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Section 12. Technical sciences
Khasanov A. S., deputy director of science head of STC SPC of production of rare metals and solid alloys doctor of technical sciences, professor Berdiyarov Bakhriddin Tilovkabulovich, senior teacher of department of Metallurgy, Tashkent state technical university E-mail: mr.berdiyarov@mail.ru
RESEARCH OF EDUCATION AND PREVENTION OF FERRITE AND SILICATES OF ZINC WHEN ROASTING SULPHIDIC ZINC CONCENTRATES IN FURNACES OF THE BOILING LAYER
Abstract: In the article considered causes and conditions of formation ferrites and silicates zinc from roasting sulfide zinc concentrates on the furnace "Boyling Layer", also in the article directed measure for decreasing formation ferrites and silicates on the process roasting. Thermodynamic impacts additional components to charge for decreasing formation components to charge for decreasing formation ferrites and silicates zinc also directed in this article.
Keywords: ferrite, cake, to make silicate, zinc, autogen, pyrite, roasting.
Now in the world about 10 million t of zinc a year are produced. Its main quantity is made in the classical hydromet-
ZnO + Fe2O3 = ZnO • Fe2O3 ZnO + SiO3 = ZnO • SiO2
allurgical way with use of furnaces of the boiling layer for the translation of non-soluble sulfides in soluble oxides [1]. This process has such advantages as:
1) specific capacity of the furnace of the boiling layer of "BL" is 3-4 times higher, than in multihearth furnaces, and 1.5-2.0 times above, than when roasting in suspension;
2) content in a candle end of sulphidic sulfur when roasting in "BL" furnaces is 0.2-0.3% instead of 0.2-0.5% when roasting in suspension and 0.3-1.0% in multihearth furnaces;
3) preliminary drying of a concentrate is not required, power supply of the "BL" furnace not only a damp concentrate, but also a concentrate pulp is possible;
4) process of roasting in "BL" furnaces easily gives in to automation;
5) considerably bigger between-repairs period and simplicity of repair.
However, also essential shortcomings are peculiar to process of roasting of a concentrate in "BL" furnaces, namely -rather low extraction of zinc in solution at the subsequent leaching of a candle end. The analysis of this phenomenon showed that education in the course of roasting of ferrite and silicates of zinc on the following reactions is the main reason for low extent of transition of metal to solution:
High-temperature roasting (t, 960-990 0C) promotes formation of these connections. These connections are very poorly dissolved in solutions of sulfuric acid and substantially pass into cake [2].
When performing research work it was established that at temperatures of oxidizing roasting of 900-950 °C on separate grains of sulphidic minerals temperature rises higher than 1200 °C that leads to fusion of these grains. As a result of fusion of mineral grains difficult connections similar to metallurgical slags (ferrite and silicates of zinc) are formed. In this regard to processes of formation of ferrite and silicates of zinc at oxidizing roasting, in some degree it is possible to apply regularities of formation of metallurgical slags [3].
According to the molecular theory of education and the structure ofliquid slags - strong oxides force out weaker which remain in fusion in a free state from chemical compounds. "Force" of acid or main oxide can be measured by the thermodynamic potential of formation of connections from oxides in fusion [4].
Than more size of thermodynamic potential (its negative value), carried to 1 mole acid oxide, especially the basis is "strong" and the more so its connection with acid oxide is strong.
In silicate fusions the strongest main oxides are - oxides of alkaline and alkaline-earth Na2O metals, K2O, CaO, MgO
and BaO; weaker main oxides are oxides of heavy metals: FeO, MnO, PbO, Cu2O, NiO and the weakest - amphoteric: ZnO, Fe2O3 Al2O3. The strongest acid in SiO2 slags.
Thus, "strong" main oxides and "strong" acid oxides form most thermodynamic strong connections, than "weak" main oxides with "strong" acid oxides [1; 2]. Proceeding from it, it is possible to assume that introduction to furnace charge of roasting of "strong" main oxides will lead to decrease in formation of ferrite and silicates of zinc.
For experimental confirmation of this assumption laboratory researches of roasting of a sulphidic zinc concentrate
At Zinc plant these concentrates unite, average and prepare furnace charge according to production requirements. Pilot studies were conducted both with separate concentrates, and with the furnace charge prepared for roasting in the "BL" furnace on Zinc plant AMMC.
For a research and studying of the reasons allowing to reduce formation of ferrite and silicates of zinc in the course of roasting entered additional additives into furnace charge -chemical clean reactants are oxides of alkaline and alkalineearth metals (CaO, BaO, Na2O, K2O).
Additional additives were entered into furnace charge of roasting in three ways:
1) the sulphidic zinc concentrate and additional additives carefully mixed up in dry before formation of homogeneous mass, further furnace charge was exposed to roasting;
2) for the purpose of ensuring good contact of components of furnace charge and the entered additional additives the furnace charge prepared on the first way was moistened with water and from it granules (pellets) with a diameter of 4-6 mm prepared. The prepared pellets dried and exposed to roasting;
3) for ensuring full contact of components of furnace charge with the entered additives prepared water suspension of the added component and mixed with a zinc concentrate. The formed pulp was pounded in a porcelain mortar and was exposed to drying. The dried-up furnace charge was crushed and exposed to roasting.
