Note: Descriptions are shown in the official language in which they were submitted.
"3~
Grinding means for producing mechanical woodpulp
The present invention relates to a grinding means for use in
the production of mechanical woodpulp or in pulping, said means
comprising abrasive aluminum oxide, wolfram carbide, silicon
carbide or the like fragments and a bond for holding them
together.
In this application9 the term grinding meansJmeans the
above-described homogeneous pulpstone, an abrasive layer
affi~ed on the separate pulpstone block, and abrasive segments.
At present, a pulpstone generally comprises a concrete or metal
body, having a separate abrasive layer or segments affixed
thereon. A segment construction is generally known in the art
and it is disclosed e.g. in US Patent publications 2 887 276
and 2 421 885.
In terms of woodpulp quality the most critical component o~ a
pulpstone is the grinding means. It must be capable of
producing a desired type of woodpulp and it must be capable of
sustaining the mechanical and thermal stresses exerted thereon
during grinding.
The bond used in presently available grinding means is nearly
exclusively ceramic. Thus, the abrasive fragments, usually
either A120< or SiC, are bonded together by a ceramic bond,
generally glass. Such a co~position is disclosed e.g. in US Patent
publication 2 769 286.
The brittleness o~ ceramic materials encourages the wearing
thereof, rasulting also in deterioration of the strength
properties of pulp. Therefore, the grinding means made with a
ceramic bond must o~ten be tooled, which rapidly wears down the
grinding means. In tooling, the surface of a ceramic bond is
machine~ wi~h a special tool ~o produce a thick ~rray of
: ~"
, ~.. ~ ' `
:, :.,
, , . ,. : .
.~:
, ,
~'7~
grooves which usually forms a 28 angle relative to the axis of
rotation of a pulpstone. The relative distance of grooves is
2...3 mrn and depth circa 1 mm. The purpose of tooling is to
provide the fibers removed from wood with a space whereby said
fibers can leave the grinding zone without becoming re-ground~
In addition, the tooling is used to proportionate the grinding
area of a grinding means properly relative to the grinding
pressure and the quality o~ pulp to be produced.
Another significant drawback, especially with presently used
ceramic grinding segments, is the splitting thereof which also
has to do with their brittleness. With the present high-duty
pulp grinders this problem is even more pronounced. Splitting
of a segment takes place when it is subjected to an intense
thermal shock. Such a situation develops e.g. when a block of
wood to be ground goes endwise against the abrasive surface.
Since an endwise piece of wood is ground a lot slower than a
crosswise one, an extremely high pressure develops at such
wood. As a consequence, also the temperature rises locally very
high which may result in damage to the segment.
Such damage often results in at least one-day production
break on pulp grinder. If the stone cannot be fixed and it must
be replaced with a new one, the breakdown will take several
days.
The brittle segments also limit deployment of a stone from room
temperature. The fact is, namely, that a pulpstone or
grindstone must then be carefully heated up to 2 days before
commencing actual grinding.
The use of a metal in a grinding means is also prior known from
e.g. SE publication prin-t 309 529. This grinding means is
fitted with a metal sur~ace, provided with semi-spherical
elevations for grinding effect.
, : :
-: .
~:~67~9~.~
Such a solution is practically impossible for several reasonsO
The nodules provided on a metal surface wear away rather
rapidly with a consequence that said metal sur~ace must be
frequently replaced. The replacement is always inconvenient and
a tedious process which accordingly decreaæes production
efficiency and overall grinding economy.
An object of this invention is to provide a grinding means used
in the mechanical production of woodpulp or in defibration~
such as a homogeneous grinding stone or abrasive segment, whose
wear resiætance and mechanical strength are both superior to
those of the prior art solutions. According to the invention,
this object is achieved in a manner that the bond consists of
sintered metal.
In view of the pulping porcess it is preferable that a sintered
metal, interspaced between abrasive fragments a~d being softer
than said abrasive fragments~ wears away more during grinding
than said abrasive fragments.
Due to the conditions prevailing in a pulping process, the
preferred metal is a corrosion-resistant or stainless metal,
such as stainless or acid-proof steel, e.g. AISI 304 or 316 or
316L. The sintered metal used as a bond can also be copper or a
copper alloy, such as brass.
In a grinding means of the invention, the abrasive or grinding
fragments can comprise any presently available abrasive
materialj such as aluminum oxide, various carbides etc.
.. ,
The proportion of abrasive fragments in a grinding means is
preferably 10-70 percent by volume and most preferably 30-50
percent by volu~e.
~ sintered grinding means, bonded with a metal matrix, can be
porous, e.g. 5-30 % of its volume.
.-
.: ',,: '~
,, : . :
.
~ ~ .
~67~
The size of abrasive fragments ls preferably circa 150~700 ~m.
Sintering is effected e.g. as follows:
The mixture of powdered metal and abrasive fragments is
compressed together with a force sufficient to give the block a
handling strength in order to convey it into a furnace without
any support means or mould. This is followed by heating the
block in the furnace to e.g. 1000-1500C, i.e. suitably below
the melting point of a metal employed in a suitable protective
gas or vacuum, if desired. Thus, the metal particles adhere to
each other building around abrasive fragments a matrix to which
said abrasive ~ragments are firmly affixed.
This is a preferable way of producing e.g. grinding segments or
sectors, which are conventionally mounted on the surface of
e.g. a cylindrical, conical or disc-shaped body made of a metal
or some other suitable material.
The analyses have indicated that the compression strength of a
grinding segment of the invention, being e.g. up to 1000 N/mm~,
is approximately tenfold and its wear resistance up to fivefold
as compared with presently available ceramic segments. Despite
the high compression strength achieved, a grinding segment of
the invention is tough and does not fracture easily the way the
available ceramic segments do if subjected to mechanical or
thermal shocks.
The improved wear resistance renders possible to make the
wearing layer of a grinding means thinner than at present.
Since the strength and toughness of a sintered metal relative
to the corresponding properties of a ceramic bond are
substantially better, the grinding can be effected with
consistencies and temperatures substantially higher than at
present whereby, e.g. in pressure grinding, the strength
characteristics of pulp are improved and the recovery of heat
becomes more effective.
:- .
:':
' . ' ' ' ' ' ~:
~6~
The construction of a grinding means of the invention has been
illustrated in th accompanying drawing, which shows the
construction of a grinding means of the invention in grinding
situation cross-sectioned perpendicularly to the longitudinal
axis of a block of wood to be ground.
The drawing illustrates the position where the abrasive
fragments 1 on the surface of a grinding means contact the
surface of wood 5 to be ground. The grinding direction of a
grinding means is designated by -the arrow. At the ~ame time,
the fragments penetrate partially into the wood compressing it
also, whereby fibers 4 come off the wood as a result of
grinding.
The abrasive fragments are bonded together by sintering
therearound a matrix 2 of metal particles. It is possible that
sintering leaves the matrix with a certain amount (0...20%) of
pores 3. When softer bond 2 wears off the abrasive surface, the
abrasive fragments will be exposed. As the fragments travel at
a high rate of speed against the surface of wood, there is
generated in the viscoelastic fiber matrix of wood a
high-frequency oscillation which generates the heat that
softens the lignin serving as the bond of fibers. This way the
fibers can be removed from wood.
A grinding means of the invention can be used not only in pulp
grinders but also in a variety of disc, conical or the like
refiners, grinders and in other equipment intended for
mechanical pulping of wood. In this case, of course, the block
design beneath the abrasive surface layer must be constructed
appropriately for each purpose. It is possible that abrasive
fragments include just one quality or a plurality of qualitics
in proper ratio to each other.
':~
