Stepwise process for recovering precious metals from solution

Precious metals such as gold and silver are recovered from aqueous cyanide solutions thereof by a stepwise process. The pH of the solution is, if necessary, initially adjusted to about 13 and thereafter is adjusted and maintained at 10 or above throughout the process. A "starter" carbonyl compound (e.g., formaldehyde or dextrose) which reacts with free cyanide ions is dissolved in the solution. Following this, an "accelerator" (e.g., hydrogen peroxide or a persulfate) is added and the solution is heated to aid in conversion of free cyanide ions to other chemical species. Finally, a "clarifier" (e.g., hydrazine or a hydrosulfite) is added and the elevated temperature is maintained for at least a brief period after the addition. The solution is then cooled and the elemental precious metal particles formed are allowed to settle and are separated from the supernatant solution.

BACKGROUND OF THE INVENTION 
The present invention relates to the recovery of precious metals such as 
gold and silver from aqueous solutions thereof. Generally, the precious 
metals, e.g., gold, are present in the form of gold cyanide complexes such 
as potassium or sodium gold cyanide. Such solutions containing precious 
metals are obtained or are the by-products of certain industrial processes 
and it is obviously economically necessary to recover the precious metals 
values therefrom. For example, in gold plating, sodium or potassium gold 
cyanide solutions are employed and it is necessary to recover residual 
gold from the spent plating solution. In mining or in the recovery of gold 
or other precious metals from scrap or waste material, the precious metal 
is usually leached from the ore or scrap by a cyanide solution which forms 
a precious metal cyanide complex. The problem dealt with by the present 
invention is the recovery of precious metals by precipitating them as 
elemental metal from solutions containing their ions. 
The prior art has devoted attention to this task. For example, U.S. Pat. 
No. 3,271,135 shows a process for recovering gold from alkali metal gold 
cyanide solution in which (1) a reducing agent (alkali metal hydrosulfite 
of hydrazine hydrate), and (2) a compound selected from a group including 
water-soluble aldehydes and other carbohydrates (e.g., dextrose and 
formaldehyde) are introduced into the solution. The addition is either 
simultaneous, or by means of a precursor compound such as sodium 
formaldehyde sulfoxylate which liberates both components (2and (2). The 
pantentee notes that component (1) may already be present in the solution 
when component (2) is added. U.S. Pat. No. 3,311,468 discloses a somewhat 
similar process specifically for recovery of silver from cyanide solutions 
thereof. 
Another prior art patent, U.S. Pat. No. 3,271,136 seeks to overcome the 
stated shortcoming of the U.S. Pat. No. 3,271,135 process, which is a 
tendency to redissolve the precipitated elemental gold in the cyanide 
solution by adding a water-soluble alkali metal alkanoate and omitting 
component (2). 
U.S. Pat. No. 3,215,524 uses a water-soluble aldehyde both to destroy the 
cyanide complex and reduce the precious metal ions to elemental metal 
particles. 
The prior art has also separately addressed itself to problems of 
destruction or conversion of cyanide in aqueous waste for environmental 
protection. For example, the DuPont Chemical Company has publicized a 
process under the trademark KASTONE which involves treating cyanide 
bearing waste water with a formulation containing formaldehyde and a 
peroxygen compound. 
Such prior art processes, when they call for more than one reagent, 
generally add the reagents simultaneously or do not ascribe importance to 
the order of addition. 
It is an object of the present invention to provide a novel, efficient and 
economical method for recovering precious metals from aqueous cyanide 
solutions thereof by first reacting free cyanide ions and then reducing 
the precious metal ions to elemental metal. 
It is another object to provide a method of recovering precious metals from 
such solutions by a novel stepwise addition of a starter, an accelerator 
and a clarifier in the order stated. 
It is a further object to provide such a method which overcomes certain 
problems associated with prior art methods and which requires only 
relatively simple equipment for its practice. 
Other objects and advantages of the invention will be apparent from the 
following description. 