Roasting in the stationary mode was carried out in the muffle furnace which is previously heated to 950 0C within
with introduction to furnace charge of additional additives were carried out.
The sulphidic zinc concentrate and the furnace charge prepared for roasting in the furnace of the boiling layer "BL" at the AMMC Zinc plant served as objects of pilot studies.
Now at zinc plant concentrates are processed imported and local zinc. For carrying out researches about 2 kg of a sulphidic zinc concentrate and furnace charge were selected.
The chemical and phase analysis of the selected tests were executed in chemical laboratory of the AMMC Zinc plant. Results of the analysis are given in (tab. 1).
120 minutes. From the tests of furnace charge prepared in various ways hinge plates which filled up in fireclay crucibles of rectangular section, layer of 5-7 mm thick were selected. Then crucibles placed in the work area of the furnace. Through an opening by means of a quartz tube blew air into the furnace. By means of the same tube hashing of furnace charge with frequency of15 minutes was carried out.
After the expiration of the set roasting time, the crucible with a candle end was taken out from muffle furnaces, cooled and weighed. The phase structure of a candle end was defined in the Zinc plant AMMC chemical laboratory.
Roasting in the dynamic mode carried out on installation laboratory the furnace of the boiling layer of "LPKS" which consists of the laboratory mine SNOL 1.6/11-IZ furnace, a coil from corrosion-proof became for heating of air, model of the "BL" furnace executed from a quartz tube with a diameter of 60 mm, the brand air compressor UK - 2M (fig. 1).
For establishment of necessary pressure of the air blown into furnace volume for the purpose of creation of the boiling layer before roasting made single experiments. Further the furnace was heated to the set temperature and in furnace volume through loading tubes filled up with certain portions of a hinge plate of furnace charge prepared in various ways. Time of stay of one hinge plate in the furnace was 1.5-2.0 minutes. The candle end from the volume of the furnace was unloaded from an output tube and gathered in a porcelain crucible.
Table 1.- The chemical composition of the selected tests on the main components
Tests Contents,%
Zn Fe S SiO2 Pb Cu
Test No. 1 Zn, Concentrate of "Hondiza" 38-50 5-8 29-31 till 4 4-4.6 2.2-2.3
Test No. 2 Zn, Concentrate of "Zyryanovsk" 50-53 4-7 28-31 1.5-2.0
Test No. 3 Zn, Concentrate of "Glubochenko" 38-39 16-18 29-32 10-14
Test No. 4 Zn, Concentrate of "Predgorniy" 45-46 8-9 29-31 1.5-3.0
Test No. 5 Zn, Concentrate of "Ust-Talovka" 38-43 8-14 29-32 3.5-4
Figure 1. LPKS installation: SNOL 1.6/11-IZ 1 - mine furnace; 2 - model of the "BL" furnace; 3 - air UK-2M compressor; 4 - LATR-2M autotransformer; 5 - a heating tube from stainless steel; 6 - a cup for reception of a candle end
When roasting zinc concentrates in "BL" furnaces interaction of components (ZnO, Fe2O3, SiO2) a concentrate with oxides of the alkaline and alkaline-earth metals entered into roasting furnace charge can proceed according to two schemes:
1) oxide of iron and oxide of silicon reacts along with oxide of zinc and with the oxides entered as additional additives
ZnO + (m + n)Fe2O3 + nMeO = mZnO • Fe2O3 + nMeO •
• Fe2O3 + (l-m)ZnO + (l-n)MeO (l)
ZnO + (m + n)SiO2 + nMeO = mZnO x SiO2 + nMeO •
• SiO2 +(l-m)ZnO +(l-n)MeO (2)
2) in the beginning ferrite and silicates of zinc which interact with in addition entered additives are formed
(3)
(4)
Regardless of according to what scheme interaction of components of furnace charge with in addition entered additives (oxides of alkaline and alkaline-earth metals) will proceed, the amount of oxide of the zinc which is not connected in the form of ferrite, obviously, will depend on the speed of formation of ferrite and the entered metal oxide. The more there will be a speed of an make of ferrite of the entered metal, the less zinc will ferritization. If process goes on a formulas (3, 4), then zinc oxide replacement from ferrite by other oxide will depend on the relative strength of the last.
The possibility of course of this or that chemical reaction and also its direction, as we know, determinate by the sign of change of size of free energy. The absence given on warmth of education and thermal capacities of ferrite, oxides of metallic does not allow to count a value of the maximum work
ZnO ■ Fe2O3 + MeO = MeO ■ Fe2O3 + ZnO ZnO ■ SiO3 + MeO = MeO ■ SiO2 + ZnO
of reaction therefore the issue of the direction of reaction of formation of ferrite of zinc had to be resolved by practical consideration.
