Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/20—Silicates
  • C01B33/24—Alkaline-earth metal silicates

Definitions

• the present invention relates to a process for pre ⁇ paring a synthetic raw material of cyclowollastonite (previously designated pseudowollastonite and represented by ⁇ -CaO-SiO ) which after crushing and grinding is an attractive additive in glazes and ceramics, in powder mixtures used as lining/barrier layers in electrolysis furnaces for the production of aluminium, in different powder mixtures used in welding and in continuous casting of steel, and in plastics, paint and paper where it may be applied as filler and/or as pigment, as per se known.
• pseudowollastonite previously designated pseudowollastonite and represented by ⁇ -CaO-SiO
• Wollastonite is a naturally occuring mineral formed by contact metamorphosis in limestone.
• the mineral has triclinic crystal structure and occurs normally in ray ⁇ like, usually fibrous aggregates.
• the compo ⁇ sition on weight basis is 48.3% CaO and 51.7% Si0 2 , but calcium may to some extent be replaced by contaminations, such as divalent iron, manganese and magnesium, as the case usually is if the wollastonite has been formed from impure limestone.
• the fibrous character is characterized by the socalled "aspect-ratio", which represents the mean ratio between fibre length and diameter.
• aspect-ratio represents the mean ratio between fibre length and diameter.
• wollastonite powder with an aspect-ratio in the range 1/15 - 1/20 is demanded and well paid for.
• the fibrous character of the powder may for health reasons be directly undesired. Examples are ceramic and metallurgic industry where there are strict restrictions with respect to the handling of dry powder materials with a fibrous grain shape.
• the rotary kiln technology is in no way simple, cf. British patents Nos. 1174919 and 1465635.
• reactive raw materials are required, preferably also addition of mineralisation reagents, careful charge preparation in the form of grinding, mixing and com- pacting, and control of residence time and temperature at a level which is close to the limit of what may be tolerated without the occurence of local melting and resulting ring formation in the kiln.
• the process must be operated optimally in order for the resulting clinker to contain more than 90% pseudo-wollastonite.
• the rest may be unreacted SiO- and silica rich glass phase, in the worst case also unreacted CaO and Ca_Si0..
• the presence of the latter two phases is particularly unfortunate, primarily due to their pronounced tendency to form hydrates.
• polymorph inversion of the disilicate between the modifications ⁇ / ⁇ which is accompanied by change in volume as per se known.
• German patent No. 568832 from 1933 which relates to a method for preparing refractory blocks with a wollastonite compo ⁇ sition intended for use as lining in glass furnaces.
• the cooling immediately after the casting should take place very slowly, but nothing is said with respect to the structure of the product.
• the melting method which accordingly may be said to be known, has also been considered as a possible alterna ⁇ tive to the sintering method as a process for preparing crystalline pseudowollastonite in both the above mentioned British patents.
• the patentee in British patent No. 1174919 asserts without any documentation that he has succeeded in the preparation of pseudowollastonite by a process which includes melting and subsequent crystalli ⁇ zation as the essential steps, but he states that the method is uneconomical.
• British patent No. 1465635 it is stated that the melting process does not give a completely crystalline wollastonite directly, but a product which has a vitreous phase associated with a "crystalline phase formed by high temperature wolla ⁇ stonite". It is further stated that this "crystallo- vitreous" form limits the commercial and industrial uses of the product, and supplementary treatments are necessary to obtain conversion to crystalline form.
• Example 1 where the melting itself was carried out in an induction heated and open graphite crucible. To ensure that all charge and particularly all lime was completely dissolved in the melt, the latter was heated to 1600°C and kept at this level for about 1 hour with periodic stir ⁇ ring, before casting was performed (see experiments 1 and
• a process for preparing synthetic wollastonite which does not contain any detectable phases other than crystalline cyclo- wollastonite and which upon crushing and/or grinding yields particles having an irregular, round-edged shape without any detectable fibrousness.
• the process is characterized by the use of purest possible CaO and SiO_ containing raw materials which are finely ground, preferably to a grain size less than 10 mm, mixed in the proportions stated below, and gradually charged to a melting furnace, fused to a homogeneous melt in the melting furnace by heating to above 1550°C, and cooled to a solidified product which is crushed/ground to the desired particle size, the amount of starting material being adjusted so that the molar ratio in the melt between s -° 2 an ⁇ divalent metal oxides, including CaO, is above 1- 0.014 • (% A1 2 0_) , where "% l ⁇ O " represents percent by weight of A1 2 0 3
• a conventional electric melting furnace is a suitable apparatus for preparing a melt having wollastonite composition.
• the furnace may be a single-phase furnace having a vertically positioned cen ⁇ tral graphite electrode and a carbon-lined furnace pot as counter-electrode as described in Example 2, or it may be a single-phase furnace having 2 electrodes, or optionally a more conventional 3-phase furnace having 3 electrodes.
• the furnace crucible may in the latter cases be cerami- cally lined, or it may be surrounded by water-cooled or oil-cooled panels so that the melt forms its own lining by solidifying on the panels.
• Graphite may equally well be replaced by cheaper self-baking carbon electrodes of the S ⁇ derberg type, or it may be chosen to replace these with plasma burners suitable for transferred arc operation - if it is essential to melt in a non-carbon environment.
