Method for vitrifying powdery and granular materials

Abstract: A material treating apparatus is used for treating powdery, granular and conglomerate materials. The material treating apparatus includes a material supply apparatus (14) for transferring the materials to a predetermined position, particularly for supplying the materials to an automatic crusher (1). The materia treating apparatus includes a crushing system having a crushing vessel transferring unit (8). The material treating apparatus further includes an analysis sample transferring apparatus particularly suitable for transferring samples formed by a press (101) to a fluorescent X-ray analyzing apparatus (102). The material treating apparatus includes a fluorescent X-ray analyzing system which is capable of automatically analyzing samples of various kinds. An analyzing method is carried out by the use of the above material treating apparatus. The analyzing method includes a method of vitrifying an inorganic material, which is preferably used for obtaining samples to be analyzed by the X-ray fluorescence analysis. The analyzing method further includes a method of pretreating samples for the X-ray fluorescence analysis.

Description: This invention relates to a method for vitrifying powdery and granular materials. In analyzing components of various materials by a method such as the X-ray fluorescence analysis, it is necessary to pretreat the material to be analyzed. Following four methods have been known as methods for making samples of inorganic materials. 1) Crushing method in which a material is pulverized in order to bring the material into an appropriate condition for the analysis. 2) Vitrifying method in which a material is added with one or mixture of an alkali metal borate (for example, Li 2 B 4 O 7 , Li 2 O·B 2 O 3 and Na 2 B 4 O 7 ) and heated to be vitrified. 3) Alkali metal carbonate decomposing method in which a material is decomposed by an alkali metal carbonate. 4) Decomposition solution method in which a material in a pressurized vessel is decomposed by a strong acid such as hydrofluoric acid to make it a solution. With the crushing method among the above methods, it has a disadvantage in that if the material is at least partially crystalline, the crushed material still includes a crystal structure so that analyzed results with high accuracy could not be obtained in the X-ray fluorescence analysis due to so-called "mineral effect". Moreover, it takes long time to crush inorganic materials including non-oxides as components such as silicon nitride, sialon, silicon carbide and the like owing to high hardness. Moreover, there is a risk of the materials to be contaminated by a material of a vessel for crushing. Therefore, the crushing method has been scarcely used other than cases not requiring high accuracy of analysis. The alkali metal carbonate decomposing method is applicable as a pretreatment method for wet analyses even for non-oxides. However, this method could not be used for pretreating materials for the X-ray fluorescence analysis owing to the fact that decomposed and melted products are high in moisture absorption and difficult to be removed from used vessels such as platinum dishes and the like. The vitrifying method using the alkali metal borate has a disadvantage in that analyzed accuracy is often lowered because vitrification only partially progresses in case of inorganic materials including non-oxides as components. It is an object of the invention to provide an improved method for vitrifying inorganic materials, which mitigates the disadvantages of the prior art and is preferably used as a method for pretreating samples for X-ray fluorescence analysis. In order to achieve the object, a method of vitrifying an inorganic material according to the invention is as set out in claim 1. In a preferred embodiment, in the step of adding with the alkali metal carbonate, boric acid is added to the inorganic material, the boric acid being 5-50 parts by weight relative to 100 parts by weight of the alkali metal carbonate and being less than 0.4 mol relative to 1 mole of the alkali metal carbonate. With the above constitution, an inorganic material to be vitrified is added with a predetermined amount of an alkali metal carbonate or preferably an alkali metal carbonate and boric acid and decomposed in heating process and thereafter the crystalline substance is vitrified by a predetermined amount of boric acid. Therefore, mineral effect does not occur in X-ray fluorescence analysis and vitrification progress uniformly so that good samples for analysis can be obtained in short time. Moreover, the vitrified substance or glass is not moisture absorptive and is easily peeled from a platinum pan or the like so that it is preferably supplied for the X-ray fluorescence analysis. The reason why the alkali metal carbonate is twice an inorganic material to be vitrified in weight in the decomposing process is that an alkali metal carbonate less than twice the inorganic material makes insufficient the decomposition of the material. Moreover, the reason why the boric acid is more in weight than the inorganic material in the vitrifying