Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
This invention relates to the recovery of metal values from a pulp
containing such values in dissolved form.
In mining operations, the mined ore is generally crushed and milled
5 and is then subjected to a leaching step with a suitable chemical
reagent in order to solubilise the metal values of interest in the
ore, the resultant product being called a pulp. Such a pulp may
contain from about 1 to about 60 percent by weight solids, but
typically contains 30 to 50 percent be weight solids. Such pulps
10 are commonly subjected to a liquid-solid separation stage prior to
the recovery of the metal values from the resulting clarified or
partially clari~ied solution. Typically such a liquid-solid
separation stage may take the form of thickening, followed by
filtration. Cycloning, centrifugation and counter-current
lS decantation may also be used in such a liquid-solid separation
stage.
Prior art methods of dealing with this clarified or partially
clarified solution resulting from a liquid-solid separation stage as
described above, cover a wide spectrum of metallurgical routes.
Thus, for example, the use of ion exchange resins and solvents,
5 adsorbents such as activated carbon, together with the chemical
precipitation and electrowinning of rnetal values, are conventional
techniques well known in the art of metal recovery.
Ihe pulp may also be treated with ion exchange resins or adsorbents
such as activated carbon in the so-called "in-pulp" processes, in
10 which no prior liquid solid separation is carried out. One of the
most successful methods currently being used is the carbon-in-pulp
process for the recovery of gold and silver values. This method
involves contacting the pulp with granular activated carbon. The
gold and other valuable rnetal values are adsorbed on to the
15 activated carbon from whence they may be recovered by a variety of
known techniques. This method is usually carried out by contacting
the leached pulp with the activated carbon in a counter-curren-t
fashion involving a series of contacting stages. In each stage the
activated carbon is agitated in the leached pulp in a suitable
20 vessel. Average pulp residence times in the process are commonly
in the region of four to eight hours while average carbon residence
times may be ten to thirty days. A high degree of interstage
carbon mixing is common to the process by virtue of the methods used
in transferring carbon from one stage to another. This carbon
25 mixing has a detrimental effect on the metallurgical efficiency of
the process.
While the carbon-in-pulp process achieves excellent recoveries of
gold and silver values, it requires large contacting vessels and is
energy intensive. The large contacting vessels are required
30 because low carbon concentrations are used to minimise mechanical
problems, e.g. inter-stage screening and breakdown of the carbon.
As -the adsorption of gold and silver are critically affected by
the carbon concentration, relatively 510w carbon and pulp movement
rates are common to the process. Such slow movement rates result
in low gold loadings on the carbon going forward to the elution
circuit as well as a high degree of metal value lock-up in the
adsorption circuit. This lock-up of metal values in a carbon-in-
pulp circuit may constitute a major indirect opera-ting cost due to
the value of the contained metals. Further, -the inters-tage screen-
ing of carbon at each contacting stage of -the process may also
present a major mechanical problem particularly on large plants
where relative screen areas are limited due to the large size of
the contacting vessels.
SUMMARY OF THE_ NVENTION
According to the present invention, there is provided a method of
recovering metal values selected from gold and silver values from
a pulp containing such values in dissolved form includes the steps
of:
(a) forming a bed comprising a random mass of particles of a
~0 material capable of removing the metal values from the pulp
selected from an ion exchange resin, a chelating resin and acti-
vated carbon, in a column having a feed end and a discharge end,
and
(b) passing a stream of the pulp through -the column from the feed
end to the discharge end so that the particles of the bed are
suspended in a substantially s-tationary non fluidised state between
the feed end and the discharge end of the colurnn.
DETAILED DESCRIPTION OF THE INVENlION
.. .... _ _
The method of the invention will now be described in more detail.
Initially the particles in the bed are at rest. Once the s-tream of
the pulp begins to be passed through the bed, the particles move
5 towards the discharge end of the column and become suspended in a
substantially stationary non-fluidised state between the feed end
and the discharge end of the column. As the stream of the pulp
passes through the bed, the metal values in the pulp are captured by
the particles, and a concentration or loading gradient is
10 established in the bed, with the particles closest to the feed end
having higher metal value loadings than the particles near the
discharge end. It is important that this ccncentration gradient be
maintained in the bed so that periodically, the particles having the
highest metal value loadings, may be removed from the bed, and
15 replaced by fresh particles. This is achieved by ensuring that the
particles are suspended in a substantially stationary non-fluidised
- state so that there is no mixing or agitation of the particles in
the bed.