• the process is carried out as a series of batchwise melting steps comprising a charging period, wherein the charge-materials are added, preferably continuously, so that the bath all the time is kept well covered by a solid charge, a melting down period, and finally a period of tapping before the charging for the next step is initiated.
• the thermal efficiency of the furnace is highest when the bath is covered, and lowest when the bath is open and the temperature is highest immediately before tapping - while at the same time the thermal load on electrode holders, roof etc. is then greatest.
• cyclic melting it is therefore desired to make the melting down period as short as possible without affecting the quality of the product with respect to homogenity and consistency of the composition, and it is desired to keep the temperature as low as possible, preferably below 1650°C, desirably at a maximum of 1600°C, but above 1550°C.
• the choice of raw material and the sizing are important parameters to take into consideration in addition to those which only concern the melting technology, which among other things include electrode submersion, bath movement and the possibility to influence this, for instance by using gas for stirring.
• the Si0 2 ⁇ source is the one which has the lowest melting point and ini ⁇ tially forms the molten phase together with dissolved CaO.
• lime particles which as such have a very high melting point, and which also in the presence of a Si0 2 - rich melt always will be surrounded by a layer of high- melting Ca 2 SiO.-phase, are dissolved more slowly than quartzite and that coarse lime particles may stay floating on the bath surface for a relatively long time after all the quartz had been dissolved. This was the reason why we in the subsequent example decided to choose the size 0-6 mm for the CaO components which then were somewhat finer than the quartzite, with a size grading of 0-10 mm.
• the grain size of the CaO component should be limited to about 10 mm due to the dissolution rate.
• the upper grain size of the Si0 2 -component should also not be essentially different.
• the melting process does not present any other specific requirements as to the physical form of the raw materials than this. If the size grading is initially acceptable, the handling of the raw material consists in that they are weighed separately, batchwise or continuously, mixed in proper proportions and transported into the furnace where they are distributed over the melt surface in a suitable and known manner without any essential occurence of any type of selective accumulation or loss on the way. Factors which will then be decisive for the choice of raw ateri- als for the melting process, are availability, price, chemical composition and chemical stability.
• Burned lime is therefore a preferred CaO-source in the melting process.
• the quartzite used as Si0 2 -source in the present investigations was chosen because it is available as a cheap and suitably localized waste product although basically a more pure material would be preferable, parti ⁇ cularly with respect to the content of A1 2 0 3 and MnO. These contaminations are entirely recovered in the product and contribute to reduce its quality, while contaminations of Fe 2 0 3 and Ti0 2 under the prevailing reducing melting conditions were found predominantly to be reduced to metallic form and were to a minor extent found as oxide in the wollastonite melt.
• the metal phase formed under such conditions will get little opportunity to separate out in the form of a uniform melt phase which may readily be isolated from the oxide melt by tapping, but is found as small metal globules suspended in the oxide melt from which they may be removed by magnetic separation after crushing. This introduces the need for a separating step making the process more expensive, and it is accor ⁇ dingly preferred that the raw materials have a low content of reduceable oxides, preferably below 0.4%.
• the melting process is suitably carried out according to the invention by fusing the added raw materials com ⁇ pletely in an oxide melt phase which upon tapping and subsequent cooling and solidification outside the furnace crystallizes and forms the purest possible cyclo- wollastonite as the only phase.
• the process is adapted to industrial operation, i.e. that ordinary contaminations of the type FeO, MnO, MgO and A1 2 0 3 , which necessarily will be brought in by using readily available and cheap raw materials, and fluctuations in the melt composition, which is frequently a result of incidental operational disturbances and un ⁇ certainty with respect to the chemical analysis of the raw materials, may be tolerated without any reduction of the product quality, for instance by the formation of detri ⁇ mental dicalcium silicate phase.
• This is attained by considering the most important contaminations in the calculation of the charge and specifying the mixing ratio so that the melt composition with sufficient margin will have a ratio between Si0 2 and divalent metal oxides ex ⁇ pressed by the term:
• N reflexMe ⁇ / o ⁇ is the sum of the molar fractions of all divalent metal oxides occuring in the oxide melt in ad ⁇ dition to CaO.
• the stated molar ratio has a calculated border value of 0.99
• the real composition has varied between a molar ratio of 1.07 and 1.12.
• the crystallization takes place completely independent of the cooling rate and independent of whether the solidification takes place in a hot or cold graphite chill mould, in a large cast iron chill mould with stationary bath, or whether the bath is stirred, for instance as a consequence of oxygen being injected during the solidification, in water or in cold copper chill moulds having thick walls, but the macro- and micro-structure of the material is of course influenced by whether the cooling takes place slowly or rapidly and whether the bath is stationary or is stirred.
• Powder prepared by direct grinding of fused wolla ⁇ stonite produced according to the process has according to measurements reported in Example 2 a whiteness or brightness which does not satisfy the requirements of the market for certain applications.
