PECULIARITIES OF POLYMETALIC SULPHIDE CONCENTRATE SOLID-STATE OXIDATION

UDC 669.053.2

N. Komkov (EKSTU, Ust-Kamenogorsk),

V. Luganov (KazNTU, Almaty)

PECULIARITIES OF POLYMETALIC SULPHIDE CONCENTRATE SOLID-STATE OXIDATION

Sulphide ores and concentrates are usually the charging feed for most metals production. Processing of the materials in order to get metals is connected with the process of sulphide oxidation. In metallurgical processes sulphide oxidation is realized in solid and liquid states.

In many cases sulphide materials roasting is the first stage of processing when solid sulphides are oxidized. In other cases, e.g. while smelting, sulphide oxidization is carried in both liquid and solid states. And finally there are some processes in which oxidization is carried entirely in liquid state. Converter blowing the matte in order to get crude copper and autoclave leaching are examples of them.

Nowadays low-grade concentrates are being processed. In applied sciences the task of finding these concentrates’ processing modes arises. That proves the great importance of studying the principles of sulphide oxidization processes in metallurgy.

The choice of thermal treatment method is determined by the character of the antecedent and subsequent processes, types of energy sources in the industry being designed and by many other factors including physical and chemical characteristics of the processed material. The tasks of material processing technology development include choosing a rational method and the optimum mode that will ensure acquiring the target product quality based on the characteristics of the processed material and on the certain industry conditions. At present time roasting in fluid-bed furnaces is a conventional method of pyrometallurgical treatment of sulphide materials.

The optimum mode is understood as the one which provides the best quality of the product with the minimum reagents and energy consumption. Temperature mode and technological parameters depend on the material characteristics. The maximum fluid-bed temperature permissible within the limits determined by the material characteristics should be provided in order to intensify the process. Off-gas temperature is specified by the kinetic peculiarities of sulphate formation.

The roasting process should be intensified with elimination of impermissible strains in the material and overheat that can cause formation of liquid phases.

As regards the intensification, the right choice of temperature and hydro-dynamical mode seems to be very important. The choice of temperature mode is predetermined by the chemical composition of the charging feed and by the processes occurring in it while being heated and roasted. Thus, before choosing the roasting mode (temperature, gas velocity, concentration and etc.) it’s necessary to study beforehand the material and end product response in a wide temperature range as in the certain change intervals of moisture (and temperature) sharp charging feed phase fluctuations caused by the temperature and the content (fluxation, melting and decomposition) can be observed. Being aware of these process peculiarities for every temperature interval it’s possible to use individual most intensive roasting mode as well as to evaluate the possible deviation of mode parameters [2]. Concentrate №1 unlike Concentrate №2 contains the minimum amount of impurities and doesn’t cause significant difficulties while being roasted that is why it was determined as the check standard.

Concentrates description

Table 1- Chemical composition of zinc sulphide concentrates

	Concentrate №	Elements
Potential, eV	Concentrate №	Zn	Fe	Cu	Pb	SiO₂	S	Sum of impurities
0.069	1	53.3	6.2	0.28	0.61	2.6	32.6	9.69
0.119	2	45.8	8.2	0.9	2.12	4.8	30.7	16.02
0.327	3	43.2	7.1	4.1	2.9	4.4	29.6	18.5
0.539	4	37.1	5.6	0.10	4.7	8.8	24.7	19.2
0.687	5	41.8	7.8	2.7	4.2	7.4	29.7	22.1
0.888	6	44.7	7.2	2.9	4.9	8.3	24.1	23.3
1.0195	7	40.8	10.5	3.2	4.7	6.5	30.6	24.9

Concentrate №1

Concentrate №1 possesses the following mineral composition,%: 79.44 ZnS, 9.99 FeS₂, 0.70 PbS, 0.81 CuFeS₂, 2.60 SiO₂, less than 0.5 of fahlore. All the minerals of the concentrate are represented as angular flakes. 40–50% of ZnS sphalerite is in free form. The rest of sphalerite has emulsion impregnation of CuFeS₂copper pyrite 1 – 5 microns in size as well as copper pyrite, FeS2pyrite, PbS galenite inclusions or marginals 10 – 20 microns in size.

Concentrate №2

Concentrate №2 possesses the following mineral composition,%: 68.26 ZnS, 10.25 FeS₂, 4.16 FeS, 2.45 PbS, 2.6 CuFeS₂, 4.8 SiO₂, less than 0.5 of fahlore.

