Theoretical Questions of Definition of Timepoint of a Cut-Off of Air at an Oxidizing Stage of Fire Refinement of Copper

Abstract

Fire refinement of copper is intended for purification of draft copper of the impurity having the increased affinity to oxygen. At the same time a part of impurity passes into slag, and a part in a gas phase. However full removal of impurity doesn't manage to be reached. Therefore the air volume demanded for the maximum removal of impurity at an oxidation stage needs to be controlled, using the combined methods. Especially it is relevant in the conditions of increased requirements to resource-, to energy saving.

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Fire refinement of copper is a closing stage of pyrometallurgical purification of draft copper of the impurity having the increased affinity to oxygen. This method is directed to the maximum removal of sulfur, oxygen, iron, nickel, zinc, lead, arsenic, antimony and the gases dissolved in fusion. At the same time a part of impurity passes into slag, and a part – into a gas phase. Further cleaning is carried out by electrolytic refinement.

From practice of fire refinement of copper it is known that process of removal of impurity takes place at an oxidation stage due to creation of necessary concentration of the oxygen given as a part of air to furnace charge fusion.

At a temperature of fusion of 1 100 °C the content of oxygen can be brought to 0,34% depending on the chemical composition of furnace charge. At the same time change of structure of primary slag phase is defined by the content of oxides in the emerging oxidation products during process (table 1).

Table 1: Structure of products of oxidation, mass % Structure of products of oxidation, % PbO FeO SnO SiO2 Al2O3 NiO ZnO CuO Beginning of oxidation 20,1 2,5 16,3 18,5 28,9 2,3 8,2 3,2 Middle of oxidation 31,7 4,8 39,9 1,2 2,7 4 9 6,7 End of oxidation 31,8 4,9 35,4 0 0 4,3 6,5 17,1

Refinement of copper by a melt purge air has the thermodynamic restrictions expressing that residual concentration of impurity in a melt depends not only on concentration of oxygen in a melt, but also on activity of oxide of impurity in slag. The activity of oxide of impurity in slag is less, the refinement due to shift of equilibrium of response of oxidation of impurity towards interaction products is deeper.

Process of refinement is complicated by the fact that on the enterprise blister copper of the variable chemical composition arrives. It leads to scope change of air at an oxidation stage from melting to melting [ 1–5 ]. Cleaning of copper from impurity is carried out for the purpose of execution enough increased requirements to its characteristics, such as conductance, the plasticity, corrosion resistance, etc., low values of these indices demonstrate that copper is unsuitable for use in production.

As pure copper is generally widely applied in the radio engineering industry, the main attention in case of its receiving is paid to electrical indices, in particular unit electrical resistance ρ which makes 0,0175 Ωꞏmm2/m at 20°С. Quite often in practice use the size of specific electric conductivity σ = 1/ρ, (m/(Ωꞏmm2)).

The physical and chemical analysis of various samples of copper shows direct dependence of specific electric conductivity by nature of substance (concentration of impurity) and temperatures (figure 1). Apparently from the figure 1, increase in concentration of impurity in copper leads to reduction of its specific electric conductivity and reduces quality electric indicators [ 6–9 ].

The dependence of the specific resistance (conductivity) of two component systems on the chemical composition of substance is described by the known rules, their application is defined by specific conditions in which there is a studied sample. In the theory of assessment of conductivity several rules use.

According to the rule Nordgeyma the specific resistance of alloy has to be approximately proportional to the work of mole fractions of two components [ 10 ].

According to the rule Linda the speed of increase in resistance with increase in concentration in the diluted alloys has to be proportional to a square of a difference of valency of components [ 10 ]. By the rule to Linda additional electric resistance ∆ρ, caused by the content of impurity of 1%, in proportion to a square of a difference of valency of pure metal and ∆z impurity:

∆ρ = a + bꞏ(∆z)2, (1) where a, b – the sizes defining properties of metal-solvent.

Many fusions don't submit to this rule.

For assessment of influence of impurity at their rather low concentration use linear dependence of a gain of specific electric resistance Δρ from concentration of Сim impurity

ρ = ρ0 + ΔρꞏСпр, (2) where ρ0 – the specific electric resistance of the main component (solvent) depending on temperature; Δρ – the residual electric resistance which isn't depending on temperature, caused by availability of impurity atoms and proportional to concentration of impurity.