The column for use in the method of the invention is preferably
20 inclined upwardly, with the feed end being below the discharge end,
and more preferably, the column is substantially vertical. In this
case, the stream of the pulp will be passed in an up-flow mode
through the bed. The bed of the particles may be confined within
the column by means of suitable screens which will allow the flow of
25 pulp therethrough, but which will prevent the passage of the
particles therethrough. When there is an upward flow of the pulp
through the bed, the flow velocity and the viscosity of the pulp
will generally be sufficient to lift and suspend the particles oF
the bed in a substantially stationary non-fluidised state.
The method of the invention preferably includes the step of
periodically stopping the passing of the stream of the pulp through
the column so that the particles of the bed move back towards the
feed end of the column while remaining in a substantially non-
5 fluidised state. This relaxation of the bed reduces any tendencyfor clogging or choking of the bed to occur.
The method of the invention preferably also includes the step of
periodically removing the particles in a selected zone of the bed
from the column. Preferably, the particles having the highest
10 metal value loadings, i.e. the particles in -the zone nearest the
feed end of the column, are removed from the bed. The removed
particles may then be further treated to recover the metal values
therefrom.
Generally these removed particles will be replaced with a similar
15 volume of fresh particles introduced into the column from the
discharge end of the column.
The method of the invention may be applied to a plurality of beds in
which each bed is discrete, or in which the beds form part of a
continuous bed. Under normal operation, the passage of the pulp
20 through the system is counter-current to that of the particles of
the material forming the bed in order to maximise metallurgical
efficiency. The movement of the particles of the bed in such an
operation may be carried out either continuously or intermittently.
The material capable of removing the metal values from the pulp may
25 be a suitable ion exchange resin, a suitable chelating resin, or an
adsorbent such as activated carbon.
:~2~ 3
~ 7 -
The rate of adsorption of metal values from solution must be such
that the inventory of particles and the loading of metal values on
the particles are within acceptable limits. Generally this implies
the use of relatively small particles which have a relatively large
5 surface area per unit volume.
Further, the particles of the material must be of a size and shape
as to allow for the passage of the pulp through the bed of the
particles. Thus, the particles may be elongate in shape and may be
solid or may have an elongate hole extending partially or wholly
10 therethrough. For example, when the material is activated carbon,
the particles may comprise rod-like pieces having a substantially
circular cross-section, with a diameter of 2 to 5 mm, which are made
by an extrusion process. However, other shapes and sizes of
particles are also suitable for the method of the invention, e.g.
15 irregular shaped pieces with a maximum dimension of 2 to 20 mm.
Essential to the method of the invention is that the metal values
are recovered from a pulp and not a clarified solution~ The pulp,
which may be formed by leaching a crushed and milled ore with a
suitable chemical reagent, will generally have a solids content of
20 10 to ~0 percent by weight, more typically 30 to 50 percent by
weight. The solids in such pulps are usually in fine particulate
form, e.g. having a typical particle size of less than 300 microns.
The method of the invention has particular application to the
recovery of gold and silver values from a pulp containing such
25 values in dissolved form. In this case, it is preferred to use
particles of activated carbon in the bed.
~xamples illustrating the method of the invention will now be given.
EXAMPLE 1
/
This example illustrates the treatment of a gold plant residue pulp
taken from the President ~teyn Gold Mine according to the method of
5 the invention.
The pulp, resulting from a conventional gold leach operation,
contained approximately 48 percent by weight solids. Approximately
80 percent of the solids had a particle size of less than 74
microns. As a considerable quantity of wood fibre was present in
10 the pulp, this was screened off using a 425 micron vibrating screen
prior to use. The pulp was contained in a 200 litre vessel fitted
with a suitable stirrer from where it was circulated through a
vertical column. lhe column was 2 metres long and had an internal
diameter of 200 mm. A wedge wire screen with a 2 mm slit gap was
15 positioned at the top of the column in order to confine the bed in
the column. Approximately 30 kg of calcined coconut shell
activated carbon, pre-screened to 3 to 15 mm in si7e, was contained
in the column, leaving a freeboard depth of about 15 mm. The pulp
was circulated through the column at a flow rate of 15 to 20 litres
20 per minute. The gauge pressure drop across the column during this
operation measured 80 to 90 kilopascals. No choking of the column
with coarse sol1ds was indicated following continuous operation
lasting five hours.
