The dissertation is devoted to the formation of new environmentally friendly phosphate binders with stable chemical and mineralogical composition, the development of simplified technology for their production and implementation of processes of forming foundry cores based on them with improved technological properties for iron-carbon alloy castings.
In order to create new inorganic binders, the analysis of physicochemical processes occurring in orthophosphoric acid systems with inorganic sodium salts was carried out, new variants of phosphate binder synthesis were theoretically established and confirmed in practice. The kinetics of binder formation in systems of phosphoric acid with carbonate, tripolyphosphate, and sodium chloride has been studied. The mechanism of formation of sodium phosphates with binding properties as a result of direct chemical interaction of sodium chloride with H3PO4 was established for the first time.
In order to determine the phase and mineralogical composition of the binder components formed by the interaction of sodium salts with orthophosphoric acid, it was found that binders formed at normal temperature or heat not exceeding 150 ℃ have the structure of orthophosphates, at 150…250 ℃ – pyrophosphates, and at 250…300 ℃ – sodium metaphosphates.
In order to study the dynamics of structural transformations during heating of binders formed in systems of H3PO4 with Na5P3O10 and Al2(SO4)3, descriptions of the mechanisms of thermal transformations have been developed. The phase composition of phosphosulfate binder at different temperatures in the range from 200 to 1000 ℃ was determined. It has been established that when heated, there is a gradual removal of crystalline water, the transformation of aluminum metaphosphate into orthophosphate, and thermal decomposition of Al2(SO4)3. It has been established that in the composition of the binder based on Na5P3O10, as part of the mixture, when heated, sodium pyrophosphate is converted into metaphosphate in the range of 200…300 °C, which, after melting at a temperature of about 550 °C and the next hardening, passes into amorphous state.
In order to study the dependence of physical and mechanical properties of core mixtures on their composition, for the first time the regularities of the influence of the crystal structure of synthesized sodium phosphates on their binding capacity in the composition of molding and core mixtures were established. It has been shown that disubstituted sodium pyrophosphate, which is formed in the system of Na5P3O10 with phosphoric acid, provides the greatest strength. This is explained by the presence of hydrogen bonds and the ratio of the ionic radii of Na+ and P2O74- equal to 0.196.
Two variants of phosphosulfate and sodium phosphate binder synthesis have been developed. The first option provides for the formation of dry binders by holding Al(SO4)2 or Na5P3O10 with phosphoric acid at a temperature of 200 ℃ with mass ratios of 10:1 and 5:1, respectively. According to the second option, the binders are formed directly during the thermal hardening of the cores (150…300 ℃) from solutions of aluminum sulfate (10…20%), tripolyphosphate (20…30%), chloride (25…30%), carbonate (25…30%) sodium in orthophosphoric acid.
In order to determine the optimal formulations of core mixtures, the planned experiments and mathematical processing of their results were carried out. Phosphosulfate binder at a content of 3.0...3.5% in the mixture provides compressive strength of 2.5...3.0 MPa, a binders based on orthophosphoric acid and: NaCl at a content of 4.5… 5.0% – 2.8…3.0 MPa, Na2CO3 at a content of 3.0… 3.2% – 2.6…2.8 MPa, Na5P3O10 at a content of 4.5…5.0% – 3.2…3.6 MPa.
Laboratory tests of the developed core mixtures were carried out and their properties were determined. Cores with phosphosulfate binder after pouring iron-carbon melts have a minimum residual strength due to thermal destruction of this binder when heated. In the composition of the cores with all the sodium phosphate binders, when heated, a melt of sodium metaphosphate is formed, which contributes to a significant increase in their residual strength. However, the water solubility of sodium metaphosphate is provided the minimum work of knocking out the cores.
The practical significance of the results of the dissertation is due to the fact that, on the basis of theoretical research, new technologies for the synthesis of phosphate binders have been created and core mixtures with improved technological properties have been developed on their basis for the manufacture of castings from iron-carbon alloys. The processes of synthesis of binders are simple, low-operational, require a minimum of materials and can be implemented as part of the core mixture during thermal hardening of the core, and before preparing the mixture at a chemical plant or directly in the foundry. Ready-made binders have a virtually unlimited shelf life; they are as environmentally friendly as possible.
