SE513273C2 - Crucible for spectral analysis of mineral samples melted with a lithium borate flux - Google Patents

Abstract

A crucible used for optical or X-ray spectral analysis of useable sample bodies by oxidative melting of the sample material in lithium borate flux comprises a ceramic material with an open porosity. At least part of the crucible that comes into contact with the sample material, flux or melt is coated with an air permeable protective layer essentially comprising boron nitride particles. The preparation of these crucibles is also claimed, comprising the formation of a crucible body from a ceramic casting material that contains a high proportion of chamotte, then drying and firing the cast at ca. 1100-1200 deg.C, and then coating part of the porous crucible with a boron nitride slurry using spraying, slurry casting, doping or other application techniques, before finally drying the coated crucible. Preferably the boron nitride slurry comprises 50-80 % volatile organic liquid, 10-30 % boron nitride particles, 1-10 % combustible organic binder, 1-10 % high temperature inorganic binder and an even smaller amount of anti-foaming agent and pH regulator. The slurry coating is pre-dried and then wetted with an aqueous silica sol before the final drying step.

Description

l0 l5 20 25 '513 273 IQ The property of bomitrid crucibles is particularly valuable in that they are not wetted by glass melts.

In our previous patent applications SE-A-9101629 and SE-A-9504180, both in connection with the above-mentioned sample preparation with borate fl uss, the production of amorphous test specimens is described, as well as a device for digesting test material and casting test specimens, comprising a crucible with holes at the bottom intended to reduce the contact time between material and crucible wall during digestion of test material with lithium borate fl uss. During further work intended to further develop and for this sample preparation technique, it has been shown that boron nitride is the practically only possible crucible material, as it does not react with lithium borate melts and also withstands high temperatures in an oxidizing environment. However, it has also been found that in the event of such digestion in the boron nitride crucible, serious losses of certain elements can occur, in particular this applies to such important elements as sulfur and copper. These losses could possibly be explained by the fact that lithium borate melts easily form a tight-fitting lid which excludes the atmospheric oxygen from the sample while the walls of the boron nitride crucible act as reducing agents and cause the formation of volatile compounds with certain easily reduced substances in the sample. the mentioned losses of S and Cu are completely avoided.

One object of the present invention is thus to provide crucibles for sample preparation, where losses of the above-mentioned kind are avoided, and another object is to provide a method for producing such crucibles.

For these purposes, the invention is characterized by the features and steps recited in the appended claims.

Crucibles according to the invention are thus made of a ceramic material and have an open porosity. The crucible or at least the part of the crucible which comes into contact with material, fl uss or melts at the digestion is coated with an air-permeable protective layer essentially consisting of born -combent boron nitride particles.

Preferably, the crucible should be provided with a hole in the bottom and the protective layer in the vicinity of this be thicker than in other parts. The boron nitride particles suitably have a grain size of approx. l tim.

The method according to the invention means that a crucible body is formed from a core casting compound with a high proportion of chamotte, dried and fired at approx. 100-1200 ° C to form a ceramic crucible body with a high degree of open porosity. This porous crucible body is coated with a slurry containing boron nitride by spraying, slurry casting or dipping, after which it is dried. A suitable sludge composition is 50-80% volatile organic liquids, 10-30% boron nitride particles, 1-10% organic combustible binders, I-10% inorganic high-temperature binders and possibly smaller amounts of defoamers and pH adjusters. The dried boron nitride-containing protective layer can be further treated by wetting a SiO 2 sol in water and then drying again.

The sludge for the ceramic crucible coating is suitably prepared from five functionally different components. The boron nitride particles give the crucible surface its chemically inert property while forming a non-wetting surface against the lithium borate melt. A liquid which is inert and slightly volatile to the boron nitride particles, such as some ketone or alcohol, constitutes a liquid phase in the sludge. A pyrolysable organic binder for the particles was used to obtain a simple layer application and a mechanically stable protective layer after the liquid phase in the sludge has dried. A small proportion of a high temperature binder in the form of LizBrOy has been added, which gives a good mechanical stability to the protective layer. after the organic binder has been burned off in connection with the calcination of the test material and borate fl uss in air at a temperature of 600-750 ÜC.

Certain performance-enhancing small additives, such as wetting agents, may also be included in the sludge. However, none of the added components in the sludge may interfere with the analysis of the prepared samples. 10 20 513 273 The composition of a typical sludge is shown in the recipe below.

Table 1: Recipe for a boron nitride sludge for coating ceramic crucibles Quantity in grams Weight% No. mm! Comment The invention will now be described in more detail in the form of exemplary embodiments illustrating the production of crucibles according to the invention, and comparative experiments with different crucibles, which show that the crucibles according to the invention meet the requirements for such crucibles when preparing test specimens.

EXAMPLE l.

