MODEL OF FORMATION OF DUST AND GASES IN THE EXPLOSION CHAMBER OF THE BLASTHOLE CHARGE IN SULPHUR-CONTAINING ORE

Vladimir Ivanovich Chernobai ^* And Dmitriy Vladimirovich Moldovan

Saint-Petersburg Mining University, 199106, Saint Petersburg, Vasilyevsky Island, Line 21, 2, Russia

*Corresponding Author:

Vladimir Ivanovich Chernobai
Saint-Petersburg Mining University, 199106, Saint Petersburg, Vasilyevsky Island, Line 21, 2, Russia
E-mail: chernobay_vi@mail.ru

Received Date: 06 April, 2017; Accepted Date: 08 April, 2017

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Abstract

The ragging processes of sulphide ore and its oxidation in the explosion chamber during the detonating of an explosive charge were considered. The theoretical model of the processes of formation of dust and gases that take place in the explosion chamber was built taking into account the regularities of chemical and phase equilibriums at high temperatures according to the universal thermodynamic method of determination of equilibrium characteristics of random heterogeneous systems based upon the fundamental principle of entropy maximum. The model was based upon the system of famous laws and equations: V.L. Kirpichev’s law, logarithmic normal distribution, the equation of energy balance, diffusion law, and the principle of entropy maximum. The results of the calculations of parameters of gas formation and inhibition were obtained. Much attention was paid to the distinguishing of the regularities of formation of sulphur dioxide gas at oxidation of the finely divided sulphide dust. The efficient inhibitive additives were determined. Jr. of Industrial Pollution Control 33(1)(2017) pp 804-808 www.icontrolpollution.com Review *Corresponding

Key words

Explosion chamber, Ragging model, Explosive dust, Heterogeneous transformations, Gas formation, Inhibition, Sulphur dioxide gas

Significance

The intensity of the dust formation and granulometric distribution of the finely-divided fractions at the explosion of an explosive charge are the main factors determining the degree of danger for working conditions and environment pollution as well. The most widely spread professional disease connected to the regular presence of a worker at the dusted working place is silicosis. And if the working place is in the underground digging of mining of the sulphur-containing ores the working conditions are worsened by the possibility of inflammation and explosion of the finely divided fractions of the explosive dust and this influence more destructively the ecological situation and decreases the life time of various equipment. Thus, the solution of the process problems of dust suppression to prevent the inflammation, combustion and explosion of the finely divided dust is still critical.

Objective

To create a physical and mathematical model of the sulphide dust ragging within the front motion of the explosive chamber. To describe the heterogeneous processes of gas formation and inhibition under pressure of the high temperature detonation products on the finely divided sulphur-containing dust. To perform the necessary calculations of this model and to reveal the key regularities.

Model

The following initial conditions and assumptions were chosen for formation of the model that are close to the real characteristics of the process of the explosive destruction of the charge in the blasthole of the known (or chosen) diameter (for example, Ø=36 mm) in the sulphur containing ores that are dangerous because of the gas and dust:

• Formation of the main amount of SO2 is the result of the combustion of sulphide dust of the explosive dispersion in the zone of overmilling at the distance of up to 3 radiuses of the charge from the axis of the extended charge (Menzhulin and Paramonov, 1998);

• The most explosive is dust of minerals-pyrite and marcasite that contain more than 50% of sulphur. All sulphide ores containing 40% of sulphur and more are conditionally referred to the explosive ones;

• The dust with fineness more than 250 microns actually becomes inexplosive;

• When the concentration of the atomic oxygen in the air is less than 8% to 9% there are no explosions of the sulphide dust of the finely divided fractions;

• The minimal explosive concentration of the sulphide dust in the air is 80 g/m³ to 100 g/m³.

• For the calculation scheme of the model, the following regularities are accepted.

• The energy spent for the ragging of rock in the considered volume is determined according to V.L. Kirpichev’s law:

(1)

where is an average size of the piece before the destruction (we accept it according to the set blocky structure according to the geological data); is an average size of the piece after destruction; σ is a strength characteristic of the rock; E is Young’s modulus; V is destructed volume.

An average size of piece after destruction is determined according to the lognormal distribution:

(2)

where the parameters of distribution are the logarithm of the average piece ln and a rootmean- square deviation of the logarithms of sizes β. The value is accepted from the condition of coincidence of the experimental law and Poisson formula. Thus, the logarithm of the average piece ln is determined from the equation:

(3)

where N=N_i is a number of the considered fraction. The number of pieces of the i-th fraction with an average diameter of the piece for the current considered layer is:

(4)

where P_i is a mass of all pieces of the i-th fraction. The equation of energy balance, used for the formation of a new free surface of the unit volume of rock, is:

(5)

where η is a efficiency coefficient (ragging) of the explosion; q is a specific consumption of explosives, kg/m³; Q is the energy potential of explosives (explosion heat), kJ/kg; A is the energy content of ragging, kJ/m²; S is the newly formed specific (in volume units) square of particles surface equal to the difference between the final total square of all pieces and initial surface,

The initial surface is determined as the side surface of the charge cylindrical chamber per 1 running meter (l=1 m) of the blasthole (well):

(6)

The final surface is determined as a sum of all small surfaces of the finely divided dust formed during the process of the explosive ragging in the volume of the explosive cylindrical chamber per 1 running meter (l=1 m) of the blasthole (well):

(7)

Where is an average size of the piece typical for the p-th destroyed cylindrical layer, for example, the external zone of the movable explosion chamber; is an average diameter of the piece typical for the smallest destroyed layer adjacent to the charging wall at the initial moment.

