Flotation process

Disclosed is a process for the flotation of a mineral concentrate comprising the steps of forming an aqueous slurry of a milled mineral ore containing particles of a desired mineral species and adding a flotation reagent which causes a desired variation in the flotation tendency of the desired mineral species present within the slurry so as to increase the efficiency of separation of that mineral species from the slurry relative to a situation where said flotation reagent is absent from the slurry. A stabilising agent is introduced to the slurry in an amount which creates electrochemical conditions conductive to separation of the desired mineral from the slurry and causes the destruction of a deleterious component from the slurry which consumes the flotation reagent thereby maintaining or improving the efficiency of separation of the desired mineral species from the slurry of milled ore.

FIELD OF THE INVENTION 
The present invention relates to flotation processes and, in particular, to 
processes requiring activation or depression of species present in a 
milled ore concentrate. 
BACKGROUND OF THE INVENTION 
Flotation is a widely utilised unit operation in mineral processing and is 
based upon the principle that different mineral species have different 
wetting characteristics. This difference in wetting characteristic can be 
used as a basis for separating the different mineral species of a milled 
ore because relatively unwetted or hydrophobic milled mineral particles 
adhere more strongly to a stream of gas bubbles, generally air, passing 
through a slurry of the milled mineral than those particles which are 
relatively wetted or hydrophilic. 
The process is generally assisted by the addition of reagents, for example, 
depressants which reduce the flotation tendency of certain minerals such 
as pyrite and activators such as copper sulphate which activate, that is, 
assist minerals to float which do not have a tendency to do so even in the 
presence of collectors. Organic collectors such as sodium ethyl xanthate 
which enhance the tendency of mineral particles to adhere to bubbles of 
gas are also widely utilised. 
The flotation operation is conducted in flotation cells and columns which 
contain a slurry of the milled ore to be separated into the constituent 
streams of concentrate and gangue. A gas, usually air, is sparged through 
the cell or column causing hydrophobic particles to selectively attach to 
air bubbles, generally with the aid of agents such as those described 
above. The hydrophobic particles collect in a froth layer at the top of 
the cell and are removed. The unfloated material is removed from the 
bottom of the cell from where it may be transferred to a further flotation 
stage in which the flotation conditions may be altered to selectively 
float the same or another desired mineral concentrate. Alternatively, the 
unfloated materials may be removed as a tails or gangue stream which may 
be used to fill desired mine shafts or for other forms of land 
reclamation. 
A typical flotation process involves the separation of the constituents of 
a mixed ore such as an ore containing the minerals galena (lead sulphide), 
sphalerite (ZnS) and pyrite (FeS.sub.2). In a first stage, galena is 
floated by adding a xanthate collector (0.05-0.15 kg t.sup.-1 ore) to 
promote the flotation of galena. Sodium cyanide and zinc sulphate 
(0.05-0.15 kg t.sup.-1 ore and 0.5-1 kg t.sup.-1 ore respectively) are 
added to depress the pyrite and sphalerite. In a second stage, sphalerite 
is activated with copper sulfate to form a copper sulfide layer on the 
sphalerite grains which allows adsorption of the xanthate activator and 
flotation of a predominately zinc concentrate. Pyrite is recovered as a 
tailing. 
Where the ore is more complex or the proportion of coarse particles is too 
high, regrinding and further flotation circuits may be required. Cleaner 
and scavenger flotation cells may also be required to maximise recovery of 
desirable mineral constituents. It is also to be noted that effective 
flotation requires careful control over chemical conditions such as pH 
which require an acid or lime to be added in conditioning stages prior to 
each flotation step. 
In spite of the above precautions, the tails stream from a flotation 
circuit often contains appreciable amounts of valuable minerals and 
therefore, if the flotation operation is to be optimised in terms of 
economic efficiency, these minerals must be reclaimed to the maximum 
extent possible. Such an objective requires careful control over the 
flotation process both through judicious use of the above described agent, 
control over pH, Eh, and, consequently, the process chemistry. It will be 
appreciated, in this regard, that the above described agent are expensive 
and over use is to be discouraged. 