Significant diversity in the concentrate structure is not observed. Singular pyrite and copper pyrite flakes are noticed.

Concentrate №3

Concentrate №3 possesses the following mineral composition,%: 64.4 ZnS, 6.77 FeS₂, 0.53 FeS, 3.4 PbS, 11.80 CuFeS₂, 4.4 SiO₂, less than 0.5 of fahlore.

It does not differ from Concentrate №1 in structure. ZnS sphalerite is mostly in free form and rarely in crystal-jams with CuFeS₂copper pyrite.

Concentrate №4

Concentrate №4 possesses the following mineral composition,%: 55.29 ZnS, 9.39 FeS₂, 1.802 FeS, 5.43 PbS, 0.288 CuFeS₂, 8.8 SiO₂, less than 0.5 of fahlore.

Concentrate №5

Concentrate №5 possesses the following mineral composition,%: 62.3 ZnS, 10.13 FeS₂, 1.13 FeS, 4.85 PbS, 7.79 CuFeS₂, 7.4 SiO₂, less than 0.5 Cu₅FeS_4. It differs from Concentrate №1 in the presence of epigenetic sulphides and Cu₅FeS₄erubescite.

Concentrate №6

Concentrate №6 possesses the following mineral composition,%: 63.17 ZnS, 7.34 FeS₂, 5.66 PbS, 7.63 CuFeS₂, 8.3 SiO₂. It does not differ from Concentrate №1 in structure. Most part of ZnS sphalerite is in free form. Inclusions are represented by emulsion impregnation of CuFeS₂copper pyrite. Sulphur deficit is 6.8 %, in sulphurite – 3.43 %, in copper pyrite – 0.72 %, in pyrite – 2.65 %.

Concentrate №7

Concentrate №7 possesses the following mineral composition,%: 60.8 ZnS, 8.3 FeS₂, 6.03 FeS, 5.43 PbS, 9.24 CuFeS₂, 6.5 SiO₂, less than 0.5 of fahlore. It differs from Concentrate №1 by the presence of epigenetic cupric sulphides, 7Cu₅FeS₄erubescite, Cu₂S chalcosine and covelline CuS.

The Results of Low Grade Zinc Sulphide Concentrates Roasting in Laboratory Furnace

The research of low grade zinc sulphide concentrates roasting was conducted in a lab pipe furnace of SUOL – 0,4.4/12 type with the temperature of 1173 – 1273 К in blast air. The blast air velocity was kept at 2 dm³ per minute by ejector regulation of exhaustion and thrust power in gas duct system and by controlling blast rate with a flow gauge. The furnace temperature was automatically kept and controlled with platinum-rhodium thermal converter and with a secondary instrument that was a millivoltmeter. Zinc sulphide concentrate of 10 gram weight quantity in a porcelain boat was placed into quartz tube installed in the furnace. The research duration was within 1600 – 7200 second duration.

Table 2 - Roasted Product of Zinc Concentrate №1

Time, s	Content,%
Time, s	Zn_o	Zn_к	Zn_в	Fe	Cu	Pb	SiO₂	S_o	S_SO4	S_s	Zn Solub., %	Desulph.,%
1800	58.1	18.9	0.21	6.76	0.305	0.66	2.83	20.5	0.45	20	32.53	37.12
3600	60.5	34.8	0.19	7.04	0.32	0.69	2.95	12.5	0.38	12.12	57.52	61.66
5400	62.5	48.5	0.13	7.27	0.33	0.72	3.05	5.3	0.38	4.92	77.6	83.74
7200	63.6	55.4	0.12	7.398	0.334	0.73	3.102	2	0.35	1.65	87.11	93.86

Table 3 - Roasted Product of Zinc Concentrate №2

Time, s	Content,%
Time, s	Zn_o	Zn_к	Zn_в	Fe	Cu	Pb	SiO₂	S_o	S_SO4	S_s	Zn Solub., %	Desulph.,%
1800	51.4	19.3	0.42	9.2	1.01	2.38	5.39	17.9	0.71	17.9	37.55	41.69
3600	53.2	30.2	0.56	9.52	1.04	2.46	5.58	11.5	0.8	10.7	56.77	62.54
5400	53.9	38	0.41	9.65	1.06	2.49	5.65	7.2	0.73	6.47	70.5	76.55
7200	55.3	34.1	0.16	9.9	1.09	2.56	5.8	4.2	0.68	3.72	61.66	86.32