SUMMARY OF THE INVENTION 
In accordance with the present invention the method for recovering precious 
metal values from aqueous cyanide solutions thereof comprises carrying out 
the following steps in the sequence stated. To an aqueous alkaline 
solution which contains cyanide ions and precious metal ions, there is 
added a first material which comprises a water-soluble compound which 
contains at least one aldehyde group and which is reactive in alkaline 
aqueous solution with free cyanide ions. Then there is added to the 
solution a second material which is one which releases hydrogen peroxide 
or oxygen in alkaline aqueous solution. Thereafter, the solution is heated 
to a temperature of at least about 85.degree. C. for at least about 
one-half hour to facilitate the reaction of the cyanide ions to other 
chemical species. There is then added to the solution a third material 
comprising a reducing agent, to reduce the precious metal ions to 
elemental particles. The solution is maintained at a temperature of at 
least about 85.degree. C. for an additional period of time after the third 
material is added. The solution is then allowed to cool and elemental 
metal particles to precipitate therefrom. The supernatant solution is 
separated from the precipitated metal particles. 
In a preferred mode of practicing the invention, an interval of time for 
chemical reactions to occur is provided after addition of the second 
material and before commencing heating of the solution. Preferably, this 
interval of time is at least about 15 minutes. Heating of the solution to 
facilitate reaction of the cyanide ions is preferably for a period of 
between about one-half hour to about one and one-half hours. 
Certain objects of the invention are obtained by the use, in the sequence 
specified by the method of three kinds of materials. The first material is 
a water-soluble compound containing at least one aldehyde group and 
reactive in alkaline aqueous solution with free cyanide ions. Generally, 
aliphatic aldehydes, aromatic aldehydes or, particularly, monosacchrides 
are preferred. For example, formaldehyde or dextrose are preferred. 
The second material is one which releases hydrogen peroxide or oxygen in 
alkaline aqueous solution. Generally, peroxy compounds and persulfate 
compounds are convenient. Hydrogen peroxide and alkali metal persulfates, 
particularly sodium persulfate, are preferred. 
The third material employed in carrying out the method is a reducing agent 
to reduce the precious metal ions to elemental metal particles. Generally, 
the reducing agent may be selected from hydrazine, hydrazine compounds, 
and hydrosulfite compounds. Sodium hydrosulfite is a preferred reducing 
agent. 
Other objects of the invention are attained by maintaining the pH of the 
solution at a value of not less than 10, preferably at a pH of about 13 or 
higher before adding the first material and a pH of about 10 or higher 
before and after each addition of the second and third materials.

DETAILED DESCRIPTION OF THE INVENTION 
The materials required for use in practicing the method of the invention 
are the first material or "starter", the second material or "accelerator" 
and the third material or "clarifier". Each of these plays a specific role 
and when used in the required sequence in the method of the invention 
provides efficient recovery of the precious metal values. 
The precious metals (and other metals) in solution are usually in the form 
of cyanide complexes such as sodium silver cyanide, Na[Ag(CN).sub.2 ]; 
potassium gold cyanide, K[Au(CN).sub.2 ]; potassium ferri cyanide, K.sub.4 
[Fe(CN).sub.6 ]; etc. Typically, other precious and base metal ions are 
also present in substantial or trace amounts. For example, platinum, 
palladium, rhodium, zinc, iron, cadmium, nickel, cobalt, magnesium, 
copper, aluminum, lead, manganese, etc., may also be present. Such 
polyvalent metal ions generally form cyanide complexes. "Free" cyanides, 
i.e., cyanides which are not bound in the metal complex, such as potassium 
cyanide, are also usually present. 
The purpose of the first material or starter is to react with the free 
cyanide ions and to convert them to other chemical species such an nitrile 
and amide-type compounds. Such compounds do not react with metallic gold 
or silver as readily as do free cyanide ions and therefore the problem of 
redissolution of metallic precious metal is avoided or minimized. 
Water-soluble compounds containing an aldehyde group, such as aromatic and 
aliphatic aldehydes, and monosaccharides are generally satisfactory. 
