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Makhmudova Nadyrakhon Abdukadirovna, Candidate of Technical Sciences, Associate Professor Tashkent Architectureand Construction Institute,
Tashkent, Uzbekistan. E-mail: shavkat-19912070@umail.uz
TECHNOLOGIES FOR OBTAINING HIGH-STRENGTH GYPSUM FROM GYPSUM-CONTAINING WASTE OF SULFUR PRODUCTION - FLOTATION TAILINGS
Abstract: Development of technology for obtaining gypsum binder from gypsum-containing wastes of sulfur production-flotation tailings is considered in the paper, it requires the investigation of the effectiveness of various methods for obtaining gypsum binders and searching for optimal technological parameters.
Keywords: mono-mineral binder, gypsum-containing waste, phosphor-gypsum, flotation tailings, organic and inorganic compounds, chemical composition, granulometric composition, thermal treatment, container.
Vast experience is gained in the production and use of gypsum mono-mineral binder, one of the most studied building materials. Moreover, in recent years, the production and use of gypsum and especially gypsum products in highly developed countries are continuously increasing.
At present the most important raw materials for the production of gypsum products are gypsum-containing wastes from chemical industry and, above all, waste from phosphorus fertilizers (phosphor-gypsum)production. In terms ofwaste utilization and ecology, the study solves an actual problem.
The need of Uzbekistan's industry for gypsum raw materials can also be met through industrial by-products. This will reduce the extraction of natural gypsum stone, as well as reduce the cost of the construction and operation of slag accumulators of gypsum-containing wastes. The use of by-products containing calcium sulfate in industry has not only economic but also environmental implications. Significant funds are spent to the disposal of waste, sometimes they reach 8-10% of the cost of the main products.
A small proportion of used gypsum-containing waste from their total quantity is due to the peculiarities of physical-chemical composition and properties that do not allow to apply traditional methods for processing natural gypsum raw materials. Flotation tailings con-
tain a small amount of semi-aqueous calcium sulphate, which is formed with the technology of obtaining sulfur from natural sulfuric stone.
The basis for the production of gypsum binders is the process of thermal dissociation of calcium sulfate dihydrate. Losing a part of crystallization water, the dihydrate gypsum passes into a semi-aquatic gypsum, which under normal conditions is chemically active with respect to water; that makes it possible to use this product as abinder.
The use of gypsum-containing wastes in the production ofbinders is dictated by economic and environmental objectives.
Except calcium sulphate, the main substance of gypsum-containing waste, they can contain impurities that significantly alter the technical properties of the binders obtained. Impurities may contain both organic and inorganic compounds, namely, compounds of silicon, fluorine, aluminum and iron, alkaline and rare earth elements, radioactive compounds, as well as certain amounts of free acids.
Mechanical impurities of organic substances, as well as the presence of easily soluble salts of sodium and magnesium, leads to the staining of gypsum, contribute to the formation of efflorescences on the surface of products, impair the setting process. Physical-chemical impurities are the most difficult removed. It is sufficient to
TECHNOLOGIES FOR OBTAINING HIGH-STRENGTH GYPSUM FROM GYPSUM-CONTAINING WASTE OF SULFUR PRODUCTION —
FLOTATION TAILINGS
rinse the raw material to remove mechanical impurities, but in order to eliminate physical-chemical impurities, a recrystallization process must be carried out along with washing. Free acids, present as impurities in gypsum-containing waste, contribute to the lengthening of the setting time, deterioration of binder adhesion and the strength of products, and cause corrosion of the equipment. In order to get rid of impurities, active washing and neutralization are necessary [5].
However, the results of more recent studies [1, 4] indicate that in the plants, designed to process natural gypsum stone, it is impossible, without the introduction of additional technological operations, to obtain from gypsum-containing waste abinder that meets the required standards.
Flotation tailings are a powdery substance of grayish-white color with a specific surface of 900-1000 cm 2/g, which are formed as a result of separation of sulfur inclusions from ore. According to the technology, sulfur ore from the quarry is ground, mixed with water; sulfur inclusions due to the difference in the density of materials are on the surface of crushed ore (pulp), which is collected by special devices and directed for the processing. Pulp with gypsum-containing waste with a significant amount of water is diverted to special collectors-sedimentation tanks, where it is dried under atmospheric influence.
The average bulk density of waste in a loose state is 950 kg/m 3, in a compacted state - 1100 kg/m 3.
Flotation tailings consist mainly of dihydrate gypsum, the total content of which varies between 80-90% by weight. In addition, they contain sulfur, silica, alumina and other substances from the original rock.
