Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a method of disintegrating
pre-crushed lump material, preferably ore and other mineral
material, with the aid of a crushing device and a primary auto-
genous or semi-autogenous drum-type grinding mill.
When processing ore to recover therefrom one or more of its
constituents, such as metal minerals, industrial minerals etc.,
the ore is normally mechanically dislntegrated in a first operation.
The main purpose of this mechanical disintegration of the ore is
to render the valuable cons~tuents accessible in a subsequent sepa-
10 ration process, in which minerals present in the ore can be sepa-
rated in dependence upon their difference in colour, shape or
density, or in dependence upon differences in surface activity, ;
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magnetic properties or other properties.
The mechanical disintegration of the ore normally takes
place initially in the mine, when the ore is blasted from solid
` rock, and is then conti`nued progressively in the form of a series
; of crushing and grinding operations, which may take different forms.
The ore is normally crushed in a plurality of successive stages ~ .
using jaw crushers or cone crushers, whereafter the thus disinte-
grated ore has been ground in rotary drums containing grinding
bodies, such as balls or rods, which are normally made of steel.
Because of the hardness of the rock, however, the grinding bodies
are subjected to intensive wear, which results in considerable
cost. In an effort to avoid this, a technique has been developed
in which the mineralization or ore itself forms the grinding bodies,
i,e. the so-called autogenous grinding technique.
The autogenous grinding technique has found wide use and is
employed to a large extent throughout the world and enables the
pre-crushing of the rock to be limited to the crushing of only
large pieces of rock and boulders to a maximum particle size
acceptable from the transport aspect. The investment costs and
operation costs of the crushing plant is therefore relatively low.
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The absence of artificial grinding bodies of high specific weight
in comparison with the material being ground, necessitates, how-
ever, mills of large diameter in order that the requisite impact
energy is obtained in the grinding charge, which increases the
investment cost and operation cost of the mill. If grinding bodies -
are -to be formed to a satisfactory extent, the material being
ground must have a high mechanical strength. If this is not so,
there is a risk that pieces of material of so-called critical size
will accumulate in the mill, it being meant by pieces of critical
size such pieces which do not exert any appreciable grinding action
and which are not appreciably-affected by the grindlng bodies.
When such accumulation takes place, the throughput falls greatly.
The fact that the material forms its own grinding bodies thus
implies that the efficiency of the grinding operation is also
dependent upon variations in the properties of the material being
ground. Since the properties of the material of, for example, an
ore deposit normally vary, autogenous grinding has been encumbered
with a large number of problems, primarily when existing grinding
apparatus are to be utilised effectlvely and when a constant through-
put is to be obtained with the grinding operation.
If, when effecting a primary autogenous grinding operation,a constant amount of pre-crushed lump material is charged to the
drum mill per unit of time, the extent to which the mill is filled
will vary, as will also the power required to grind the material
when the physical properties of the material or its constituents,
such as its mechanical strength, hardness, toughness, elasticity
etc., vary. It was discovered during tests carried out in conjunc-
tion with the advent of the present invention that the size distri-
bution of the lumps of material charged to the mill also greatly
influences the constitution of the grinding charge formed, and
therewith the grinding process. Changes in the size distribu-tion
of the material charged to the mill influences the degree of fill-
ing of the mill and the power requirement, as does also thephysicaI properties of the material and in practice it is con-
sequently always necessary to compromise, i.e. both to over-size
the mill and to vary the feed, in order to avoid excessively wide
variations in the degree of fllling of the mill and in order to
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- be able to maintain a relatively uniform throughput. A uniform
- throughput is d~esirable in order to~prevent disturbances in process
stages preceding and fo~llowing the grinding operation. It will be
understood from the~aforegoing~-that an;autogenous grinding mill is
seldom operated at~opt~imum conditi~ons, i.e. at the highest possible
throughput. A relatlvely~signlflca~nt reserve capacity must always
be available in order to even out those variations occurring in
the throughput as a r;esult of variatlons in the properties of the
~- material being ground.
