Note: Descriptions are shown in the official language in which they were submitted.
~2~.3~34
The invention relates to a process for stabilizing
pellets of pulverulen-t, granular or powdery mineral materials,
for example, ores, dolomite, carbonate or phosphate salts for
the chemical industry, the glass or ceramics industry, agri-
culture and the building trade or the like, which are unstable
after being agglomerated, due to redrying, hygroscopicity or
reaction with -the atmosphere.
Large amounts of pulverulent, granular or powdery
materials turn up in industry as raw materials or as scrap. For
various reasons, the storage, transport and further processing
of these materials in the existing form create considerable
difficulties. These ma-terials include dusts from gas purifica-
tion plants, cement dusts, and other similar dusts, which are
formed during loading, transporting and dumping and therefore
represent losses and a considerable contamination of the
environment. They also include hygroscopic materials, such as,
mineral fertilizers and building materials which, depending on
the degree of -their hygroscopicity
~ 3734
and on their water-bonding capability, sometime become lumpy.
Often, in the case of the more hygroscopic materials, they
may dissolve and actually become useless for transporting and
for further possible processiny. Also, included in this group
are materials which react chemically with oxygen or other com-
ponents of the air and undergo a change in their material com-
position during prolonged storage, which may also make them
useless for further processing. Finally, these materials in-
clude those, for example, smeltable ores, which are obtained,
to a considerable extent, as dusts or finely divided materials.
These can only be processed further, or are preferably processed
further, in vertical kilns, however, only or at least preferably
in the form of larger pieces or aggregates.
Depending on the nature of the problem, various
measures are taken in practice to counteract the above-noted
disadvantages. On the whole, however, these measures are very
expensive and sometimes do not satisfactorily solve the prob-
lems. For example, hygroscopic materials, and those which
react wi-th components or the atmosphere are stored and trans-
ported in laminated bags. This results in considerable
expenditures for packaging and transporting, in comparison to
the usual method oE transporting dust or grain materials by
belt or pipe conveyors. Also, the results are not completely
satisfactory since the contents of the opened package, which
~213734
are not used immediately, are once again~exposed to air and
therefore of moisture which is undesirable. Storage and
transport in sheet metal containers, which can be permanently
locked, is prohibitive for cost reasons.
The manufacture of nitrate-containing fertiliæers
with higher acid contents for the purpose of avoiding ammonia
losses in mineral fertilizers also does not lead to a satis-
factory result. The oxides of nitrogen, which are formed in
these mineral fertilizers represent a loss of nutrient.
~dditionally, they are extremely injurious to health and form
corrosive compounds which disintegrate the packaging material
ater a short time.
The agglomeration of these materials by compacting
and pelletizing with a pelletizing liquid, usually water, also
does not lead to a satisfactory result. This is because with
hygroscopic and/or materials which react chemically with the
atmosphere, such properties have not been eliminated. While
these disadvantages do not exist for a large portion of the
other materials referred to, such as, ores, coal dust, and
most of the dustwaste, these materials are still not amenable
to compaction pelletizing. This is because, after being
pelletized, the pelletizing liquid escapes into the atomsphere
and the pellets disintegrate into the original dust form under
the sllghtest compressive stress.
~3~3~
SUMMARY OF THE INVENTION
I have discovered a process for making stable pellets
for materials which are not normally amenable to the formation
of permenent huildup agglomerates. In this connectiorl "build-
up agglomerates" are understood to be those granulates which are
formed from grain or dust material by a mixing or rolling mo-tion
of the material in the absence of or wit.h the addition of a
liquid phase as a granulating liquid. The present process is
effective irrespective of the consistency of -the material to be
processed. More particularly, the process of the present in-
vention comprises granulating the finely grained or dust-like
material using known procedures with the addition of a pelletiz-
ing liquid. Thereaf-ter, the pellets formed are coated wi-th a
mineral ma-terial which is insoluble or sligh-tly soluble in water
selected from the hydroxides of metals of the second main group
and sub-group of the Periodic System.
I'he invention provides a process for pelletizing
materials whi.ch are amenable to agglomeration with a pelletiz-
ing liquid but do no-t Eorm a stable agglomerate. This is due to
either subsequen-t drying, the subsequent absorption of water
because of their hydroscopicity, or chemical reactions with
substances contained in the surrounding atmosphere. This
5 --
I
.; ~-,
. i
~2~L3739~
~an result in rapid changes in consistency and des-truction of
the agglomerates. With the present invention, access of mois-
tures or other components of the atmosphere to the pellet
material is reliably prevented by coating the surface of the
pellets or granulates formed with a layer of material, which is
insoluble or slightly soluble in water. Consequently, not only
the physlcal properties of the pellet, especially its strength,
but also, its chemical stability are permenen-tly secured.
