« PreviousContinue »
The results indicate that a eutectic mixture is formed with about 90 per cent feldspar and 10 per cent biotite. The feldspar alone deformed at about cone 9, whereas the eutectic mixture deformed at about cone 7, indicating that biotite causes a pronounced lowering of the deformation temperature. The rate of deformation is not noticeably affected. The coloring action of the biotite is a complete barrier to the use of biotite-bearing feldspar in most ceramic industries, as 1 per cent of biotite colors the fired feldspar a pronounced gray and 2 per cent produces a dark gray-brown color. The mixtures containing 5 per cent biotite show a tendency to boil when deformation begins, a characteristic of mixtures containing an excess of magnesium. EFFECT OF KAOLIN ON PROPERTIES OF COMMERCIAL
The presence of kaolin and kaolinized material in commercial feldspar makes a knowledge of the effect of kaolin on the properties of the feldspar important. Such an investigation has been made by Hewitt Wilson, ceramic engineer, in preparing data for a thesis at Ohio State University, and those parts of the papera which bear on this subject are given here.
DEFORMATION TESTS OF FELDSPAR-KAOLIN MIXTURES.
KAOLINIZATION OF FELDSPAR AND SIGNIFICANCE.
The kaolin impurity in most commercial feldspars averages from 1 to 3 per cent and usually is the result of the decomposition of more exposed portions of the bed. A description of this "decomposition of feldspar and the recombination of a portion of the decomposition products of kaolin" has been given by Watts. Briefly it is this:
The molecular formula for orthoclase or microcline feldspar is KO.1Al2O3.6SiO2. By water solution the K2O is lost, leaving Al2O3.6SiO2 as the insoluble decomposition products. The Al2O3 and 2SiO2 of this recombine with the absorption of 2H2O to form kaolin, leaving 4 molecules of silica as free quartz. Thus the presence of kaolin and quartz impurities in a bed of feldspar is partly accounted for.
The increase of the combined water and the decrease of the potash content are thus indicators of the degree of kaolinization. As the water content increases, the deformation temperature also increases. By plotting the water alkali curve alongside this other, a corresponding deformation temperature can be obtained for each percentage of alkali in the feldspar. We, therefore, may say that generally the more kaolinized is the feldspar-that is, the higher the water and lower the alkali content-the higher will be the deformation temperature. However, when the feldspar contains less than 16 per cent kaolin, it has been found that there are indications of the formation of a eutectic, so that the rise of the deformation temperature is not as rapid as with higher percentages of kaolin.
Simonis (Sprechsaal, 1907, vol. 2) gives a deformation temperature determination for Zettlitz kaolin-feldspar mixtures and it was decided to check this curve with our kaolin and feldspar.
a Wilson, Hewitt, Deformation study of feldspar-kaolin mixtures: Trans. Am. Cer. Soc., vol. 15, 1913, pp. 217-232.
b Watts, A. S., Mining and treatment of feldspar and kaolin in the southern Appalachian region: Bull. 53, Bureau of Mines, 1913, p. 16.
Watts a found that there was an apparent eutectic with 90 per cent feldspar and 10 per cent quartz. As most commercial feldspars contain both quartz and kaolin impurities it would be nearer actual working conditions to have both present in this study, after the mixture of feldspar with quartz and with kaolin had been investigated separately.
METHOD AND PROCEDURE.
The chemical analysis of the feldspar and also of the theoretical microcline are as follows:
The purest kaolin obtainable was used.
per cent.. 13. 22
..do.... 39. 07
The crude, unwashed clay was blunged in a small household churn, passed through a 200-mesh screen to remove all the larger quartz and mica particles, and then sized in a Schultz elutriation apparatus
Four sizes of grain were separated, dried, and their average size determined with a microscope:
Jar No. 1=0.0032 to 0.0040 inches.
Most of the clay grains were taken from jar No. 3, and after the relative size of grain study this size was used altogether for all the cones.
* * *
The feldspar and kaolin ingredients of the cone mixtures were weighed out and the two were mixed dry by passing twice through a 150-mesh screen and enough dextrine added to permit its being formed * * into a standard size cone * * Each cone was stamped with its serial and clay content number and baked in a constant temperature oven at 109° C.
From 24 to 27 hours were used in reaching cone 8; rapid firing maintained until cone 6 to 7 was reached and then several hours intervening until cone 8 should start.
a Watts, A. S., Op. cit., pp. 26, 27, 28.
DATA AND RESULTS.
To determine the effect of size of grain of the kaolin, a series was made up of 0.2, 5, 7.5, and 10 per cent of each of the sizes Nos. 2, 3, and 4 of kaolin and fired as a preliminary burn. No difference could be distinguished between the three, and so kaolin No. 3 was used throughout the rest of the determination.
Kaolin was added to feldspar in 1 per cent intervals, from 0 to 16 per cent. Beyond this the kaolin percentages were 25, 30, 35, 40, and 45. The average of 9 burns is given in figure 8.
This chart indicates that
1. The pure feldspar deforms between cone 8 and cone 9, being closer to the latter. 2. Around 2 and 3 per cent kaolin the start of deformation is late, the rate is fast, and the end is before that of pure feldspar.
3. At 5 and 6 per cent the rate decreases until it is about that of the pure feldspar, but the start and end of deformation is behind that of the latter.
4. In the 9 to 11 per cent region there is another slow starting, a rapid rate, and quick finish to deformation. The rate here is faster than any other.
5. Beyond 10 per cent kaolin, normal conditions are gradually resumed, the start and finish progressing toward higher temperatures with the increase of kaolin.
Cones in which the kaolin impurity varies only by 1 per cent do not show much difference in their deformation. Hence regions, instead of definite percentages, must be used in terming these indicated eutectics.
FIGURE 9.-Diagram showing relation of rate of deformation to proportion of kaolin. Vertical distance between lines represents rate of deformation of pure feldspar.
Figure 9 shows in a more general way the rate of deformation, that being designated by vertical distances between the two curves. The normal rate-that is, that of pure feldspar-is represented by the vertical distance between the two dotted lines dropping to the right as the kaolin content increases.
The results of these two charts show that there are two indicated deformation eutectics, the lesser in the region of 2 to 3 per cent kaolin and 98 to 97 per cent feldspar, and the greater at 9 to 11 per cent kaolin and 91 to 88 per cent feldspar. Correspondingly, 5 to 6 per cent kaolin and 95 to 94 per cent feldspar is a region which may be termed a deformation compound. When we speak of regions, it is meant that in the limits given there is a definite percentage at the maximum or minimum point which should be termed the deformation eutectic, or compound, but that it has not been accurately determined.
A final determination was made to this phase of the study by grinding up the deformed cones to pass 150 mesh, mixing with dextrine, reforming into cones and firing again; 0, 2, 4, 6, 7, 5, 10, 12.5 per cent of kaolin were used. The results as given in figure 10 in general check the above results, the deformation, however, taking place from two to three cones lower-close to cone 6. Pure feldspar started about the same time as the others, but was slow in deforming. Two, 4, 7.5, and 10 per cent were practically the same and deformed more rapidly than pure feldspar. Six per cent kaolin was slightly behind 2 and 10 per cent and dropped more like 0 per cent, thus indicating the higher region of 4 to 6 per cent.