EXAMPLE 2
~ = ~ . v
25 This example illustrates the treatment of a gold plant residue pulp
similar to that used in Example 1 according to the method of the
invention.
~L2-~4~3
The same column as used in Example 1 was used again, but the column
contained approximately 30 kilograms of extruded activated carbon.
The extruded activated carbon was tube-like in shape having a
diameter of 3 mm with particle sizes varying from 5 to 15 mrn long.
5 The leached pulp was contained in a 200 litre vessel fitted with a
suitable stirrer from where it was circulated through the column a-t
a flow rate of 30 litres per minute. The gauge pressure drop
-across the column during this operation measured 80 kilopascals.
No choking of the column with coarse solids was indicated following
10 continuous operation lasting several days.
EXAMPLE 3
This example illustrates the gold recovery obtainable utilising the
method of the invention.
Leached pulp containing 6 to 11 milligrams per litre of gold in
15 solution was taken from the existing gold plant circuit at the Vaal
Reefs Exploration and Mining Company (South Division) and pumped in
an up-flow mode into a column containing activated carbon. The
pulp resulting from a conventional gold leach using cyanide,
contained approximately 40 to 50 percent solids by weight having an
20 average particle size analysis as follows: 2,5 percent + 150
microns; 26,4 percent + 74 microns and 71,1 percent - 74 microns.
The leached pulp also contained approximately 200 milligrams per
litre cyanide as NaCN, and had a pH in the range 10 to 11,5. A
considerable quantity of wood fibre was also present in the pulp and
25 this was screened off using a 425 micron vibrating screen prior to
use. The column was 10 metres long and had an internal diameter o~
185 mm. A wedge wire screen with a 2 mm slit gap was positioned at
the top of the column in order to confine the activated carbon in
the column. Approximately 150 kilograms of 3 mm diameter extruded
30 or molded activated carbon was contained in the column leaving a
-- 1 0 --
free-board bed depth of about 300 mm. The pulp was pumped into the
column a-t a flow rate of 15 litres per minute which corresponded to
a superficial flow velocity in the column of 0,56 metre per minute.
The counter-current movement of carbon in the column was carried out
5 on an eight hour shift basis. Loaded carbon was discharged from
the bottom of the column, while a similar volume of virgin carbon
was added to the top of the column. During normal column operation
the line pressure to the column averaged 300 to 500 kilopascals
(gauge pressure). The counter-current movement of carbon as
10 described above nad a most beneficial effect on reducing the line
pressure to the column.
The column was run in the mode described for a period of 32 days
with operational time including carbon movement approaching 97
percent. During this time the recovery of soluble gold in the pulp
15 averaged 99 percent with the soluble feed to the column averaging
9,6 milligrams per litre of gold. When carbon was moved at a rate
corresponding in volume to a bed depth of one third of a metre per
eight hour shift, loaded carbon from the column contained 10 to 11
kilograms of gold per tonne of carbon. Likewise, when carbon was
20 moved at twice this rate, the loaded carbon contained approximately
5 kilograms of gold per tonne of carbon.
EXAMPLE 4
..,
Residue pulp containing 0,1 to 0,4 milligrams per litre of gold in
solution was taken from the existing gold plant circuit at the Vaal
25 Reefs Exploration and Mining Company ~South Division) and pumped
into a column as described in Example 3.
-- 1 1 --
In the first test of 9 days duration, using a 10 metre long column,
the gold in solution was reduced to 0,001 milligrams per litre. In
a second test of 15 days duration, where the column was shortened to
5 metres, the gold in solution was reduced to 0,005 milligrams per
5 litre. The respective gold recoveries were 99 percent and 95
percent. The pulp conditions in both these tests were similar to
those described in Example 3. Duriny normal operation on the 5
metre column, the line pressure to the column averaged 100 to 200
kilopascals (gauge pressure). In both tests the carbon movement
10 rate was maintained at one third of a metre per 8 hour shift. The
carbon removed was loaded with 150 to 200 grammes of gold per tonne
of carbon. Any reduction in the carbon movement rate would result
in higher gold loadings on the carbon.
The method of the invention has several advantages. Firstly, using
15 the method of the invention, much lower pulp residence times in the
column are obtainable, when compared with the carbon-in-pulp me+hod.
For example, pulp residence times for the method of the invention
are a matter of minutes compared with residence times of several
hours for the conventional carbon-in-pulp method. Secondly, it is
20 possible to obtain higher metal value loadings on the metal-
capturing particles.