r~ It is often necessary to finely grind the material prior to
the~separation process, and consequently the primary autogenous -
grinding stage is normally~followed~by a further stage, a so-called
secondary grinding stage. This~ secondary~grinding stage in auto-
genous grinding processes ofté~comprises ~a pebble~mill, in which
~; 70 pebbles screened from the grinding charge of the primary mill form
r~ ~ thé grinding bodies. The material belng ground is given its final~ ;
~;~ particle size distribution in this secondary grinding stage. In
~order to~;obtain~the best~ return f~or the capital invested, the
throughput must be uniform and, of course, as high~as possible
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or both grindlng stages.
The object of the present invention is to provide a novel
and useful grinding method in which all the advantages afforded
by the autogenous grinding technique are retained whilst, at the
' same time, substantially eliminating the aforementioned disadvan- -
tages.
To this end there is proposed in accordance with the inven-
tion a method of the type mentioned in the introductlon in which
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the lump material is divided into a coarse and a finer fraction
in which the coarse fraction is charged to the drum mill whilst
the finer fraction is charged to a crushing device and is crushed
therein to a largest particle size in the order of magnitude of
at most one third of the smallest particle size of the coarse
fraction, and is thereafter charged directly to the drum mill and
is ground therein whilst utilising the aforementioned coarse frac-
tion as grinding bodies. By dividing the lump material and crush-
ing the separated finer fraction to said relationship between the
material fractions charged to the mill, there is obtained a uni-
form and high throughput in the mill without any appreciable
tendency of the pieces of material of critical size to accumulate
therein and without any appreciable tendency of variations in the
degree of filling of the mill whilst maintaining the amount of
material charged to the mill per unit of time constant. In this
way the need of substantial over-sizing the mill is obviated, with
considerable saving in cost as a result thereof, at the same time
, as the process apparatus arranged upstream and downstream of the
mill can be dimensioned to work with substantially constant quanti-
ties of material per unit of time without requiring the arrangementof storage bins etc. upstream or downstream of the mill. The
material ground in the drum mill may be subjected, to advantage,
to a secondary, further grinding operation in a drum-type secondary
mill downstream of said drum mill.
By "largest particle size" is meant here and in the following
a measurement which corresponds substantially to the smallest slit
width of the crusher, and by "the smallest particle size of the
coarse fraction" is meant a measurement which corresponds substan-
tially to the smallest mesh size or the like of a screen device
capable of being used to separate the desired coarse fraction from
the remain~er of the ingoing lump material.
In a primary autogenous grinding operation,when applying the
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method according to the invention, the smallest particle size
of the coarse fraction is suitably approximately 80-150 mm, al-
though this range is not critical. On the other hand, the lump
material should be divided, if possible, in a manner such that
the coarse fraction is substantially consumed during grinding of
the finer fraction. The amount of the coarse fraction obtained ~ -`
may be sufficient, however, to form in the mill an intermediate
fraction which can be screened therefrom and used as grinding
bodies in a subsequent, drum-type secondary mill working in ac-
10 cordance with the autogenous grinding technique. e.g. in accord-
ance with the method disclosed in the U.S. Patent Specifications
3,924,814 and 3,942,727. An excessively large percentage of
coarse fraction in the primary mill should be avoided, however.
The throughput will normally decrease when the coarse fraction ~;
comprises re than 40% by weight of the grinding charge. Ac-
cordingly, the lump material is conveniently divided so that said
; coar~e fraction comprises approximately 0.15-0.35 times the total ~ ~
guantity of the original lump material, optimal, and therewith ~ -
preferred conditions, being obtained at a value of approximately
0.2S. Should the smallest particle size of the coarse fraction
have been selected such that the amount of coarse fraction is in-
suffi¢ient to provide the desired grinding of the goods arriving
from the crusher device, said division of the lump material may
be modified in a manner such that the requisite quantity of o~se
fra¢tion having a somewhat smaller least partic~e size isobtained,
or further grinding bodies which consist of a forelgn material,
e.g. steel, can be charged to the drum mill to the extentre~i~d.