Monosaccharides are preferable because of the irritating odor of aldehydes 
including formaldehydes. Dextrose in particular has another advantage in 
that the formation of colloidal gold particles, which tend to remain in 
solution and not settle, appears to be minimized when dextrose is used, 
particularly as compared to formaldehyde. This is a very important 
advantage since it tends to maximize the overall recovery of gold or other 
precious metal. 
While not wishing to be bound thereby, it is believed that the reactions 
which occur in the solution upon addition of the first material are 
typified by the following: 
##STR1## 
The free cyanide ions are seen from the above to react to form nitriles 
and amides. The starter material, at the pH and temperature conditions 
employed, does not significantly attack the precious metal cyanide 
complex. 
The second material added is the accelerator which is essentially an 
oxidizing agent which releases hydrogen peroxide or oxygen in alkaline 
aqueous solution. The second material generally may be a peroxy or 
persulfate compound such as alkali metal pesulfates, alkali metal 
peroxides and hydrogen peroxide. These reagents release or form H.sub.2 
O.sub.2 and/or O.sub.2 in alkaline aqueous solution. Specifically, 
sodium-, potassium-, and ammonium Persulfate and sodium peroxide are 
included. 
Hydrogen peroxide, usually in the form of a water solution thereof is a 
useful second material. However, one difficulty with hydrogen peroxide is 
its tendency to cause excess foaming. Foaming is a particular problem when 
the solution to be treated is relatively high in free cyanides and 
metallic ions of other than precious metals, for example metallic ions 
such as iron, copper, nickel, zinc, tin, lead, etc. 
The foaming problem is often so severe that hydrogen peroxide must be added 
slowly and in small increments. This increases time and labor costs. On 
the other hand, potassium persulfate (K.sub.2 S.sub.2 O.sub.8, standard 
nomenclature, potassium peroxydisulfate) greatly alleviates the foaming 
problem particularly with high cyanide or metallic ion solutions as 
described b above and for this reason is a preferred second material. 
Without wishing to be bound thereby, it is believed that the reactions 
which occur in the solution upon addition of the second material are 
typified by the following: 
##STR2## 
It will be noted that the cyanide reacts with the second material and is 
converted to other chemical species such as carbon dioxide, ammonia, and 
ammonium hydroxide. 
The amount of first and second material to be added is determined by the 
free cyanide content of the untreated solution. The first and second 
materials should each be used in at least the stoichiometric amount 
required to react all of the initial free cyanide content of the solution. 
As a practical matter, an excess, say 5-50% or more, over the 
stoichiometric amounts should be provided both to favor complete reaction 
of the free cyanide ions and to provide for reaction with at least some of 
the cyanide ions which may be released upon destruction of the precious 
metal cyanide complex. 
Finally, the third material or clarifier, which is a reducing agent, is 
added. The third material may be hydrazine, a hydrazine inorganic salt 
such as hydrazine -chloride, -iodide, -bromide, -sulfate, -nitrate, etc. 
or an alkali metal hydrosulfite. The third material, acting in a solution 
in which substantially all the free cyanide has been reacted, reduces the 
precious metal ions to elemental metal. It follows that the quantity of 
third material required is at least the stoichiometric amount necessary to 
reduce all the precious metal ions to elemental metal. As a practical 
matter, an excess over the stoichiometric amount is provided. 
Because it is relatively easy to handle, and has been found to yield 
excellent results, sodium hydrosulfite is the preferred third material. 
While not wishing to be bound thereby, it is believed that the reactions 
which occur in the solution upon addition of the third material are as 
follows: 
##STR3## 
As the above reaction scheme shows, the gold and/or other precious metals 
are reduced to the elemental state and precipitate out. 
In carrying out the process, a sample of the solution from which the 
precious metal, say gold, is to be recovered is taken and the gold content 
thereof is determined by conventional and well known testing means. The pH 
of the solution is also measured with a pH meter. If the pH of the 
solution is below 13 a caustic such as sodium hydroxide or potassium 
hydroxide should be added in amounts sufficient to raise the pH of the 
solution to 13 or higher. A representative sample of the solution is then 
taken and the free cyanide content thereof is determined by well known 
test means. One method of determining the free cyanide content is to add 
potassium iodide solution to the sample and titrate the solution with 
silver nitrate until an end-point showing a faint yellowish turbidity, 
which persists even after swirling thoroughly, is reached. 