Flotation tailings also contain a small amount of semi-aqueous calcium sulphate formed when sulfur is produced from natural sulfuric stone as a by-product of the technology adopted at the plant.
It is known that each type of gypsum binder has a certain structure of the crystal lattice. Studies show that according to granulometric composition, the ordinary building gypsum consists, as a rule, of fine and porous grains of detrital nature, which determine the high water intake of gypsum during mixingand the low strength of the products obtained from it. All this is due to the free removal of hydrated water from the gypsum.
Despite a significant number of studies devoted to the technology of obtaining high-strength gypsum bind-
ers, the researchers do not have a single opinion on the technological parameters, the methods of preparing raw materials for hydrothermal treatment, and on comparative effectiveness of using various methods for obtaining high-strength gypsum.
In the production of gypsum binders from gypsum-containing waste, the technology can undergo certain changes related to the material composition of the raw material, i. e. its morphology.
In general, the process of obtaining high-strength gypsum from gypsum stone includes the following technological methods: crushing ofgypsum stone, autoclave treatment of crushed gypsum stone, drying the obtained calcium sulfate hemihydrate and grinding the finished product.
Gypsum-containing raw material -flotation tailings of sulfur production differ from known gypsum-containing wastes by the composition of impurities, the state of calcium dihydrate and other features.
In this regard, the actual problem is the development of technology for obtaining high-strength gypsum from gypsum-containing waste of sulfur production — flotation tailings; this development requires investigation of the effectiveness of various methods for obtaining gypsum binders and searching for optimum technological parameters.
Due to the powdery state of flotation tailings, it is necessary to determine the possibility of its thermal treatment in an autoclave in its natural state
One meter high container is filled with a slightly moistened initial product. After autoclaving, samples of obtained binder from different points along the height and width of the container are taken. The experimental data are presented in Table 1.4.
As can be seen from the table, the deeper (down to 40-50 cm) samples are taken, are lower the strength parameters. Studies have shown that autoclaved treatment at an optimum vapor pressure of 0.2 MPa, a steaming time of 20 hours and a drying time of20-22 hours, the formation of the necessary number of centers of recrystallization of the dihydrate into the hemihydrate in the entire volume of the material occurs only to a depth of 20 cm.
We have constructed a special stack with trays 20 cm high (on the basis of above mentioned experiments), into which slightly moistened flotation tailings in a loose state were poured. The thermal treatment of the product was carried out according to the optimum mode.
Table 1. - Influence of the height of the waste layer on the properties of gypsum binder at thermal treatment
Height of raw material layer in autoclave container, cm Strength MPa under Time of setting, min Water - gypsum ratio
compression bending beginning end
10 8.3 3.6 12 2103 0.37
20 8.6 3.8 12 20 0.37
40 2.9 1.6 25 70 0.55
50 2.7 1 37 120 0.55
60 2.7 1 40 unset 0.55
Despite the high values of strength of gypsum binder obtained with this technology, it should be noted that certain production processes, in particular, unloading and labor-consumption, increase the net cost of the binder.
In this regard, the finely ground flotation tailings should be coarsened into granules or briquettes and heat treated.
As noted earlier, the waste of sulfur production -flotation tailings, as gypsum-containing by-productsatsulfur production, is a pulp with a significant amount of water (up to 45%), and in a dried state (in case of dehydration in natural conditions)- a powdery greyish-white substance. It is impossible to obtain wastes with a certain moisture-content. This makes difficult to use wastes
of sulfur production without additional procedures for their conditioning.
A number of methods for conditioning (granulating) gypsum-containing wastes are known from the literature [2; 3]; these methods are developed mainly for phosphor-gypsum. They have several drawbacks, which make them inapplicable for the conditioning of waste products of sulfur production-flotation tailings, since they demand either the introduction of additives as binders that are not used primarily in production; that significantly complicates the conditioning process, or making it uneconomical, complex and multistage.
All this necessitates the development of more economical and simple methods for conditioning raw materials.
References:
1. Manufacture and Application of High-strength Gypsum Binders // Ref.Inf. VNIIESM. MISI. Industry of autoclave materials.
2. Gordoshevsky, Properties P. F. and Possibilities of Using Phosphor-gypsum // Building Materials. - 1960. -No. 12. - P. 32-34.
3. Simyanovskaya R. E. Investigations in Chemistry and Technology of Air Binders Obtained from Phosphor-gypsum // Gypsum and phosphor-gypsum. - Moscow. - 1960. - P. 67-75.
4. Peredery I. A. High-strength Gypsum GP. - 198 p.
5. Skramtaev B. G., Bulychev G. G. High-strength Gypsum by self-steaming method. - Moscow. Stroyzdat. - 117 p.
