes. 37% strength hydrochloric acid (15 cc; d=1.19) is 
added. Stirring and heating are continued for 90 minutes. Filtration leads 
to the recovery of a reddish pasty precipitate (0.508 g) containing 0.3% 
of rhodium, which is recovered by combustion. 
The volume of the filtrate is adjusted to 500 cc by adding water. 
2. Electrolysis 
Pretreated solution (250 cc) is introduced into an electrolysis cell 
without a separator diaphragm. A copper cathode and 2 graphite anodes with 
a surface area of 25 cm.sup.2 are immersed on each side of and at an equal 
distance from the cathode. The voltage at the circuit terminals is set at 
2.66-2.9 volts. The solution is stirred continuously. After the passage of 
2530 coulombs, the electrolysis is stopped. The copper cathode is covered 
with a grey deposit containing a high content of rhodium. 
3. Results 
The initial solution contained 184 mg of rhodium and the final solution 
after electrolysis contains 25 mg thereof. The efficiency of recovery is 
86%.