The equation (2) is called Matissena-Fleming's rule [ 11 ]. According to the rule Matissena, full specific resistance is equal to the sum of the specific resistance answering to each of mechanisms separately.

Matissen's theory also assumes increase in specific resistance of pure metals with growth of temperature (figure 2). Apparently from the figure 2, specific resistance sharply increases in a copper melting temperature point.

The temperature dependence of electric resistance of metals submits to the following law:

ρ(t) = ρ0ꞏ(1 + αꞏΔt), (3) where ρ0 – unit resistance at a normal temperature, for example, 20 °C; α – coefficient of thermal resistance Δt – the difference of temperatures, as it can be considered a difference between 20 ° C and a melting temperature.

Physical and chemical regularities of receiving electrotechnical copper are studied rather fully now. But despite it, in the conditions of increased requirements to resource-and energy saving, poor quality of raw materials and products with the variable chemical composition there is relevant a task of search of optimum conditions of refinement.

From the above material it becomes obvious that the private task of optimization is search necessary and enough oxygen as a part of air at a stage of oxidation [ 6 ]. 2

Integrated approach to definition of concentration of oxygen in the course of anode melting

In this operation the theoretical questions of determination of timepoint of a cut-off of air at an oxidation stage based on application of integrated approach are provided, using mathematical and instrumental methods.

Application of integrated approach is caused by the fact that only the experimental determination of concentration of oxygen in the course of anode melting is a difficult task, especially in the conditions of high temperatures and complex multicomponent raw materials of the variable chemical composition.

The decision of this task can be broken into several parts conditionally: 1. calculation of theoretically necessary amount of air for a melt purge; 2. wiring design of monitoring of unit resistance of a melt; 3. creation of an algorithm of automatic control of the system of air supply for oxidation of impurity. In the first part, using stoichiometric dependences, calculated: 1. on mass of impurity as a part of furnace charge mass of oxide of Cu2O copper (I); 2. on Cu2O mass necessary value of theoretical mass and % (on mass) oxygen; 3. amount of the air in tons and m3 given to a melt.

This information comes to the microcontroller.

The second part provides the wiring design of monitoring of unit resistance of a melt based on strain measurement by the compensation and difference method (figure 3). Two couples of electrodes 1,1’ and 2,2’ are for this purpose applied. One couple is dipped in a reference melt (a melt of electrotechnical copper) with unit resistance ρr, the second couple in a melt of blister copper with unit resistance ρin. The stabilized voltage sources of E1 and E2 are opposite connected to each couple of electrodes. On the one hand two electrodes of different couples are connected among themselves. The load resistances of identical value of Rl1 and Rl2 are provided in a circuit.

Between the free ends of electrodes 1 and 2 the digital microvoltmeter from which signal comes to the microcontroller is switched on. Through each couple of electrodes currents of I1 and I2 proceed. Currents cause falling of tension on reference ρr and initial ρin resistance.

For improving of the quantitative indices of measurement in a circuit use of the amplifier is allowed.

In a figure 4 the theoretical nature of change of voltage drop between electrodes is shown (points 1 and 2) in case of course of currents of I1 and I2 in the counter direction in the conditions of change ρin due to operation of oxidation of impurity. mkV 2 + 1 ρe 1' ρin

2’ I1 +

Е1 = Е2

Rl1 ΔU ΔUmax

The nature of change of tension on this site demands additional researches. Only the fact that the value of tension decreases in process of removal of impurity from fusion is authentically known

t I2 Rl2

Beginning of a purge

End of a purge

have a melting temperature below, than the base refractory metals, but also can be considered as electrodes under certain conditions.

For lowering of temperature of an electrode it is possible to provide cooling with system water from two coaxial steel pipes. Cold water which passes into an external pipe comes to the central pipe, cooling it. To prevent influence of the slags floating on a surface electrodes it is necessary to cover on top with heat resisting and electrical insulation. The alternating current of industrial frequency is applied to preventing of sticking of ions to one electrode.

The metal choice problem also consists in that, as in the atmosphere, and in a copper melt in case of a purge there is an oxygen which at the given temperatures oxidizes metal. So, the tungsten as the most high-melting of metals (more high temperature of melt only carbon has) at a temperature of a red kaleniye slowly is oxidized in oxide of the tungsten (VI) WO3, losing the functions. A molybdenum begins to be oxidized at a temperature of 400 °C. In case of achievement of temperature over 600 °C a molybdenum quickly is oxidized to oxide of molybdenum (VI) MoO3. Titanium in case of heating up to temperature of 1 200 °C lights up a bright white flame with formation of oxide-coated phases of variable composition TiOx. Inert platinum in case of heating up reacts with oxygen with formation of volatile oxides.