A suitable titration method is to take 10 cc of the solution and add to it 
100 cc of distilled water and 5 cc of 10 percent by weight potassium 
iodide solution. This mixture is titrated with 0.1 N silver nitrate 
solution until the end point is reached. The free cyanide content of the 
solution in avoirdupois ounces per gallon is equal to 0.07 times cc of 0.1 
N silver nitrate required in the titration. (One ounce avoirdupois per 
gallon is equal to 7.47 grams per liter). 
With the pH properly adjusted, and the materials and free cyanide content 
known, addition of the appropriate material in the specified sequence may 
be begun. 
The starter or first material, say dextrose (d-glucose) is added to the 
solution at room temperature with stirring or moderate agitation to 
promote dissolution of the first material in the solution. After the first 
material has dissolved in the solution, the pH should be checked. If it is 
below about 10, it should be adjusted to about 10 or higher with 
additional caustic. 
The accelerator or second material is then slowly added with constant 
stirring or moderate agitation to disperse and/or dissolve the second 
material. If necessary, in order to control foaming, particularly when the 
second material is hydrogen peroxide, it is advisable to add the second 
material in increments of the total amount required, with continuous 
stirring. After the required amount of the second material has been added 
to the solution, the pH of the solution is again checked and, if 
necessary, adjusted to about 10 or higher. Continued stirring or moderate 
agitation may be carried out for a further brief period, up to about 
one-half hour, preferably, to promote mixture and reaction of the 
ingredients. The reaction scheme shown in reactions (1) - (8) is 
exothermic and the solution temperature may increase to as high as 
60.degree. or 70.degree. C or C. due to the evolved heat of reaction. 
Generally, 15 to 30 minutes is sufficient to allow these reactions to 
proceed before commencing heating of the solution. 
After the reaction, the solution is heated by suitable means to a 
temperature to at least about 85.degree. C. This temperature is maintained 
for a further reaction period, preferably between about one-half to one 
and one-half hours. Thus, a total of up to about 2 hours is allowed for 
reactions (1) through (8) to take place, with the solution being heated 
through the latter major portion of the reaction period. The cyanide ions 
are converted to other chemical species, notably ammonia and carbon 
dioxide gases. 
Thereafter, the clarifier or third material is added to the solution, with 
stirring or moderate agitation as required. After this addition, the 
temperature of the solution is maintained at the elevated temperature of 
at least about 85.degree. C. for a further brief period to promote the 
reduction reaction. Generally, up to about one-half hour or less, say up 
to 20 minutes, suffices as the period during which the elevated 
temperature of the solution is maintained after addition of the third 
material. 
The heating is then stopped and the solution allowed to cool as 
precipitated metal particles settle. The solution is allowed to stand 
until the supernatant liquid is clear. For a 100 gallon or larger batch 
this usually occurs in a matter of hours. Typically, the solution is 
allowed to stand overnight and is ready for separation from the 
precipitated metals the next morning. The supernatant liquid may be 
decanted from the settled metal particles. Tests show that the supernatant 
liquid usually contains 10 parts per million gold or less. 
The process is advantageously carried out in a reactor equipped with a 
heater, an agitator or stirrer and an exhaust chimney and hood to carry 
off the evolved gases. 
The amounts of first, second and third materials required to most 
efficiently precipitate precious metals from a solution of given 
composition may vary somewhat but is easily determinable by small batch 
trials. The following table shows amounts of some materials which have 
proven to be satisfactory for recovering gold from a gold strip solution 
using the method of the invention. 
Table 
__________________________________________________________________________ 
For recovering gold from a solution containing 7.467 grams per liter (1 
oz 
av. per gal.) of free cyanide and 7.467 grams per liter of gold. 
Amount Required 
Material Per Liter of Solution 
Per Gallon of Sol. 