Material which satisfies the above conditions can be
used is selected from hydroxides of metals of the second main
group and subgroup of the periodic system which are parti-
cularly advantageous. On the one hand, these hydroxides are
also mineral materails which cause no interfering reactions in
most processing procedures and can even be used to advan-tage.
On the other, they can be applied -to the pellets in an extre-
mely simple manner in that the material, which is first of all
to be agglomerated, is prepared with an addition of pellet.iz-
ing liquid, and the coating material is applied as the dry metal
oxide and hydroxylated by surface moisture or the addi-tion of
water.
In the case of the above-described embodiment of the
inventive process, the selection of coating material depends on
the nature of the pellet material, as well as on its fur-ther
use.
6 -
~L373~
For e~ample, calcium hydroxide (Ca(OH)2) can be regarded as a
preferred coating material, which can also be used to advantage
for mineral materials in most areas of application, as in
agriculture for neutralizing acidic soils, in the building trade
as a binder, and in metallurgical smelting processes as a base
material for slag formation~
In a further embodiment of the inventive process,
coating can also be accomplished by spraying an agent in
liquid, dissolved, or dispersed form, which reacts with the
pellet material to form a compound, which is insoluble or
slightly soluble in water. This forms a protective sheath
in which prevents the diffusion of moisture. Depending on
the pellet material and the nature of the further processing,
dilute acid or alkali, organic solvents, such as, gasoline,
benzene, methanol, ethanol, carbon tetrachloride, paraffin or
the like can be used as the agent. Depending on the particular
case, the agent, which reacts with the pellet material, can
be added in an amount so that there is only partial conversion
of the pelletized material during the reaction. An example
of the application of this embodiment of the process is the
pelletizing of natural phosphates, for example, phosphorite
and apatite, which are used in agriculture, or the pelletizing
of dolomite (calcium magnesium carbonate). In these cases,
coating is advantageously accomplished by spraying dilute
~2~;~73~
sulfuric acid on the pellets or by using dilute sulfuric acid
directly as the pelletizing llquid in the final phase of the
pelletizing process. The amount of sulfuric acid added is such
that the tertiary phosphate is converted only to the calcium
hydrogen phosphate (CaHPO4), anhydrite (CaSO4xH2O) being formed
at the same time. The sheath formed in this manner, consists
only of calcium hydrogen phosphate, which is soluble in mineral
acids, and the insoluble anhydrite. Thus, although the access
of moisture to the core of the pellet and the diffusion of the
moisture to the atmosphere are prevented, the grain can never-
theless be solubilized because of the solubility of the calcium
hydrogen phosphate in the mineral acids of the soil.
The coating process is advantageously carried out in
a plough share mixer at a drum circumferential speed of 0.120
to 0.180 and, preferably, of 0.165 m/sec. and at an operating
temperature of 30, the temperature increase during the bonding
phase being limited to a maximum of 10 by controlling the
addition of li~uid. In this procedure, the amount of bonding
binder, ~or example, of CaO or CaSO4, added to l mole of
water per mole of substance, is 2 to 5 weight percent and
preferably 2 to 3 weight percent, based on the total weight
of finished pellets.
~LZ~373~
The following examples illustrate the invention.
EXAMPLE l
Dolocate, a mixture of dolomite and calcium carbonate
(40.0 kg) is weighed into a pelletizing plow share mixer of
300 L capacity and, after addition of 7.0 kg of water as the
pelletizing liquid, is processed into pellets. After a
mixing time of 15-20 minutes, 3.0 kg of quick lime are added
and mixed intensively with the pellets until there no longer
i.s an increase in temperature. Hydroxylation according to
; 10 the reaction CaO + H2O = Ca(OH~2 is then finished and durable,
solid, storable and transportable pellets are obtained for
use as fertilizer or in the fermentation industry.
EXAMPLE 2
Calcium phosphate (50 kg) and 11.25 kg of water as
pelletizing liquid are weighed into a pelletizing plough share
mixer of 300 L capacity and processed into pellets. Now 2.75
kg of concentrated sulfuric acid (98.0-99.5 weight percent)
are added and dlstributed unformly over the surface of the
pellets by mixing at 50-60C. The formation of the sheath
proceeds on the surface according to the reaction Ca3(PO4)2 +
H2SO4 = CaSO4 ~ 2CaHPO4 and then CaSO4 + H2O = CaSO4 x H2O.
lZ~L373~
The resulting pellets are mechanically solid, stable, storable,
as well as trar.sportable and are suitable for the normal
practical use.
--10--
