Note: Descriptions are shown in the official language in which they were submitted.
10629~8
This invention relates to the milling of gold bearing
graphitic or carbonaceous ores.
The extraction of gold from graphitic ores has been a
problem for many years. As commonly known, cyanide solutions
are used to treat gold bearing ores to recover gold and it is
common practice to grind gold bearing ores in the cyanide solu-
; t.ion. However, when grinding graphitic ores in the cyanide so-lution, a substantial amount of gold in the ore and in solution
in the cyanide solution is absorbed by the graphite and go to
waste. The absorption of gold is proportional to the amount of
graphite in the ore and results in extreme losses when milling
ores having a high content of graphite.
To obtain adequate gold recovery from gold bearing
graphitic ores, the common practice to date has been to shut off
~ completely the circulation of the cyanide solution in the grin-i ding circuit and to replace such cyanide solution by raw water.~ Before starting grinding operation on raw water, the grinding
`: mills were started and thoroughly washed out with water. The
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primary and bowl classifiers, which form part of the grinding ;
circuit, were also drained individually and washed out manually.
} ~rom thereon, the entire grinding circuit was run on
~ raw water. Kerosene was also added to all grinding mills at the -
$ rate of 1 pound per ton of ore. The graphitic ore was then pas-
.~ sed through the grinding circuit and pumped into three separate
; primary thickeners where the pulp settled and the water and some
slime overflow went directly to a disposal area. At this point,
there was no gold recovery at all in the thickeners since there
was no cyanidation. The discharge from these graphite thickeners
~' was then pumped to agitators where the graphitic ore had its
.~` 30 first contact with cyanide. The normal cyanidation procedure
was followed from thereon.
The raw water which accompanied the graphitic ore
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- through the circuit caused excessive calcium carbonate scale
~, build-up in all the mill piping and in the clarifier bags~ fil- '~
ter canvas and press cottons and canvas which form part of the
''. regular gold precipitation circuit. The cold water, especially
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in the winter months, lowered the cyanidation circuit temperatu-
, re resulting in lower extraction. For every ton of raw water
, introduced, it was necessary to waste a ton of barren solution
' running 0.35 pound per ton of cyanide and 0.40 pound per ton of
,` lime because there was no room for it in the mill circuit.
' 10 With this procedure, only certain types of graphitic '
ores could be milled. The types of graphitic ores that could be
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milled were determined by sampling the graphite ore stopes un-
' derground. These samples were sent to the mill to determine how
active they were. A representative head sample was sent for a
' gold assay. A second sample was cut and diluted 3:1 with gold
bearing cyanide solution, 1.5 pound per ton KCN and 1.5 pound
~' per ton CaO. This second sample in turn was bottle agitated for ,
~,,,' a 24-hour period. Upon completion of the cyanidation process
' this second sample was filtered, washed with water, dried and
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,, ,20 sent for gold assay. Any stope sample that gave a recovery ''
,' higher than 78% was mined. If the graphite stope showed a reco-
:.
'i~ ' ver,y lower than 78~i, this stope was not mined, as all the values
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. would be precipitated out prematurely in the cyanidation circuit
~,' and would go out to the disposal tail area. '
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:~j' It is therefore the maln ob jec~-- o . the~present in- '
vention to provide a process for milling gold bearing graphitic
, ores wherein all types of gold bearing graphitic ores may be mil-
led with an economic recovery. '!'
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,; The process, in accordance with the invention, consists
in grinding the graphitic ore in the barren solution, which re- , ,
"' mains after removal of the gold from the gold bearing cyanide '~
~; . .
` solution, to 85~i minus 200 mesh and adding a graphite inhibitor
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such as kerosene simultaneously to such solution.
- It has been found that some thiocyanates, cobalt and
other metal values from the barren solution as well as lime and
soluble sulphides are absorbed by the graphite in the ore and go
to waste during precipitation of the gold, thus substantially
minimizing fouling of the barren solution. This permits
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~ recycling of the barren solution when milling graphitic ores.
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" Although not fully proven in practice, it is believed
that the absorption of the above substances from the barren so-
j~ lO lution together with the coating effect of kerosene neutralize
`~ the effect of graphite on the gold during grinding of the gra- ~ ~-
phitic ore. Such gold is therefore not absorbed by the graphite
:~ and proceeds forward to precipitation eficiently.
It became also apparent during experimentation that,
,, with gold bearing graphitic ores, the gold was occluded in the
pyrite and that, in order to extract it, a finer grind than is
~; normally used with gold bearing non-graphitic ores was needed.
; It was found that with grinding to 85~ minus 200 mesh, the finer
grind freed more of the graphitewhich could be coated with kero-
sine and other substances in the barren solution and thus prevent
~X,j pre-precipitating gold. .;
~ The pH of the barren solution should be between lO and
'iit- ll to obtain optimum results.
~ The invention will now be disclosed, by way of example,
,, ; with reference to the accompanying drawing illustrating a flow
i,' sheet of a typical cyanidation mill with flotation and roasting
. and incorporating the modifications made for milling gold bearing
~' graphitic ores.