process is that the boric acid less than the inorganic material makes insufficient the vitrification. Excess alkali metal carbonate and boric acid in the vitrifying process do not cause any problem. However, in order to obtain high accuracy in the X-ray fluorescence analysis, higher intensity of X-ray is preferable. For this purpose, preferable upper limits of the alkali metal carbonate is of the order of four times the inorganic material to be vitrified and the boric acid is of the order of ten times the inorganic material. The boric acid is preferably 5-50 parts by weight relative to 100 parts by weight of the alkali metal carbonate and is preferably less than 0.4 mol relative to 1 mol of the alkali metal carbonate in the decomposing process. The reason is that the melted product has a flowability during the decomposition so that the melted product can be easily handled. The addition of boric acid less than 5 parts by weight gives only less flowability making difficult the handling the melted product. The addition of boric acid more than 50 parts by weight may cause incomplete decomposition or make the decomposition impossible. In order that the invention may be more clearly understood, preferred embodiments will be described, by way of example. The vitrifying method for preparing samples to be analyzed according to the invention will be explained as one example hereinafter referring to a case that an inorganic material including silicon nitride is the inorganic material to be vitrified. The inorganic material including silicon nitride is referred to hereinafter simply as "silicon nitride". First, powder of the silicon nitride or coarse grains of sintered silicon nitride (for example 3 mm, 5 mm and the like) are prepared by weighing. Lithium carbonate or the like as an alkali metal carbonate, and boric acid if required are prepared by weighing. The prepared silicon nitride, lithium carbonate and boric acid are mixed in a dish made of platinum-gold alloy and then progressively heated by a Bunsen burner and if required a Méker burner. In this case, the heating is started from low temperature and the heating power is progressively increased to effect a decomposition resulting from the heating. Such a heating and decomposing process is completed at an instant when bubbles of CO 2 , N 2 and the like generating from the mixture disappear. After the mixture is cooled by leaving it in atmosphere, the mixture is added with a predetermined amount of boric acid and heated by the Meker burner. The heating is started from low temperature and the heating power is progressively increased. At the last stage of the heating, the mixture is intensively heated at about 1,000°C to achieve a vitrification of the mixture. In the vitrifying process, the melted mixture is intensively heated while the mixture is rocked. After the vitrification, compressed air is jetted onto the dish of the platinum-gold alloy to forcedly cool the mixture. Thereafter, the melted mixture is stripped from the dish. In order to prevent the detrimental influence by segregation of the vitrified products, the melted products are pressed after grinding to obtain formed bodies for the X-ray fluorescence analysis. It takes about two hours from the preparation of the sample to the X-ray fluorescence analysis. If required, sodium carbonate or potassium carbonate may be used as the metal carbonate instead of the lithium carbonate. Moreover, other dishes may be used such as a crucible made of platinum-gold alloy, platinum or graphite or a dish of platinum. Actual examples will be explained hereinafter. In order to carrying out the vitrifying method according to the invention, silicon nitride samples No. 1-7 were prepared which were different in contained amount of silicon nitride and included lithium carbonate and boric acid of respective percentages shown in Table 1. Decomposing states in the heating and decomposing process and vitrified states in the vitrified process were reviewed. Those who are completely decomposed and whose glasses are transparent and easily stripped are indicated by o ○. Although completely decomposed and glasses are easily stripped, those who are somewhat difficult to be stripped are indicated by ○ . Those which are incompletely decomposed and vitrified are indicated by × in Table 1. Moreover, silicon carbide samples Nos. 21-26 were prepared and tested which were different in contained amount of silicon carbide. Results are shown in Table 2, which are estimated in the same manner as in Table 1. Among these samples, the silicon nitride sample No. 4 and the silicon carbide sample No. 23 are powdery raw material, while the other samples are coarse grains of sintered materials. Silicon nitride samples Nos. 8-11 and silicon carbide samples Nos. 27-29 were separately prepared which included lithium carbonate or boric acid whose amounts were out of range according to the invention. These comparative samples were vitrified. With the silicon nitride samples Nos. 1-11 and the silicon carbide samples Nos. 21-29, respective ten samples were vitrified and estimated. As can be seen from the results in Tables 1 and 2, the samples Nos. 