• Reduced whiteness may be due to the presence of socalled dark oxide contaminations such as MnO and FeO, but it may also be due to a certain content of elementary carbon in the material, in the order of magnitude of about 0.1% C.
• the latter component may be effectively removed by annealing the powder in air, and thereby the whiteness is improved considerably.
• the oxide contaminations must be eliminated by choosing purer starting materials for the melting, or with respect to FeO, by carrying out the melting under sufficiently reducing conditions. Reducing melting conditions may also be attained in a carbon-free environment, for instance by adding metallic silicon or ferrosilicon in small amounts.
• the batches were heated to 1600°C in open graphite cru ⁇ learners which were heated by induction and kept at the temperature for about 1 hour until everything had melted, while at the same time the bath was carefully homogenized by periodic stirring with a graphite rod.
• the x-ray reflex at the d-value 3.764 is for instance not detectable in the diagrams which represent Experiments 1 and 2, but is barely visible on Fig. 2 which relates to Experiment 3. Extended annealing of the latter sample in air did not result in any change in the x-ray diagram of the sample. Accordingly, the conclusion is drawn that the lack of conformity is caused by contaminations present in the cyclowollastonite structure in our samples. However, there are no traces of other crystalline phases in any of the diagrams, and nor are there any traces of glass phase which would normally be recognized by a displacement of the base line over a certain range in the x-ray diagram.
• Example 2 The same raw materials as used in example 1, with the exception that limestone with a size distribution between 0 and 6 mm, had now been chosen, were mixed in approxi ⁇ mately the same proportions as in Example 1 and melted in a single phase electric furnace with transformer capacity of 150 kVA.
• the furnace used was stationary and equipped with a vertical adjustable center electrode of graphite and with an open carbon-lined furnace crucible as counter- electrode and with a tapping hole in the bottom.
• the melting campaign comprised two experiments. The first one started with cold furnace by short circuiting the electrode against the bottom, whereafter a charge of 2-3 kg was placed around the electrode tip.
• a molten pool was rapidly formed in the arc crater, and more charge could be added so that the bath became more or less covered by solid charge, and the electrode very soon became submerged in the melt. This continued until all charge, i.e. 80 kg, had been added, whereafter the cover layer was melted down, and the bath temperature was measured and adjusted up to about 1700°C before the melt was tapped from the bottom and directly into a 60 liter conical cast iron chill mould, where the melt was allowed to cool without further treatment.
• Fig. 2 Thin polished test samples (thin sections) were made from compact lumps of both product types, and these were examined in light microscope connected to a picture analy ⁇ zer.
• Fig. 3 is the product from Experiment 1 which consisted of long prismatic, partly fibrous, possibly lamellar crystals which generally were several mm long. With increasing magnification as illustrated on Fig. 4, it will be seen more clearly that there are nickings in the form of hairline cracks across the longitudinal direction of the crystals.
• the mean characteristic crack distance was for the present sample measured to 100 ⁇ m, while the mean thickness of the crystals was measured to 56 ⁇ m. It was therefore not unexpected that this material upon crushing and subsequent grinding in a jet mill to ⁇ 10 ⁇ m yielded powder particles having a round-edged shape with ⁇ out any sign of fibrousness. Pictures taken by means of a scanning electron microscope (SEM) confirm this, see Fig. 5 which illustrates an arbitrarily chosen area of the sample support at 2000x magnification.
• SEM scanning electron microscope
• the brightness of the material may be influenced, for instance by annealing ground powder in air. It may for instance be mentioned that a product sample which after grinding in a jet mill had an ISO brightness of 70.3%, acquired an improved brightness to 84.9% after annealing the sample in air for 12 hours at 800 C.
• a thin section of solidified material was prepared with orientation parallell with and perpendicular to the solidification front which here is assumed to have moved parallell with the vertical chill mould wall in the first case and parallell with the horizontal plate in the second case.
• Fig. 7-10 illustrate the material which was cooled at the fastest rate, viz. where the melt was allowed to solidify in a thin layer directly against a cold copper surface.
• Fig. 7 and 8 illustrate with different magnification a homogeneous area located some distance above the boundary surface against copper and consisting of fibrous or lamellar aggregates varying between 100 ⁇ m and 2 mm in length. Individual fibres in these agglomerates have a mean thickness of 6 ⁇ m, and there is a relatively long distance between observable cracks across the longitudinal axis of the fibres.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
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• General Life Sciences & Earth Sciences (AREA)
• Geology (AREA)
• Inorganic Chemistry (AREA)
• Silicon Compounds (AREA)
• Medicinal Preparation (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)
• Glass Compositions (AREA)

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Citations (1)

• 1990
  • 1990-10-31 NO NO904732A patent/NO171497C/no not_active IP Right Cessation
• 1991
  • 1991-10-30 JP JP3518223A patent/JPH06502380A/ja active Pending
  • 1991-10-30 EP EP19910919631 patent/EP0555320A1/en not_active Ceased
  • 1991-10-30 WO PCT/NO1991/000133 patent/WO1992007794A1/en not_active Application Discontinuation
• 1993
  • 1993-04-29 FI FI931931A patent/FI931931A/fi not_active Application Discontinuation

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