__________________________________________________________________________ 
(First Material) 
Formaldehyde (37.5 wt. %), or 
.0265 liter .0265 gal. 
Dextrose 46.4 grams 5.65 oz. troy 
(Second Material) 
Hydrogen Peroxide (35 wt % 
.0265 liter .0265 gal. 
aqueous solution); or 
Potassium Persulfate, or 
55.1 grams 6.75 oz. troy 
Sodium Persulfate 43.1 grams 5.25 oz. troy 
(Third Material) 
Sodium Hydrosulfite 39.6 grams 4.82 oz. troy 
__________________________________________________________________________ 
The amounts of first and second material required are adjusted 
proportionally to free cyanide content of the solution to be treated. The 
amount of third material is adjusted proportionally to gold content of the 
solution. 
Some specific operating examples showing the efficacy of the present 
invention are as follows: 
EXAMPLE 1 
A 45 gallon (170.6 liter) batch of gold stipper solution has the following 
analysis at room temperature. 
Au: 6.06 grams per liter 
pH: 12.7 
free CN.sup.-: 7.17 grams per liter 
It is treated as follows: 
__________________________________________________________________________ 
Time (minutes) 
Steps and data 
__________________________________________________________________________ 
0 Add 4108.3 grams dextrose. 
5 Add 1000 cc H.sub.2 O.sub.2 (35 wt. %). 
10 Add 1000 cc H.sub.2 O.sub.2 (35 wt. %). 
55 Solution temperature = 39.4.degree. C. 
57 Add 1000 cc H.sub.2 O.sub.2 (35 wt. %), strong NH.sub.3 odor. 
63 Solution temperature = 43.3.degree. C. 
64 Add 1000 cc H.sub.2 O.sub.2 (35 wt. %). 
66 Solution temperature = 46.1.degree. C. 
70 Solution temperature = 48.8.degree. C. 
78 Solution temperature = 51.7.degree. C. 
83 Add 3368.1 grams dextrose. 
85 Solution temperature = 51.7.degree. C. 
86 Add 1000 cc H.sub.2 O.sub.2 (35 wt. %). 
90 Solution temperature = 55.6.degree. C. 
100 Heat Solution with steam. 
105 Solution temperature 79.4.degree. C. 
109 Solution temperature 87.8.degree. C. Decrease heating 
steam flow rate and maintain temperature at 
82 - 85.degree. C. 
195 Solution temperature 83.3.degree. C. Increase steam flow 
rate and raise solution temperature to 93.3.degree. C. 
210 Solution temperature 93.3.degree. C. Add 4994.7 grams 
Na.sub.2 S.sub.2 O.sub.4 to reactor. Gold metal became visible 
in solution in 1 or 2 minutes after Na.sub. 2 S.sub.2 O.sub.4 
was 
added. 
240 Solution temperature about 93.3.degree. C. Steam flow rate 
shut off. Allow solution to stand overnight. 
18 hrs, 45 min. 
Solution temperature 72.2.degree. C. 
Decanted solution: Au less than 1 ppm. 
__________________________________________________________________________ 
EXAMPLE 2 
A 250 cc. silver plating bath has the following analysis: 
Ag: 82.1 grams per liter 
Hcn: 82.1 grams per liter 
STEPS AND DATA 
Add 53 grams of dextrose to solution. 
Heat solution to 82.2.degree. C. 
Add 21 grams of Na.sub.2 O.sub.2 in small increments. Some foam and 
spattering. Continue heating for 15 minutes. 
Add 10 grams Na.sub.2 S.sub.2 O.sub.4. Allow solution to stand for 1 hour. 
Filter off silver precipitate. 
Filtered solution: Ag at 38 ppm. 
As indicated by the above examples, the sequence of addition of the 
materials may be departed from somewhat after initial additions have been 
made in the prescribed sequence. For example, after addition of an 
incremental portion, say one-fifth or more, of the total of the first 
material, some of the second material may be added and then the balance of 
the first material. Other similar variations may be followed without 
departing from the scope of the invention.