A problem arises with certain minerals of economic importance, for example 
sphalerite (zinc sulphide), pyrite (iron (III) sulphide), arsenopyrite 
(iron arsenosulphide) and stibnite (Sb.sub.2 S.sub.3) in that such 
minerals have a poor tendency to float even in the presence of collectors. 
In these instances, it has been necessary to employ an activator such as 
copper sulphate to encourage flotation. The copper sulphate achieves this 
objective by encouraging the formation of a surface layer(s) of copper 
sulfide, a mineral which does have a tendency to float. In the case of 
sphalerite, the formation of this surface layer follows the chemical 
reaction. 
EQU ZnS+Cu.sup.2+ .fwdarw.CuS+Sn.sup.2+ (I) 
Unfortunately, it has been found that copper sulphate must often be used in 
excess of the theoretical quantity required to enable the formation of 
sufficient coverage of the zinc sulfide with copper sulfide. As the 
operation is conducted at alkaline pH there is a tendency for hydroxylated 
copper species to form which may also react with other species such as 
cyanide and complex sulphated anions causing the activation process to 
become less efficient. Similar behaviour may be observed with other milled 
ones. 
SUMMARY OF THE INVENTION 
Therefore, it would be of advantage to the mineral processing industry to 
provide a flotation process which enables the flotation reagent, for 
example, an activator to be used to best effect, that is, by reducing the 
species responsible for preventing (or deactivating) activation and 
ideally, simultaneously creating a conducive chemical environment for 
flotation. Therefore, the object of the present invention is to maximise 
the benefit of such reagents. 
With this object in view the present invention provides a process for the 
flotation of a mineral concentrate comprising the steps of: 
(a) forming an aqueous slurry of a milled ore containing a desired mineral; 
(b) adding a flotation reagent which causes a desired variation in the 
flotation tendency of the desired mineral present within the slurry to 
obtain at least partial separation of the mineral from the slurry; and 
(c) adding a stabilising agent to the slurry in an amount which creates 
electro chemical conditions conducive to separation of the mineral from 
the slurry and causes destruction of a deteterious component in the slurry 
which is chemically reactive with and consumes said flotation reagent to 
reduce separation of the desired mineral from the slurry. 
Advantageously, the desired mineral is a sulfide mineral contained within a 
milled sulphide ore. 
Conveniently, the flotation reagent may be soluble in the aqueous phase of 
the slurry being, for example, an activator such as copper sulphate or a 
depressant such as sodium or potassium cyanide and other depressants 
containing hydroxyl, sulphite or sulphide radicals. 
The stabilising agent may be, for example, an oxidising agent such as 
permanganate and peroxide or an oxidising gas containing elemental or 
molecular oxygen with the proviso that the oxidising agent is not 
exclusively air where the oxidising agent is added to the flotation cell. 
Oxidising gaseous agents, such as oxygen, may be found to be especially 
suitable but species such as ozone and oxidising gases and mixtures of 
such gases may also be of benefit. 
The deleterious component to be destroyed can either exist in dissolved 
form within the aqueous phase of the slurry or on the surfaces of the 
mineral grains. Destruction involves removal by dissociation or either 
mechanism which involves loss of integrity of the deleterious component. 
In the specification and the claims, "destruction" demands the removal of 
the component from the slurry by chemical reaction or other means. In this 
regard, metallic components are not destroyed, they merely remain in a 
metallic state or in a different oxidation state. Such variation in 
oxidation state does not, of itself, constitute destruction. 
Conveniently, the stabilising agent is also inert with respect to the 
desired flotation reagent, though situations may be envisaged where the 
stabilising agent reacts with the flotation reagent to form a flotation 
reagent of acceptable or greater performance with respect to separation 
efficiency. By "inert" is indicated that reaction of flotation reagent and 
stabilising agent does not proceed to an extent where separation 
efficiency is economically hindered with respect to the situation where 
the stabilising agent is absent from the slurry. 
Advantageously, the presence of stabilizing agent in the slurry should be 
conductive to the creation of chemical conditions favourable to flotation. 
