Laboratory work № 4 Definition of hygroscopic moisture
In dry weighed small cup with lap-fitted cover weighed in analytical balance about 2,0 - 3,0g of kaolin, clay or grind field spar are placed and dried at 100 - 1100 tin a drying case during 2 hours. Having finished drying, small cup is cooled in desiccator and weighed. After that the drying proceeds 1 hour, then again is cooling and weighing follow.
Drying and weighing are repeated till receiving constant weight. The (rejections) can make no more than 0,0001g.
Amount of hygroscopic moisture in the specified materials is calculated under the formula:
W = a ∙ 100 /c , ٪
Where W - moisture content ٪
а - loss of weight of a material at drying, g;
с – initial weight.
Devices and reagents
1. Small glass with friction-fit lid
2. Analytical balance
3. Desiccators
4. Exiccator
5. Kaolin, clay or feldspar
Question for self-checking:
1. Moisture as one of basic properties of raw materials.
2. What correlation exists between absolute and relative moisture?
3. Gravimetric methods of moisture definition.
4. In what limits oscillation of moisture not braking normal flow of the technological process are admitted?
5. Tolerances of moisture rejections complicate the development of automatized and instrumental methods of control in production?
Laboratory work № 5 Definition of silica regeneration degree
Aim of the work - to define density substance
Theoretical information
A normal and practically important oxide of silicon is dioxide or silica – SiO2. Silica has complex polymorphism. The polymorphism is the phenomenon , when substances of one and the same chemical composition have various crystal condition.
14 crystal forms of SiO2 and one аmorphic - quartz glass are known now. Many of crystal forms of silica occur naturally and quartz and its varieties are more distributed. The content in earth crust of these minerals is about 12% of mass.
In products of silicate technology 3 forms occur more often: quartz, tridymite, crystobalite.
Figure 2 - Diagram of a condition of system SiO2.
In figure 2 one - component hypothetical diagram of SiO2, constructed by Fenneris given. On the diagram basic widely spread forms of SiO2 are presented. Three of them - more frequently occuring enantiothropic forms - quartz, tridymite, crystobalite, each of which has low-temperaturе varieties.
It is convenient to give a sequence of phase transitions in the system of SiO2, schematically in equilibrium conditions in the following way:
α-quartz
α- tridymite
α-christobalite
melt
↨575 ↨163 ↨230
β-quartz β- tridymite β-christobalite
↨120
γ- tridymite
As it is seen in the scheme, all shown transformations are enantiothropic (i.e. convertible). However, the character of these transformations at change of temperature is various. The transformations within the limits of the given main form (vertical) occur easily and quickly, explained by similarity of structure within the limits of one and the same identical form. As opposed to this, the transformation between the main modifications (horizontal) proceeds hardly and very slowly, as it is connected with deep reorganization of the structure.
The transformation of high-temperature modification in low-temperature occurs without significant reorganization of the structure, only to some displacement and rotation of tetrahedronal groups relatively each other, changing their symmetry.
The transformations between the main modification of SiO2 are considerably accelerated in the presence of some additives called mineralizators. At mineralizators, for example, compounds of alkaline and alkali-earth of metals can be used.
The great changes accompanied with change of density of SiO2 modification at mutual transformation are rather important for practice. As transition from low-temperature, forms to high-temperature, density decreases, and specific volume increases. From of SiO2, β- quartz (density 2650 kg/м3), is more dense; less dense- - crystobalite (density 2200 kg/м3). Great changes at phase transitions in the system of SiO2 (expansion when heated, compression when cooled) can reach a significant size (about 16 %) and cause destruction of products. The transformations: ↔β- quartz; - quartz in - tridymite and - crystobalite; β in - tridymite are the most dangerous.
All modifications of SiO2 have frame crystal structure differing by the sheme of tetrahedron [SiO4] in three-dimensional space.
Quartz SiO2. It is assumed, that the name of minerals is originated from sakson’s word "Querxlutfeve", meaning ore of cross veins.
In the structural relation the high-temperature - quartz differs from β- form by highes symmetry.
Curves of DТА (differential thermal analysis) show endothermic peak at the temperature 573 0С, corresponding to enantiothropic transformation of the low-temperature quartz in high-temperature. Heat of transition - 360 кJ/mol.
Tridymite SiO2 The name is given in connection with natural accretion of crystals as Y-tube.
Crystobalite SiO2 is named according to the origin in Sierra-san-Crystobal (Mexico). Transformations of low-temperature crystobalite in high-temperature occurs at the temperature 2180С + 20С and accompanied by increase of volume at 3,7 %.