1-7 and Nos. 21-26 are substantially good in decomposition and vitrification, whose amounts of lithium carbonate in the heating and decomposing processes are more than twice those of the silicon nitride samples or silicon carbide samples and whose amounts of boric acid in the vitrifying processes are equal to or more than those of the silicon nitride samples or silicon carbide samples according to the invention. On the other hand, the decomposition and vitrification are insufficient with the comparative samples Nos. 8-11 and Nos. 27-29 which do not fulfil the above conditions according to the invention. Moreover, ten samples were prepared by crushing sintered silicon nitride without vitrification, and further ten samples were prepared by crushing after vitrification according to the manner as in the sample No. 3 in Table 1 of the invention. These samples were subjected to the X-ray fluorescence analysis to review calibration curves (relations between intensities of X-ray fluorescence analysis and concentrations of the respective components). Table 3 indicates accuracies of the calibration curves of the respective components with the aid of the following equation. σ = Σ( C chem - C x-ray ) 2 n - k σ : accuracy of calibration C chem : analyzed vlaue by chemical analysis wt% C x-ray : analyzed value by X-ray fluorescence analysis wt% n : the number of reference samples used for obtaining calibration curves k : the number of parameters used in presumption for regression equations Moreover, accuracies of calibration curves with silicon carbide crystals were measured in the same manner as above described, the results of which are shown in Table 4. In both cases of the silicon nitride and silicon carbide, accuracies of analyzed values by the X-ray fluorescence analysis with samples vitrified according to the invention are one figure or place higher than those with samples crushed without being vitrified. Analyzed component With vitrification (third embodiment) (wt%) Without vitrification (crushing method) (wt%) Si 0.04 0.8 Al 0.008 0.05 Fe 0.02 0.02 Ti 0.002 0.002 Ca 0.001 0.001 Mg 0.02 0.1 K 0.002 0.002 Na 0.002 0.002 Sr 0.01 0.1 Ce 0.02 0.3 Zr 0.02 0.4 Y 0.02 0.6 Analyzed component With vitrification (wt%) Without vitrification (crushing method) (wt%) Si 0.04 0.9 Al 0.002 0.03 Fe 0.01 0.01 Ti 0.002 0.002 Cr 0.008 0.009 The invention is not limited to the embodiments above described and various changes and modifications may be made in the invention. Moreover, although the above embodiments have been explained in consideration of the X-ray fluorescence analysis, it is possible to use them as samples for a wet analysis as atomic absorption spectrometry, inductively coupled plasma atomic emission analysis and the like by solving vitrified products or their crushed materials in an acid. As can be seen from the above explanation, an inorganic material is added with a predetermined amount of alkali metal carbonate, preferably alkali metal carbonate and boric acid and decomposed in the heating and decomposing process and thereafter the mixture is vitrified with a predetermined amount of boric acid. Therefore, when the sample is analyzed by the X-ray fluorescence analysis, any mineral effect does not occur so that high accuracy analysis can be effected in comparison with the crushing method. In the X-ray fluorescence analysis using samples vitrified according to the invention, the time required for the analysis can be shortened to about one tenth of that in the chemical analysis. The inorganic material vitrifying method according to the invention is advantageously used for producing or pretreating samples for the X-ray fluorescence analysis as a high speed analysis so that the industrial value of the invention is inestimable. Attention is drawn to EP-A-0306276 (the publication at eighteen months of the present application), in which additional, related methods and apparatus are described.

Claims: A method of vitrifying an inorganic material containing at least 30% by weight of silicon nitride or silicon carbide, comprising the following steps in the following order:- (i) adding to the inorganic material an alkali metal carbonate in an amount of at least substantially twice that of said inorganic material in weight; (ii) heating the material to decompose it to obtain a melted product; (iii) adding into the melted product boric acid in an amount of more than that of said inorganic material in weight; and (iv) heating the melted product to vitrify the melted product. A method as set forth in claim 1, wherein said alkali metal carbonate is a material selected from lithium carbonate, sodium carbonate and potassium carbonate. A method as set forth in claim 1 or claim 2, wherein in step (i) boric acid (additional to the boric acid added in step (iii)) is added to the inorganic material in an amount of 5-50 parts by weight relative to 100 parts by weight of the alkali metal carbonate and less than 0.4 mol relative to 1 mol of the alkali metal carbonate.

Inventors: NARUKAWA AKIRA, NARUKAWA, AKIRA
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1988
Main classification: G01N23
IPC codes: G01N23, G01N1, C03C1