Table 4 - Roasted Product of Zinc Concentrate №3

Time, s	Content,%										Zn Solub., %	Desulph., %
Time, s	Zn_o	Zn_к	Zn_в	Fe	Cu	Pb	SiO₂	S_o	S_SO4	S_s	Zn Solub., %	Desulph., %
1800	46.9	13.1	0.39	7.71	4.45	3.15	4.78	20.3	0.44	19.86	27.93	31.42
3600	48.3	17.8	0.17	7.94	4.58	3.24	4.92	16	0.35	15.6	36.85	45.94
5400	49.9	25.8	0.11	8.2	4.74	3.35	5.08	10.6	0.4	10.2	51.7	64.19
7200	50.3	34.2	0.07	8.27	4.77	3.38	5.12	4	0.53	3.47	67.99	86.49

Table 5 - Roasted Product of Zinc Concentrate №4

Time, s	Content,%										Zn Solub., %	Desulph.,%
Time, s	Zn_o	Zn_к	Zn_в	Fe	Cu	Pb	SiO₂	S_o	S_SO4	S_s	Zn Solub., %	Desulph.,%
1800	48.4	23.8	0.26	7.31	0.13	6.13	11.48	12.2	0.82	11.38	49.17	50.61
3600	50.7	32	0.3	7.65	0.14	6.42	12.03	8.1	0.96	7.14	63.12	67.21
5400	51.6	34.3	0.42	7.79	0.14	6.54	12.24	7	0.95	6.05	66.47	71.66
7200	51.9	44	0.17	7.83	0.14	6.58	12.31	1.9	1	0.9	84.78	92.31

Table 6 - Roasted Product of Zinc Concentrate №5

Time, s	Content, %										Zn Solub., %	Desulph.,%
Time, s	Zn_o	Zn_к	Zn_в	Fe	Cu	Pb	SiO₂	S_o	S_SO4	S_s	Zn Solub., %	Desulph.,%
1800	42.8	13.7	0.45	7.99	2.76	4.3	7.58	19.7	0.54	19.16	32.01	33.67
3600	50.3	25.9	0.25	9.39	3.25	5.05	8.9	12.8	0.41	12.39	51.49	56.9
5400	50.2	26.8	0.21	9.38	3.24	5.04	8.89	11.5	0.36	11.14	53.39	61.28
7200	51.1	35.7	0.13	9.54	3.3	5.13	9.05	6	0.28	5.72	69.86	79.798

As it’s obvious from the Table 1 the concentrates are enumerated in the order of increasing the amount of impurities in their content. In the result of the conducted experiments on roasting concentrates in laboratory conditions the following figures have been obtained: according to sulphur oxidization degree (remaining sulphide sulphur) the concentrates can be lined in the following sequence: 4,1,6,3,2,5,7; according to the desulphurization degree the concentrates can be lined in the following sequence: 1,4,3,2,6,5,7; according to the solvability degree the concentrates can be lined in the following sequence: 1,4,6,5,3,2,7; according to the sulphur oxidization velocity the concentrates can be lined in the following sequence: 1,2,3,5,4,6,7; according to the desulphurization velocity the concentrates can be lined in the following sequence: 1,4,2,3,6,5,7; according to the solvability velocity the concentrates can be lined in the following sequence: 1,4,6,5,3,2,7.

Table 7 - Roasted Product of Zinc Concentrate №6

Time, s	Content,%										Zn Solub., %	Desulph.,%
Time, s	Zn_o	Zn_к	Zn_в	Fe	Cu	Pb	SiO₂	S_o	S_SO4	S_s	Zn Solub., %	Desulph.,%
1800	50.4	16.4	0.37	8.12	3.27	5.53	9.36	18.8	0,31	18.5	32.54	21.99
3600	52.7	27.9	0.26	8.49	3.42	5.78	9.78	12.4	0,25	12.5	52.94	48.55
5400	54.3	37.3	0.32	8.75	3.52	5.95	10.08	7.2	0,3	6.9	68.69	70.12
7200	55.3	45.1	0.13	8.91	3.59	6.06	10.28	3.6	0,22	3.38	81.56	85.06

Table 8 - Roasted Product of Zinc Concentrate №6

Time, s	Content,%										Zn Solub., %	Desulph.,%
Time, s	Zn_o	Zn_к	Zn_в	Fe	Cu	Pb	SiO₂	S_o	S_SO4	S_s	Zn Solub., %	Desulph.,%
1800	45.9	10.5	0.24	11.81	3.6	5.29	7.31	23.3	0.4	22.9	22.88	23.86
3600	46.3	7.9	0.23	11.92	3.63	5.33	7.38	23	0.33	22.6	17.06	24.84
5400	48.3	13.8	0.15	12.43	3.79	5.56	7.69	13.5	0.38	13.1	28.57	55.88
7200	49.2	25	0.14	12.66	3.86	5.67	7.84	13.3	0.37	12.9	50.81	56.54

The Concentrate №1 has the best results of roasting quality when the Concentrate №7 has the worst ones. That is regular and obvious as the Concentrate №1 has the least number of impurities while the Concentrate №7 has the most.