Formation of ferrite and silicates of zinc when roasting a zinc concentrate was studied depending on a ratio of sulfide of zinc and the entered oxides of alkaline and alkaline-earth metals. At the same time temperature and duration of process was supported by a constant about 950 0C, process duration -2 hours.
Regardless ofa way ofpreparation offurnace charge for roasting as additional additives chemically clean Na2O, K2O, CaO the following percentage from the mass of furnace charge, were entered into%: 0.5; 1.0; 2.0; 3.0; 4.0; 5.0; 7.0.
Results of experiments on studying formation of ferrite and silicates when roasting in the static mode showed that additive in furnace charge even of a small amount of oxides of alkaline and alkaline-earth metals causes noticeable reduction of ferrite and silicate of zinc in a candle end (fig. 2 of line 2, 4). The best indicators on decrease in content of ferrite and silicates of zinc in a candle end are received when roasting the furnace charge prepared on the third way. It can be explained with the fact that at a wet way of a charge preparing there is almost full enswathe oxides of alkaline and alkaline-earth metals of particles of a zinc concentrate. Results of roasting of the furnace charge prepared on the third way are given in (tab. 2).
When roasting furnace charge in the dynamic mode on the laboratory LPKS installation decrease in content of ferrite and silicates of zinc was not observed (fig. 2 of line 1, 3), it can be explained with the fact that it when
roasting in the boiling layer is not reached full contact between particles of a concentrate and oxides of alkaline and alkaline-earth metals.
According to the results received experimentally it is visible that additive of oxides of alkaline and alkaline-earth metals leads to decrease in formation of ferrite by 1.5-1.7 times.
Figure 2. Influence of the additional additives entered into furnace charge of roasting on formation ferrite and a silikate at the static and dynamic modes of roasting: 1 content of ferrite in a candle end when roasting on the LPKS installation; The 2nd content of ferrite in a candle end when roasting in the laboratory muffle furnace; 3 content of silicates in a candle end when roasting on the LPKS installation; 4 content of silicates in a candle end when roasting in the laboratory muffle furnace
Table 2.- Results of roasting of the zinc furnace charge prepared on the third way
О Experience The name of additives Concentration of additives in% of the mass of furnace charge Phase structure candle end (relatively)
Zntotal ZnSO, ZnO ZnО ■ ■ SiO2 ZnS ZnSO4 ■ ■ FeA
1 2 3 4 5 6 7 8 9
1 Without addition 56.6 0.24 38.14 6.75 0.32 11.5
CaO 0.5 56.45 0.23 38.08 7.28 0.33 10.53
Na20 56.50 0.20 37.88 7.07 0.27 11.08
K,0 56.47 0.23 37.66 7.19 0.31 11.10
2 CaO 1.0 56.43 0.34 39.24 7.37 0.29 9.19
Na20 56.58 0.29 38.70 7.21 0.51 9.87
K20 56.54 0.24 38.65 7.55 0.22 9.88
3 CaO 2.0 55.87 0.19 39.12 7.26 0.23 9.07
Na20 56.21 0.42 39.02 7.18 0.48 9.1
K,0 55.94 0.47 38.68 7.23 0.50 9.06
4 CaO 3.0 56.84 0.19 40.53 6.65 0.57 8.90
Na20 56.52 0.23 40.11 6.79 0.38 9.01
K2° 55.90 0.39 40.21 6.10 0.31 8.89
5 CaO 5.0 56.88 0.14 42.33 6.72 0.52 7.17
Na20 56.03 0.28 40.41 6.46 0.32 8.56
K,0 56.48 0.21 41.87 5.97 0.41 8.02
1 2 3 4 5 6 7 8 9
CaO 56.11 0.15 43.30 5.30 0.36 7.00
6 Na2O 7.0 56.58 0.23 42.75 5.50 0.32 7.78
K2O 56.34 0.19 43.34 5.36 0.34 7.11
On the basis of the conducted researches on studying of influence of various additives in furnace charge of roasting and a way of a charge making on the content of ferrite and silicates of zinc in a candle end it is possible to draw the following conclusions:
- introduction to furnace charge of roasting of «strong» main oxides, such as CaO, Na2O, K2O, due to formation of strong thermodynamic connections with «strong» acid oxides (SiO2) leads to decrease in formation of ferrite and silicates of zinc;
- the optimum quantity of the entered additional component (CaO, Na2O, K2O) at which reaches minimum content of ferrite and silicates of zinc in a candle end, 7% of the mass of furnace charge are, at the same time decrease in ferrite and silicates of zinc by l.5-l.7 times is observed;
- an optimum way of preparation of furnace charge of roasting is mixing of water suspension of the entered additional additives with a zinc concentrate (a wet charge making) that provides almost full enswathe with oxides of alkaline and alkaline-earth metals of particles of a zinc concentrate.
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