In particular, where an oxidising gas is used, this will be conducive to 
the creation of optimal electrochemical conditions for flotation through 
its influence over the oxidation potential (E.sub.h) of the slurry. One 
aspect of the invention is predicated on the basis that careful control 
over E.sub.h creates flotation conditions conducive to high separation 
efficiency and to the destruction of oxygen consuming deleterious 
components which become unstable in an oxidising environment. As an 
example may be mentioned sulphur containing anions such as the complex 
sulphide anions which form when sulphide minerals are exposed to an 
alkaline environment. Such sulphide anions, being oxygen consuming 
species, can be converted by oxidation to the thio sulphate radicals and 
ultimately the divalent sulphate anion which does not consume flotation 
reagents with consequential decline in separation efficiency. If such 
species are allowed to remain in the slurry, the activation is 
particularly affected, since hydroxylated copper species not amendable to 
adsorption of collectors form. In the case of a separation involving zinc, 
formation of hydroxylated copper species cause an inevitable consequential 
fall in grade and recovery of the zinc concentrate. 
Conveniently, the slurry containing the milled mineral ore is treated with 
the oxidising agent prior to entry of the slurry to the flotation cell, 
preferably in a conditioning stage. The adjustment of pH during the 
conditioning stage should preferably be such as to obtain an alkaline 
environment which causes depression of pyrite and therefore is more 
conducive to separation of sulphide minerals.

DETAILED DESCRIPTION OF THE INVENTION 
The invention will be better understood from the following description of a 
preferred embodiment thereof, made with reference to the following 
examples. 
EXAMPLE 1 
FLOTATION OF A ZINC CONCENTRATE FROM A LEAD/ZINC ORE 
In this example, a milled lead/zinc sulphide ore was subjected to a 
flotation process to separate the lead and depress zinc and 
gangue(pyrite). The tailings from this separation was subjected to a 
further flotation process incorporating the addition of pure oxygen gas to 
the flotation cell. Oxygen was supplied by sparging gas through the 
flotation cell at rates of 1 liter/minute and 5 liters/minute for periods 
of 65 minutes, 80 minutes and 90 minutes respectively and the results 
compared with the situation using a conventional flotation method to 
enable separation of lead and zinc sulphides. The oxidation potential of 
slurry in the flotation cell was also measured upon attainment of rest 
potential and the results tabulated below. 
______________________________________ 
O.sub.2 at 1 l/minute 
O.sub.2 at 5 l/minute 
Standard Method t = 65 minutes 
t = 80 min 
t = 90 min 
______________________________________ 
Oxidation Potential 
3.7 151 87 144 
(mV) 
Grade (% by weight 
46.43 46.96 50.24 47.27 
zinc) 
Recovery (% zinc 
56.61 75.83 67.01 66.81 
from milled ore) 
______________________________________ 
The addition of oxygen at lower flowrates may or may not be effective 
depending on the oxygen uptake rate of the milled ore which in the case of 
the above ore varies between 0.4 and 3.0 mg/l/min, a very high oxygen 
demand ore. This uptake rate must be satisfied before the benefits of 
oxygenation are gained, the uptake rate is therefore an important 
parameter in the residence times selected for oxygenation and the oxygen 
supplied to the flotation cell. 
It is to be noted that the feature of higher oxidation potential reflects a 
decrease in the presence of reactive sulphides which interfere with 
flotation processes as described above. 
The oxidation of pyrite causes the pH of the slurry to fall during the 
above flotation process so it is important to add sufficient quantities of 
an alkaline agent such as lime to the slurry during flotation or, where 
the above operation is undertaken during conditioning, during conditioning 
to maintain pH in the range 10.5-11.5 where separation efficiency is 
optimal. 
EXAMPLE 2 
FLOTATION OF A ZINC CONCENTRATE FROM A LEAD/ZINC ORE 
120 tph of a tailings stream as described with reference to Example 1 and 
having a solids density of 40% and analyzing 0.4% Cu, 0.9% Pb and 13.38% 
Zn was fed to the zinc separation stage of the concentrator and subjected 
to a flotation process in five stages involving the addition of 16 m 
.sup.3 /hr oxygen (10 m.sup.3 /hr of this oxygen being supplied in the 
form of air) to conditioning cells, pH was maintained in the alkaline 
range by the addition of sufficient lime to maintain pH at 11.0. The 
results are tabulated below. Comparative results for standard running 
without oxygen are included for comparison. With the exception of 
oxygen/air addition, the flotation process is conventional. 