In the third part creation of an algorithm of automatic control of the system of air supply for oxidation of impurity on the basis of data of the first and second part is supposed [ 12–14 ]. The microcontroller normalizes indications on a formula (4) and compares to the previous indications. In a figure 5 process of a cut-off of air in time is shown. In case of the difference of these indications less preset value ΔUn the microcontroller gives a command on an air cut-off (figure 6). The value ΔUn depends on the frequency of inquiry of the microvoltmeter and the expenditure of air on a purge which is also controlled by the microcontroller.

Air, m3

Vc  [Vcur](t)  0 

∆[V](t) = 0 ∆[V](t)

Conclusion In this operation questions of methodology of integrated approach to determination of timepoint of a cut-off of air at an oxidizing stage of fire refinement of copper in the conditions of high temperatures and complex multicomponent raw materials of the variable chemical composition are considered.

Theoretical questions of preparation of the scheme of control of specific resistance of fusion are shown by a compensation and differential method.

The main restrictions imposed on use of materials as electrodes are considered. The possibility of creation of an algorithm of control and management of the air given to copper fusion at an oxidation stage is shown.

The offered methodology can be considered as the first development stage of the complex scheme of automation of process of fire refinement of draft copper.

1. Furnace charge loading

F

Reverberatory furnace F FFC

Cut-off signal

ΔUн ≤ ΔUнзад Air [Vfl](t) 3. Oxidation

Microcontroller

Calculation of volume

of air [Vc]

Calculation of value ΔUн

Exit of anode

copper

References

1. Davenport W.G. , King M. , Schlesinger M. , Biswas A.K. Extractive metallurgy of copper, fourth edition . - Oxford: Elsever Sci. Ltd ., 2002 . - 481 p.

2. Biswas A.K. , Davenport W.G. Extractive metallurgy of copper . - Oxford: Pergamon, 1996 . - 290 p.

3. Gerlach J. , and Herfort P. The Rate of Oxyden Uptake by Molten Copper // Metall. 1968 . 22 ( 11 ). - Р . 1068 - 1090 .

4. Bugs of Accusative, Spitchenko V. S., Novokreshchenov S .A., S.I. Cold . Copper refinement. - Yekaterinburg: URFA, 2010 . - 317 p.

5. Bugs of Accusative, Skopov G. V., S.I. Pirometallurgiya's Cold of copper . - Yekaterinburg: OURO RAHN , 2016 . - 632 p.

6. Bogoroditsky N.P. , Stepsons V. V. , Tareev B.M. Electrotechnical materials . - L.: Energoatomizdat , 2000 . - 304 p.

7. Wren V.T. Electrotechnical materials . - L., 1984 . - 269 p.

8. Koritsky Yu .V., Stepsons V. V. , Tareev B.M. Reference book on electrotechnical materials . - M.: Energoatomizdat, in 3 t., 1986 - 1988 . - 368 p., 464 p., 728 p.,

9. Livshits B.G. , Kraposhin V.S. , Linetsky Ya .L. Physical properties of metals and alloys . - M.: Metallurgy , 1980 . - 320 p.

10. Vilson D.R. Structure of liquid metals and the alloys / translation from English L.A. Koledov; under the editorship of the prof . V. M. Glazov . - M.:. Metallurgy, 1972 . - 247 p.

11. Okazaki K. Posobiye on electrotechnical materials / Kiyesi of Okazaki; edition of L.R. Zayonts . - The lane with яп . M.M. Bogachikhina , I.B. Reuta - M.: Energy , 1979 . - 432 p.

12. Krupsky V.N. Theory of algorithms . - M.: Academy , 2009 . - 208 p.

13. Akho A. V. Structures of data and algorithms / lanes from English and an edition of Minko A.A. / Akho A.V. , Hopkroft D. E. , Ullman D.D. - M.: Williams , 2001 . - 382 p.

14. Kormen T.H. , Leyzerson Ch.I. , Rivest R.L. , Stein K. Algorithms : construction and analysis . - M.: MTsMNO, 1999 . - 960 p.