Introduction.It is known that chloride sublimates of oxidizing-chloridizing roasting of lead-zinc ore contain 28.9% of chloride-ions, including 38.3% of ZnCl2 and 6.85% of FeCl2 [1,2]. When phosphoric-acid processing of these sublimates to obtain phosphating anticorrosive coatings four-component (quaternary) systems FeCl2-H3PO4-H2O and ZnCl2-H3PO4-H2O are formed. Formation of five-component (quinary) system FeCl2-ZnCl2-H3PO4-H2O is possible as well. Researched processes represent complex chemical-technological systems, including both chemical and phase transformations in equilibrium conditions. Therefore the technology of processing of chloride sublimates with a certain composition using phosphoric acid is impossible to develop without complex physical-chemical analysis of these mutual systems and setting regularities of phenomena taking place in the systems. The analysis is performed by studying properties of a heterogeneous system depending on its composition and parameters and imaging these dependences on state diagrams.
Materials and methods of research.In this connection we studied state diagrams of above mentioned quaternary systems, which are the foundation to develop theoretical basis of technological processes, proceeding during phosphoric-acid processing of chloride sublimates. On the basis of constructed solubility diagrams for mutual quaternary systems FeCl2-H3PO4-H2O and ZnCl2-H3PO4-H2O one can choose and justify optimal conditions of phosphoric-acid processing of lead production sublimates and to calculate theoretical output of production concentrate. Besides that obtained results would like to bring weighty scientific contribution in fundamental data about solubility in above mentioned systems and regularities of processes proceeding in complex physical-chemical systems.
Solubility was investigated by isothermal method consisting in solution mixing at constant temperature with solid phase excess up to equilibrium reaching. The chemical interaction takes place in systems under investigation between initial reagents – phosphoric acid solution with a certain concentration and ferrous chloride (II) or zinc chloride up to equilibrium reaching according to reaction equations:
3FeCl2 +2H3PO4=Fe3(PO4)2+6HCl (1)
3ZnCl2 +2H3PO4=Zn3(PO4)2+6HCl (2)
Depending on acid saturation degree mono-, di- or trisubstituted phosphates are formed. However to reach saturation state and to determine system equilibrium composition at some temperature it is necessary to input chlorides in amount exceeding stoichiometric norm for given reactions. The sample of ferrous chloride (II) or zinc chloride (reagentgrade) is added to the phosphoric acid solution with a certain concentration in amount to saturate solution with a salt to the reaction end. This state is defined by excess amount of a salt stayed in solid phase. After saturation reaching samples of liquid phase were sampled in every 30 minutes using thermostated sampler. The samples were subjected to chemical analysis to determine chloride-ions content in liquid phase. We judged about equilibrium reaching time by constant content of chloride-ions in last 2-3 samples. We sampled simultaneously solid phase samples to determine chloride-ions and Р2О5 content in it. Chemical analyses of preliminary dried crystals were carried out using standard techniques.
Solubility was calculated on the basis of 3-4 parallel experiments results at allowable discrepancy less than 0.5% between two parallel analyses of liquid phase in every experiment. It is established experimentally that equilibrium in systems FeCl2-H3PO4-H2O and ZnCl2-H3PO4-H2O is reached in 2.5 hours at any temperature. Solubility is studied in phosphoric acid concentration interval from 5 to 55% of H3PO4 at temperatures 25, 60 and 80ºС. The last ones correspond to conditions of chloride sublimate processing.
Results and discussion.Considered mutual quaternary systems consist of water and two salts without common ion, therefore to image such systems a tetragonal pyramid is used. The vertex in the pyramid represents figurative point of water composition, and the base is the reflection of composition of waterless system consisting of salt pairs 3FeCl2- 2H3PO4 and Fe3(PO4)2-6HCl, 3ZnCl2-2H3PO4 and Zn3(PO4)2-6HCl . The solubility is more visually imaged in such systems using projections of isothermal sections of space diagram on the pyramid base as a square (quadrate). Herewith only points of composition of solution and solid phase salt mass are plotted on the diagram, and such projection doesn’t contain any information about water content in the system. To reflect water content it is necessary to construct additionally water projection diagram.
The calculation of processes of monosubstituted ferrous and zinc phosphates preparation is performed on the basis of constructed phase diagrams of systems FeCl2-H3PO4-H2O and ZnCl2-H3PO4-H2O. According to the rule of connected line the process of chlorides and phosphoric acid interaction is imaged on the diagram by line BD, connecting points of initial pure reagents composition. To form the target product – ferrous or zinc dihydrophosphate it is necessary to carry out the process with such mass ratio of initial compounds when the figurative point of system composition is to be on the crystallization ray of salts Fe(H2PO4)2 andZn(H2PO4)2, which connects points of their composition F1 and eutonic points . To obtain given system composition we determined the consumption of phosphoric acid solution according to the rule of segments and then more exactly by means of mass balance calculation. As a result of calculations we determined output of target products – salts Fe(H2PO4)2 and Zn(H2PO4)2. Thus we established optimal consumption of phosphoric acid solution for given product production out of a unit of mass (1 kg) of ferrous chloride – 2.69 kg and of 1 kg of zinc chloride – 2.115 kg. It is also determined maximal possible output of target product in given conditions of the process out of a unit of mass of ferrous chloride – 0.977 kg, and out of a unit of mass of zinc chloride – 0.818 kg.
Conclusion.Calculated on the basis of experimental results Yeneke indexes and constructed phase state diagrams of systems FeCl2-H3PO4-H2O and ZnCl2-H3PO4-H2O for temperatures 25, 60 and 80ºС represent new scientific data about solubility in mutual quaternary systems. These data widen essentially the area of knowledge in physicalchemical analysis of multicomponent systems and they are the theoretical base for analysis and justification of optimal parameters of processes, proceeding in researched systems.
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