According to the influence of impurities on the quality factors one can come to the following conclusions:

Impurities influence differently on different quality factors.

The sum of Fe, Cu, Pb, SiO₂with correlation rate -0.95 and Pb with correlation rate -0.90 have the most significant impact (regeneration) on sulphur oxidization velocity. Cu with correlation rate -0.51 has the least significant impact.

The sum of Fe, Cu, and Pb with correlation rate -0.84 and Fe with correlation rate -0.93 have the most significant impact (regeneration) on desulphurization increase velocity. SiO₂ with correlation rate 0.19 has the least significant impact.

Table 9 – Zinc Concentrates Roasting Figures

Concentrate title	Velocity			Impurities content, %
	Sulphur oxidization, gps*10^-4	Desulphurization, %/s*10^-2	Zn solubility, %/s *10^-2	Fe	Cu	Pb	SiO₂	Sum
	Sulphur oxidization, gps*10^-4	Desulphurization, %/s*10^-2	Zn solubility, %/s *10^-2	Fe	Cu	Pb	SiO₂	Fe,Cu	Fe,Cu,Pb	Fe,Cu, Pb, SiO₂
Con.№1	4.3	1.3	1.21	6.2	0.28	0.61	2.6	6.48	7.09	9.69
Con.№2	3.75	1.2	0.86	8.2	0.9	2.12	4.8	9.1	11.22	16.02
Con.№3	3.63	1.2	0.94	7.1	4.1	2.9	4.4	11.2	14.1	18.5
Con.№4	3.3	1.28	1.18	5.6	0.1	4.7	8.8	5.7	10.4	19.2
Con.№5	3.33	1.11	0.97	7.8	2.7	4.2	7.4	10.5	14.7	22.1
Con.№6	2.88	1.18	1.13	7.2	2.9	4.9	8.3	10.1	15	23.3
Con.№7	2.46	0.78	0.7	10.5	3.2	4.7	6.5	13.7	18.4	24.9
Correlation rate of sulphur oxidization velocity depending on impurities content				-0.62	-0.51	-0.90	-0.72	-0.65	-0.88	-0.95
Correlation rate of desulphurization velocity change depending on impurities content				-0.93	-0.54	-0.47	-0.19	-0.85	-0.84	-0.69
Correlation rate of Zn solubility change depending on impurities content				-0.92	-0.53	-0.19	0.06	-0.84	-0.71	-0.48

The sum of Fe, Cu, and Pb with correlation rate -0.84 ÷ -0,71 and Fe with correlation rate -0.92 have the most significant impact (regeneration) on desulphurization increase velocity. SiO₂ with correlation rate -0.92 has the least significant impact.

With the potential difference of 0.069 eV at sulphide-oxide boundary, sulphur oxidization, desulphurization and soluble compounds formation velocities will be equal correspondingly: 4.3*10^-4 gps, 1.3*10^-2 %/s, 1.21*10^-2 %/s.

With the potential difference of 1.0195 eV at sulphide-oxide boundary, sulphur oxidization, desulphurization and soluble compounds formation velocities will be equal correspondingly: 2.46*10^-4 gps, 0.78*10^-2 %/s, 0.7*10^-2 %/s.

If the potential difference at sulphide-oxide boundary increases, sulphur oxidization, desulphurization and soluble compounds formation velocities decrease.

Zinc sulphide concentrate which is a raw material in zinc production and which undergoes roasting on the purpose mainly consists of heavy metals sulphides. All the sulphides are semiconductors. Semiconductors in their turn have positive or negative charges. If we roast a semiconductor with negative charge, in the result we obtain an oxide with positive charges prevailing. In the contact point of semiconductors with negative and positive charges potential difference occur and double electrical layer that increases diffusion resistance in roasting appears.