,;; The ore is delivered by a suitable conveyor to bins
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; 30 illustrated by block lO. The ore from these bins is discharged
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,~ into conveyor belts (not shown) which are speed regulated to ;
control the belt weight. A weightometer (not shown) is used to
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determine the tonnage in known manner. The ore from -the bins 10
.~ is fed to a rod mill 120 A gold bearing solution (pregnant solu-
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tion) is also fed to the rod mill 12. This pregnant solution
originates Erom a later part of the mill circuit and will be dis-
closed later. The output of the rod mill is discharged to a
primary classifier 14 in closed circuit with a ball mill 16.
Lime slurry is added to the primary classifier to give a strength
of 0.3 pound per ton. The classifier sand is fed to ball mill 16
and the ball mill discharge flows back to the primary classifier
14 to be reground to adQquate mesh size. The primary classifier
overflow is pumped to a bowl classifier 18 in closed circuit with
a regrind ball mill 20. The bowl classifier sands are fed to
regrind ball mill 20 and the mill discharge is pumped back to the
bowl classifier for further regrind. The bowl classifier over-
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flow is typically at 80~ minus 200 mesh. This is the size of the
. final product of the grinding circuit when milling gold bearing
non-graphitic ores.
v The bowl classifier overflow is pumped to typically
~, three primary thickeners 22 in which the sand settles at the bot~
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tom and the gold bearing solution remains on top. The overflow
` from the primary thickeners flows by gravity to a pregnant solu- ;;
;,~ tion tank 24 into which is stored the gold bearing cyanide solu-
tion. At this stage, about 50% of the gold is removed from the
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~- ore. Diaphragm pumps deliver the discharge of the thickeners at
` a dilution of 0.8:1 to a series of agitators represented by block ` -
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~ 26. Rake type agitators with centre air lifts are normally used - ~
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and a typical unit may contain about seven of them providing 46
hours of agitation at a dilution of 0.85:1. A calcium or potas-
sium cyanide slurry is added to the first agitator. These agita- ~-
tors mix the cyanide slurry with the sand originating from the
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, primary thickeners. The discharge from the last agitator flows
by gravity to three first stage C.C.D. lcounter current decanta-
tion) thickencrs 28. Diaphragm
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; pumps deliver the discharge from the first stage C.C.D. thicke-
ners to three second stage C.C.D. thickeners 30 and similarly
' the discharge of these thickeners is delivered to filters 32 at
a dilution of 0.6:1.
The overflow solution from the second stage thickeners
30 ~low by gravlty to a C.C.D. tank 34 and is pumped to the first
stage thickeners 28 as a wash solution. The solution from the
first stage thickeners 28 flows by gravity to the grinding solu-
, tion tank 36 and is pumped to the rod mill 12 as mentioned pre- ~;
'~ lO viously to be used as a grinding medium for the ore.
,, Six drum type filters provide one stage of filtration.
They are run at a speed of about 4 minutes per revolution. In
these filters, the gold bearing solution is sucked from the cen-
~ ter of the drum through a canvas placed on the drum and fed to
,~ ~ the grinding solution tank 36. The gold bearing solution recove-
~' red in tank 36 represents about 40% of the gold for a total of
about 90%. A one inch thick cake is formed on the outside of the
canvas of the filters and this cake is mixed with water and bar-
~ ren solution in filter repulpers (not shown) to give a dilution
'~ 20 of 1:1 and the tailings from all the filters run by gravity into
` a common box (not shown) and finally discharged into hydrocyclo-
` nes 38. The hyarocyclones are used to recover coarse sand and
pyrite larger than 66% minus 200 mesh and the slimes sent to was-
te. The output of the hydrocyclones is fed to a flotation and ~-
roasting circuit 40. The flotation and roasting circuit will not
~; be disclosed in detail because it is not necessary to the under-
~-` standing of the present invention. It will only be mentioned
that the calcine coming out from the roasters is fed back to bowl
` classifier 18 for grinding and recovery of the gold therein.
-~ 30 This procedure permits recovery of another 6 to 8% of gold.
The gold bearing cyanide solution stored in the pre- -
gnant solution tank 24 is fed to typically two precipitation
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units. ~ach precipitation unit contains a clarifier,42 for ta-
king out the fine sludge from the solution, a vacuum tower 44 for
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removing air from the solution and three presses 46 wherein the
gold removal reaction takes place. The solution coming out from
the presses isthe so-called barren solution which contains trace
(about 0.30 pound per ton) amount of cyanide. Such barren solu-
tion is stored in tank 50. The pregnant solution coming out of
the tank 24 is contacted with zinc in the clarifiers and zinc
reacts with the cyanide in the presses to liberate gold. The
precipitate in the presses 48 is scraped from the frames of such
presses and fed to furnaces 52 which are used for melting the
precipitate. The gold is recovered in buttons which are then
remelted in a graphite crucible and poured into bar moulds. The
slags are remelted in a separate furnace with silicon carbide
and poured in the form of so-called speiss bars.