______________________________________ 
Zinc Grade and Recovery. 
Oxygen Addition 
Standard Zinc Zinc 
Zinc Recovery 
Zinc Grade Recovery 
Grade 
Stage (%) (%) (%) (%) 
______________________________________ 
1 55.35 52.00 65.46 52.60 
2 70.22 50.68 79.82 51.71 
3 88.85 47.10 93.26 45.97 
4 93.09 44.25 96.37 41.44 
5 94.62 42.14 97.35 39.01 
______________________________________ 
Zinc recovery was appreciably higher at acceptable grade, the degree of 
improved recovery having substantial economic value on an annualised 
basis. 
EXAMPLE 3 
FLOTATION OF A ZINC CONCENTRATE FROM A LEAD/ZINC ORE 
Plant conditions are the same as Example 2, with throughput 14.0 m.sup.3 
/hr oxygen being supplied to the conditioning cells as air, rather than as 
described above. 
______________________________________ 
Zinc Grade and Recovery. 
Standard Oxygen Addition 
Zinc Zinc Zinc Zinc 
Recovery Grade Recovery Grade 
Stage (%) (%) (%) (%) 
______________________________________ 
1 68.80 54.40 68.76 54.40 
2 80.05 52.19 81.87 52.77 
3 93.25 47.52 94.39 46.92 
4 95.01 43.05 95.85 42.73 
5 95.56 40.21 96.32 40.57 
______________________________________ 
Again, as discussed with respect to Example 3, zinc recovery was 
appreciably higher at acceptable grade. 
EXAMPLE 4 
FLOTATION OF A ZINC CONCENTRATE FROM A LEAD/ZINC ORE 
The plant conditions are as in Example 2. 
______________________________________ 
Zinc Grade and Recovery. 
Standard Oxygen Addition 
Zinc Zinc Zinc Zinc 
Recovery Grade Recovery Grade 
Stage (%) (%) (%) (%) 
______________________________________ 
1 55.35 52.00 65.73 52.70 
2 70.22 50.68 80.40 51.36 
3 88.85 47.10 92.69 44.89 
4 93.09 44.25 95.89 40.88 
5 94.62 42.14 97.01 38.46 
______________________________________ 
Recovery is appreciably higher using oxygen at acceptable grade. 
EXAMPLE 5 
FLOTATION OF A ZINC CONCENTRATE FROM A LEAD/ZINC ORE 
The plant conditions are as in Example 3. 
______________________________________ 
Zinc Grade and Recovery. 
Standard Oxygen Addition 
Zinc Zinc Zinc Zinc 
Recovery Grade Recovery Grade 
Stage (%) (%) (%) (%) 
______________________________________ 
1 68.80 54.40 73.84 52.40 
2 80.05 52.19 85.03 51.68 
3 93.25 47.52 95.65 44.47 
4 95.01 43.05 97.14 40.38 
5 95.56 40.21 97.67 37.77 
______________________________________ 
With respect to design of the flotation and conditioning cells, the present 
invention is amendable to inclusion within plants containing conventional 
flotation cells of the Agitair type or other type known to those in the 
art. Similarly, the method of delivery of reagents, whether of solid or 
gaseous type, to flotation and conditioning cells is well known to those 
skilled in the art. Nevertheless, where an oxidising gas is employed, the 
gas delivery equipment should be such as to ensure high oxygen 
dissolution. Therefore, equipment which promotes swarming of fine 
micron-sized bubbles of the gas is to be preferred From this point of 
view, pressurised delivery of a gas is to be preferred though this is not 
essential. 
It is to be noted that while the foregoing description has focussed on the 
use of oxygen, being a widely and economically available gas, other gases 
and oxidants may be used without departing from the scope of the present 
invention.