If, in this latter case, the primary ground goods are further
ground in a subsequent drum-type secondary mill, the ~rrangement
des¢ribed in the aforementioned U.S. Patent Specifications
3,924,814 and 3,942,727 can be used to advantage, thereby ren-
dering it unnecessary to separate the aforementioned for~ign
grinding bodies from the primary ground material in a
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specific separating operation.
In order for the grinding bodies formed by said coarse
fraction to have an optimal effect on the material obtained by
crushing the aforementioned finer fraction, said material should
be crushed to a largest particle size ranging from a fifth to a
tenth, preferably at most approximately a sixth of the smallest
particle size of the coarse fraction.
The finest particles of the finer fraction, and also those
particles of small size obtained during the crushing operation,
have, in some instances, a tendency to partially clog the crusher
in a manner such as to considerably reduce its capacity. In order
to circumvent this disadvantage, in accordance with the invention
material having a maximum particle size of at most one third of
the smallest particle size of the coarse fraction can be separated
from the said finer fraction prior to crushing the same and charged
directly to the drum mill. When grinding in the presence of water,
however, it is preferred to avoid this clogging of the crushing
device by charging at least a part of or the whole of the amount
of water required for the wet-grinding process to the crushing
device, the water being caused to accompany the crushed material
to the drum mill. In -this way the crushing devic~ is flushed clean
by the water charged thereto, said water forcing the material
being flushed through the crushing device in a manner such as to
improve the capacity of said crushing device instead. The water
can also be used as a transportation medium for carrying the
crushed material to the drum mill. In this latter method, there is
therefore no need to separate the finest particles of the said
finer material fraction in a separate screening device, whilst the
arrangement provided for conveying the crushed material from the
crushing device to the drum mill can be greatly simplified.
The invention will now be described in more detail with
reference to a preferred plant illustrated schematically in the
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accompanying drawing.
The plant illustrated in the drawing comprises a screen 10,
a crusher ll and a drum mill 12 for primary autogenous or semi-
autogenous wet-grinding. ~ pre-crushed lump material, e.g. ore,
is charged to the screen 10 in the manner indicated by arrow 13.
The material is divided by the screen lO into a coarse fraction,
the amount of which comprises 15 to 35% by weight of the total
amount of said material. ~s shSown by the arrow 14, this coarse
fraction is passed directly to the mill 12, whlle the finer frac-
tion passing through the screen is charged to the crusher ll, asshown by the arrow 15, in which crusher said fraction is crushed
to a particle size which is at most one third of the smallest
particle size of the coarse fraction. ~s indicated by the arrow
16, at least part of the water required for the wet-grinding
operation is charged to the crusher 11, said water being effective
to force the material through said crusher. The crushed material
and water departing from the crusher are collected in a chute as
shown at 17, which chute conducts said material and water directly
to the mill 12, as indicated by the arrow 18, in which mill the
goods are ground whilst utilising said coarse material fraction as
grinding bodies, and is thereafter discharged from the mill, as
indicated by the arrow 19. The mill 12 may be included as a primary
grinding stage in a grinding plant of the type illustrated in US
Patent Specification 3 942 727, wherewith the discharge end may be
modified to advantage in the manner illustrated in US Patent
3 924 ~14.
The invention, however, is not limited to use in conjunction
with the illustrated and described plant. Thus, as indicated by
means of the arrow 20, water and/or a given quantity of foreign
grinding bodies may be charged directly to the mill 12. Further,
as indicated by means of dash-lines 21, the screen 10 may be
arranged to separate from the said finer fraction small particles
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capable of passlng through the crusher 11 without being crushed
therein. As indicated by means of the arrow 22, these finer
particles in the illustrated embodiment are charged directly to
the chute 17, to which water can be charged, as shown at 22, to
convey these particles into the mill 12.
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