The above disclosed mill is well known and is regularly
used for milling gold bearing ores which do not contain graphite. ;
For milling graphitic ores, it has been the usual practice to
~ date, as disclosed previously, to shut off the circulation of the
`~ 20 gold bearing cyanide solution in the grinding circuit by closing ~:
off of valve 54, and to feed the grinding solution in tank 36
into pregnant solution tank 24 by opening a valve 55. The entire
~- grinding circuit was then thoroughly washed out with water and -~
- the solution was replaced by raw water by opening valve 56. As
also mentioned previously, kerosene was added to the solution in -~
the grinding mills by opening a valve 58 in a suitable kerosene
supply line or otherwise. In addition, all solution lines were
; switched over so that all gland wash lines and spray lines were
on raw water. The solution lines going into the bottom of the
primary thickeners were also fed with raw water. The sump pumps
were also disengaged so that -they could not be started accidently
by the operator on shift because the sumps were filled with cya-
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~L0629~L8
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nide solution and gold bearing sand.
In accordance wi-th the present invention, it is now
~ proposed to shut off the circulation of the pregnant solution in
!~,, the grinding circuit by closing valve 54 and to grind graphitic
; ore in barren solution. The solution in the grinding tank 36 is
,~ fed to the pre~nant solu-tion tank 24 through valve 55 and the
barren solution is then pumped from tank 50 and fed to the grin-
ding circuit through valve 60. Keros~ne is also added to the in-
. put of ball mill 12 through the opening of valve 58.
It is also proposed to grind graphitic ore to a finer
si grind then the non-graphitic ore by varying the adjustment of the
ball mills 16 and 20. As mentioned previously, the gold is oc-
~;' cluded in the pyrite and in order to extract it, a finer grind is
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,,?,~, required. The finer grind liberates more of the graphite which
!~ is coated with kerosene and some of the materials in the barren
solution to neutralize the effect of the graphite on the gold
present in the barren solution.
` Grinding gold bearing graphitic ore in barren solution
,~- and kerosene has shown that most of the graphitic ore in appli-
~ 20 cant's mine can be milled with a recovery as high as 90%. The
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following Table I illustrates typical results obtained-from a
number of test which were done in the p~ant:
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TABLE I
_ :
,`~ Head Agita- Tail
Assay Kerosene Grinding Dilute tion Assay %
.~ Sample oz gold/ pound/ton solution solution time oz gold/ Reco-
ton ~hrs) ton very -
~475 .53 1.0 Barren Pregnant 24 .04 92.5 ~
~475 53 1.0 water 24 08 85 ;
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~`~ It will be seen from the above Table I that the use of
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the barren solution instead of water for grindiny graphitic ores
has given a recovery of about 92.5%, which is much higher than
the recovery obtained using water as a grinding solution.
Tests were also made on various types of graphitic ores
which could not be milled before because gold assays performed on
them showed a recovery lower than 78%. One typical example is
the following using a sample which was nearly pure graphite:
TABLE II
Head Agita- Tail
~ Assay Kerosene Grinding Dilute tion Assay %
!': Sample o~ gold/ pound/ton solution solution time oz gold/ Reco-
ton (hrs) ton very
~.,, . ... '":',' ,
,~ Nearly
pure .541.0 Barren Pregnant ~8.12 78%
Graphi- 54_ _ _ water 481.08 nil
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,, The above Table II shows that, in fact, there was a higher
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amount of gold in the cyanidation tails than at the input of the
grinding circuit. This is due to the fact that when using water
j 20 as a grinding solution gold is absorbed by the ore instead of
being removed therefrom, giving a negative recovery. On the con-
trary, by using barren solution as a grinding medium a recovery
of approximately 78% of gold has occured. The above tests permit
' to conclude that many tons of graphitic ores which could not be
` economically mined before can now be treated with the present
`` process. -
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To determine the chemical reaction which likely occurs
when the barren solution is circulated through the grinding cir-
cuit, a sample of barren solution was assayed for its content.
~: 30 The graphite ore sample was ground in a barren solution for a
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thirty minute period, and the filtered solution was assayed. As-
says were as follows:
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~L~62~8
Normal sarren Solution sarren Solution after 30 minute grind
,
KCNS - 300 ppm 234 ppm
, S04 - 775 " 850 " -
, FE - .70 " 19.4 "
~, Ni - 10.5 " 10.5 "
Co - 1.03 " 95 "
, Cu - 27.5 " 27.7 "
According to the above results, the graphite particles
are absorbing some quantities of potassium thiocyanate, cobalt
and probably other metal values as well soluble sulphides. It
is also known that lime is absorbed by the graphite. It is be-
lieved that the absorption of potassium thiocyanate, cobalt and ;
` other metal values, as well as lime and soluble sulphides by the
graphite together with the coating effect of kerosene neutralize ;
the effect of graphite on the gold during the grinding of the
graphitic ore and permit the gold to go forward to precipitation
~, efficiently.
, It is also important to note that the absorption of
h the above substances from the barren solution cleans the solu-
~: tion and permits recycling thereof when milling graphitic ores. ;
Although the above description mentions kerosene as
~ 20 the graphite inhibitor, it is to be understood that other types
; of graphite inhibitors could be used instead of kerosene.
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