Technique of definition of silica regeneration degree
Whereas the polymorphic transformations of silica are accompanied by changes of properties, in particular density, for definition regeneration degree it is enough to define the density of silica before and after burning.
Density of substance can be defined by:
- a method of hydrostatic weighing (law of Arhimed);
- a picknometric method (for powder substances);
- selection of liquid equal with density of researched substance.
Density can be measured by a picknometric method with accuracy up to 0,01 kg / м3 that is an advantage of this method.
Definition of tridimitization degree (regeneration) of quartz sand is brought to its burning during an hour at the temperature 12000С and definition of density before and after burning. The definition consists of the following operations:
1. Crushing sand into a thin powder. Approximately 10 g of Quartz sand is
crushed first of all in metal mortar, and then in agate mortar up to a subtlety powder (at grinding a powder rough grains) must not be between fingers.
2. Powder is divided into two parts. One part of the test is to define.
Density before burning, spent by a picknometric method in two parallel tests.
3. Second part of test is placed in fire-clay and subjected to burning in
the laboratory furnace at the temperature 1200 0С during an hour.
4. After cooling it is again crushed and density is defined by a picknometric method after burning.
5. The degree of regeneration (trimidisation) is determined under the formula:
Where:
X -a degree of regeneration of quartz in tridymite,
d1- density of a material befor burning, kg / м3
d2 - density of a material after burning, kg / м3
d3 - density of tridymite is accepted as equal 2310 kg / м3. The picknometer is a glass flask whith narrow neck, whith notch, and glass fuse. The method of density definition consists in the following.
Dry and pure picknometers (definition is made in two parallel tests) with a fuse are weighed in analytical balance to 0,001 g (first weighing). After that every picknometer with powder is weighed (second weighing). A difference between results of the first and second weighing is size of weight of silicon oxide of powder. After the second weighing in picknometer with powder liquid (distilled water) is poured up to a level little below the neck. Then to remove air from powder open picknometer is placed in the vacuum device and its contents are vacuumed (instead of it sometimes boiling of contents of picknometer is employed on water bath during 0,5 hours). After boiling, the picknometer with powder and water is cooled water is added. Up to a level little bit below notch closed picknometer is immersed up to the neck in water thermostate, having constant temperature, at which the definition of density (usually 15 0С) is made and kept in water thermostate 20-40 minutes to equal the temperature. After keeping, the picknometer is dried, level of water is set precisely up to notch and weighed (third weighing). Then the picknometer is broken free from water and powder, carefully washed out, filled approximately up to notch by one distilled water, again thermostated as mentioned above, and after setting water level exactly up to notch is weighed (fourth weighing).
The processing of results. Density is defined under the formula:
Where: d -density, kg / м3
Р1 – weight of empty picknometer (first weighing), kg
Р2 - weight of picknometer with the weight (second weighing), kg
Р3- weight of picknometer with water and weight (third weighing), kg
P4- weight of picknometer with water (fourth weighing), kg.
The results of two parallel experiments must not differ from each other more than 5 kg / m3.
It is necessary to point out, that the definition of density by a picknometrical method is rather difficult operation.
To get correct results it is necessary to weigh precisely by meniscus to add level of liquid in picknometer to the notch, it is recommended to add water and then absorbd with rolled filtrated paper. It is necessary with clean dry hands for neck to take picknometer. Even small discrepancy in weighing or setting level of a liquid can result in essential distortion of definition of results.
The results of definition are written down under the following form:
Date |
№ of sample |
Weight of empty picknometer |
Weight of ppicknometer with the sample
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Weight of picknometer with the sample and water
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Weight of picknometer with water
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Density, kg/m3 |
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Devices and reagents
1. Picknometers
2. Electric heater
3. Water bath
4. Analytical balance
5. Water thermostat
6. Sand
7. Distilled water
Question for self-checking:
1. Polymorphism as physical phenomenon.
2. Write the scheme of polymorphic transformations of silica. Specify, what of transformations proceed quickly, what slowly.
3. What are polymorphic transformation of silica accompanied by?
4. Explain the notion of density and specific volume?
5. What methods do you know to define density of pieced and powdered material? Bases of methods. How is method the density of silicate melting defined?
6. For the brick-work of ardent space and arch tanks glass melting furnaces and steel meltings furnaces (such as open-hearth) the silica refractory. What temperatures are dangerous for silica brick-work at removing (starting) of these furnaces, which should be taken into account at drawing up the diagram of removing?
7. The importance of polymorphic transformations of silica in character of silicates formation.