Calculation of Alternating Double Electrical Layer Value Depending on Impurities Amount

Firstly, the conductivities of concentrates and roasted products are determined. After that, we calculate the Hall remainder of potential differences.

Having got the Hall potential differences value one can calculate the Hall coefficients for each product.

V_H= RjB₀d (1)

Modifying the formula we get:

R= V_H/(jB₀d) (2)

Where B₀= B_M*χ/ε - magnetic induction in a semiconductor sample, gauss;

B_M – magnet magnetic induction, 800 gauss;

j – current density in a sample, а/sm²;

d – sample width, sm;

V_H – Hall potential difference, volt;

R – Hall coeficient;

ε – relative magnetic permittivity (for sphalerite -7.9; for zinc oxide – 8.5);

χ - magnetic susceptibility (for sphalerite 0.7*10^-6; for zinc oxide – 0.33*10^-6 см²/г);

If we know Hall coefficient, we can determine the concentration and charge sign in the semiconductor.

Hall coefficient for electrons:

R = -1/(e*n) (3).

Hall coefficient for electron vacancies:

R = 1/(e*p) (4)

where e – electron charge, 1,602*10^-19 К;

n – electrons concentration in a semiconductor, 1/sm³;

p - electron vacancies concentration in a semiconductor, 1/см³.

Thus, electrons concentration in a semiconductor equals:

n = -1/(e*R) (5).

And electron vacancies concentration in a semiconductor equals:

p = 1/(e*R) (6).

The obtained calculation results are consolidated in Table 2.5.

We calculate the contact potential difference value according to the formula:

eU_K= k₀Tln(n/p) (7).

When metal phases occur, semiconductors become ligated. In this case contact potential difference can reach the maximum value.

eU_K = ∆E_g (8).

Thus, when metal phases occur, contact potential difference increases up to the forbidden band.

We calculate the forbidden band value of concentrates and roasted products.

(9)

where k – Boltzmann constant, 0,86*10^-4 ev/K.

Modifying the equation (9) we get the following one for concentrates:

∆E_g = (ln C – ln n)/(2kT) (10),

and for electron vacancies:

∆E_g = (ln C – ln p)/(2kT) (11).

To calculate the forbidden band we determine С coefficient:

(12).

The obtained results are in the tables below:

Table 10 - С coefficient

Temperature, К

298

373

473

573

673

773

873

С coefficient

2.4E+37

3.3E+37

4.7E+37

6.3E+37

8.0E+37

9.8E+37

1.2E+38

Temperature, К

973

1073

1173

1273

1373

1473

1573

С coefficient

1.4E+38

1.6E+38

1.8E+38

2.1E+38

2.3E+38

2.6E+38

2.8E+38

Cupric, ferric, lead, zinc sulphides as well as their oxides are semiconductors.

In the result of the research pyrite, copper pyrite and galena in low grade zinc sulphide concentrates appeared to be semiconductors with electron conductivity prevailing while their oxides after thermal treatment and oxidization appeared to be semiconductors with electron vacancies conductivity.

So, in the contact point of semiconductors with electron and electron vacancy conductivities contact potential difference occur. Metal phases, in their turn, increase contact potential difference value as they are conductors with electron conductivity.

Product title	Conductivity, Ohm^-1sm^-1	Hall coeficient	Charge carriers concentration, 1/sm³	Contact potential difference, eV	Forbidden band value, eV	Current density , amp/sm²	Hall potential difference, В
Concentrate № 1	4.74*10^-9	-3.39*10⁸	1.84*10¹⁰	0.07	3.2	4.74*10^-5	-1.125
Concentrate № 2	7.48*10^-9	-4.81*10⁷	1.297*10¹¹	0.12	3.1	7.48*10^-5	-0.252
Concentrate № 3	4.64*10^-8	-1.437*10⁴	4.344*10¹⁴	0.33	2.7	4.64*10^-4	-4.667*10^-4
Concentrate № 4	2.88*10^-7	-3.74	1.667*10¹⁸	0.54	2.3	2.88*10^-3	-7.54*10^-7
Concentrate № 5	1.14*10^-6	-1.16*10^-2	5.37*10²⁰	0.69	2.0	1.14*10^-2	-9.257*10^-9
Concentrate № 6	7.05*10^-6	-4.65*10^-6	1.342*10²⁴	0.89	1.6	7.05*10^-2	-2.295*10^-11
Concentrate № 7	2.77*10^-5	-2.696*10^-8	2.315*10²⁶	1.02	1.3	2.77*10^-1	-5.228*10^-13