Preparation of boron nitride-coated ceramic devils A number of crucible bodies were prepared by molding a shaped calcium aluminate casting compound (Höganäs, H54) with a high proportion of chamotte. This mass can withstand temperatures up to 155000 E fi er forming and drying, the crucibles were fired at 150 ° C for about 8 hours. The crucibles are relatively strong and showed a porosity after firing of about 40%. Subsequently, a boron nitride-containing sludge (BN sludge) was prepared. The selected BN powder (HC. Starck Grade A01) consisted of high-purity, lpm large BN particles. The only significant pollutants in the BN powder are B20; and B4C3. Boron, carbon and oxygen are not included in the elemental analysis and thus do not interfere. The liquid for the slurry of the BN powder was a mixture of acetone and ethanol, which is chemically inert to bomitride, relatively non-toxic, and which can dissolve a variety of organic binders. BN powder additives of 15-20% give the sludge a suitable viscosity and quickly build up a suitable layer thickness at the sludge base fl on a suction ceramic vessel fl el. As the thematic layer binder a methylcellulose derivative which is soluble in the acetone-ethanol mixture has been used. A binder addition of 0.3-1% was also made. After drying in luñ, the BN layer had a good adhesion to the crucible and also a good strength. To obtain a uniform layer strength after calcination of the sample, a high temperature binder consisting of a mixture of a granular lithium tetraborate and lithium acetate powder (LiAc) has also been used. LiAc has a certain solubility in the acetone-ethanol mixture, which gives a very good distribution of LiAc on the BN particles, when the solution evaporates in connection with the BN layer drying. In the subsequent calcination, LiAc decomposes and reacts partly with B; O ;, which is always present on the BN particles and partly with the Li2B4O1 additive, whereby the strength of the BN layer increases at the same time as the organic binder is burned. Foam dampers were also added to improve the handling properties of the sludge. A slight excess of HAc in the solution reduces the risk of precipitation of LizCOg upon exposure to C02 in air, which provides storage and handling benefits of a pre-prepared BN sludge.

To obtain a good mixture of the constituents of the BN sludge, the mixture was shaken with an equal amount of Al 2 O 3 cylpebs for about 10 minutes in a turbulent mixer. The finished fired ceramic crucibles were ultrasonically washed in deionized water and then rinsed with distilled water to remove loose material. The crucibles were then dried in a heating cabinet at 120 ° C for about 1 hour. The manufactured BN sludge was shaken for about 30 seconds before use. The BN layer was applied in the simplest possible way by filling the cone up to the edge with the sludge while the tap hole was sealed. After a 10-5 second suction time, the excess sludge was then allowed to drain out through the tap holder. The tip of the cone-shaped crucible was then quickly dipped into the sludge, so that even 5 mm of the lower part of the outside of the cone was treated. Due to the open porosity of the ceramic, a BN layer was quickly sucked into its walls, whereby about 0.8 g of BN stuck per treatment.

Some reinforcement of the tap hole was made by dipping the crucible in the sludge again after a short time of air drying. A completely renewed coating can be done after 10 minutes of air drying. The layer or layers were then dried to odorless in air and pre-dried in an oven at 200 ° C for about 30 minutes. After cooling, the crucibles were ready to be sent with samples. 10 15 20 513 273 EXAMPLE 2 Comparative experiments with different crucibles.

A copper concentrate was melted with lithium borate in a hot-pressed gas-tight boron nitride crucible. The experiment was repeated on three occasions, samples CCuBN-94, -95, -96.

The table below shows some interesting analysis results.) Table 2: Analysis of specimens during melting in a dense BN crucible (intensities ic / s) As can be seen, the Cu intensity has decreased dramatically, while the intensities for other elements have increased due to the enrichment that took place after removal. copper.

An analysis of a mechanically extruded bottom sample from the BN crucibles used showed only traces of copper and does not explain the amount, estimated at about 100 mg, that has disappeared. An analysis performed on the same copper concentrate, firmly performed in an air-permeable BN-coated crucible ( BN-U) according to inventions, does not show any copper losses when compared to the original sample. fl

Claims (6)

lO 20 513 273 PATENT REQUIREMENTS 1. l Crucible for the production of specimens useful for optical spectral and X-ray spectral analysis by oxidative melt digestion of lithium borate fl-us sample material, characterized in that it is made of ceramic material and has an open porosity and that at least the part of the crucible which comes into use during contact with sample material, fl uss or melt is coated with a lu fi permeable protective layer consisting essentially of boron nitride particles. 2. A crucible according to claim 1, characterized in that it has a hole in the bottom and that the protective layer in the vicinity of this hole is thicker than in its other parts. 3. A crucible according to claims 1 and 2, characterized in that the boron nitride in the protective layer has a particle size of about 1 μm. A method of making a crucible according to any one of claims 1-3, characterized in that a crucible body is formed from a ceramic casting compound with a high proportion of chamotte, that the shaped crucible body is dried and fired at about 100-1200 ° C to form a ceramic crucible body. with a substantially open porosity, that the burnt porous crucible body is at least partially coated with a boron nitride-containing sludge by spraying, slurry casting, dipping or any similar method of application and that the thus sludge-coated crucible body is dried. Process according to Claim 4, characterized in that the boron nitride sludge contains 50-80% of good organic liquids, 10-30% of boron nitride particles, 1-10% of organic combustible binders, 1-10% of inorganic high-temperature binders and possibly also smaller amounts of defoamers and pH adjuster. Process according to Claims 4 and 5, characterized in that the boron nitride-containing protective layer is first dried after application, then thoroughly sieved by a SiO 2 sol in water and dried.