The impulse of explosion I is determined via an integral of the pressure function on the front of the denotation wave from time p(t), during which the denotation products impact (τ) on the massive of rocks takes place:

(8)

Besides, when the charge length is l, the impulse of charge is also connected to the explosive detonation speed according to the dependence:

(9)

Where ρ_D is the density of the detonation products; D is the detonation speed; R0 is a radius of charge; l is the length of charge.

The radius of the maximal enlargement of the cavity can be estimated on the base of the approximate dependence (Baum, et al., 1975):

(10)

Where σ₀ is a stress on the wave front, generated during the charge explosion; σ_s is the compressive strength of rock. Thus, the maximal radius of destruction r_k, used in the equation (7) is accepted as r_k=R_max.

The destroyed parts of rock in the charge chamber are subjected to the intensive heating and oxidation during the closing time. An average integral temperature along the thickness of every finely divided particle of rock is determined by the following (Menzhulin and Paramonov 1998):

(11)

Where R is a radius of the finely divided dust determined in the ragging zone by the distribution law (2) and equal to a half of the diameter of the average piece in the equation (3)

The combustion process of the finely divided particle takes place on the border of the phase division; therefore, we consider the dissociation of dust in the explosion chamber as a heterogeneous reaction with the corresponding regularities.

The diffusion law is a flow of substance coming into the reaction zone that is proportional to the difference of the concentration of reacting substance in the volume and the zone of reaction (Kaganovich, et. al. 2007):

(12)

where P is a flow of substance, kmol/m²sec; β is a coefficient of substance diffusion or coefficient of mass transfer, s^-1m; C₀, C_x are initial and final concentrations of the reacting substance in the volume and zone of reaction.

The speed of chemical reaction u in the zone of the chemical reaction and interrelation of concentrations is determined from the main idea of the chemical kinetics:

(13)

Where 1/k is a chemical resistance of the chemical reaction. The formal scheme of a heterogeneous reaction is:

(14)

Where (x^*–x) is a number of active centres that can absorb the oxygen. If by the moment of time t, x moles are fixed on the surface, the number of active centres is (x^* – x).

A general view of the main reaction of sulphide oxidation is:

(15)

The principle of entropy maximum S in the conditions of equilibrium of the random heterogeneous systems is:

(16)

Where is a number of moles of the i -th component.

Problem Setting

Mixture: the detonation products + inhibition additive + sulphur-containing dust + air. The mixture is in the explosion chamber (formula (10)), length l = 1m.

It is necessary to determine the thermodynamic parameters in the explosion chamber, the chemical composition and the percentage of the heterogeneous reaction of all substances that were obtained as a result (in the gaseous and solid form).

The additional data necessary for solution. The mass of air in the set volume at the normal conditions is:

where ρ is the air density at normal conditions; ρ=1.205 (g/l)=1.205 (kg/m³); V is the volume under study equal to: where 0.018 m is a radius of the blasthole.

The sulphur-containing dust CuFeS₂ of the explosive dispersion is up to 250 microns.

The simplified air content is: N_53.91O_14.48Ar_0.3204 (nitrogen-75.5%, oxygen-23.1%, argon-1.4%).

The conditions of thermodynamic heterogeneous reactions correspond to the temperature and pressure typical for the condition of the denotation products in the explosion chamber, for example, at the explosion of Granulite AC-8:

The initial pressure of the studied state of mixture: 800 MPa.

The initial temperature of the studied state of mixture: 3300°K.

Separately studied additives in 1 kg of explosive: Li₂CO₃, CaCO₃, CaMgC₂O₆, K₂CO₃, (NH₂)₂CO, Na₂CO₃ should be reasonably used according to the content 5, 10, 15, 20% of the mass of explosive to maintain the efficient operating capacity of the explosive charge. We consider that during the detonation process the additive decomposed completely and was ready for the further reactions.

The Results Of Calculations Of The Developed Model And Definition Of Regularities

Most The mass of dust formed in the zone of overmilling was approximately 1 kg per a running meter of the extended charge of Granulite AC-8. 40% of the total mass of the most sulphide ores contain sulphur. About 50% of the sulphur containing dust have explosive mass (dispersion is not more than 250 microns). As a result, the mass of the studied sulphide dust in the considered volume was 0.2 kg.

The smaller part of the combustion results of the finely divided dust in the thermodynamic conditions of the brisant zone of explosion of the blasthole charge (initial pressure 800 MPa, inflammation temperature of 3300°K) with the further spontaneous isoentropy enlargement up to the termination of the gas formation is shown in Tables 1 and 2.

Table 1: Mass fractions of the products of combustion reaction of the sulphide dust at the explosion decomposition of the charge of Granulite AC-8 without additives

Table 2: Mass fractions of products of combustion reaction of the sulphide dust at charge decomposition of Granulite AC-8 with additives up to 10%

The best inhibition properties were demonstrated by the additive CaCO₃, the dynamics of its gas suppressing capacity in the explosion chamber in comparison with the dolomite powder is shown in the graphic (Figure 1).

The regularities of the inhibition activity of the dolomite and chalk additives in the explosion chamber with isoentropy change of temperature were obtained by the method of polynomial approximation:

(explosive without additive)

(explosive contains 5% dolomite)

(explosive contains 10% dolomite)

(explosive contains 5% chalk)

(explosive contains 10% chalk)

Discussion

Within the range of 10% in the explosive content, the studied additives do not influence significantly the denotational capacities of the charge.

All studied additives demonstrated Li₂CO₃, the worst result was shown by carbamide ((NH₂)₂CO) using which revealed he significant increase of the output of the hydrogen sulphide that is more dangerous than SO₂.

Almost the same inhibition ability to decrease the emission of the sulphur dioxide gas was shown by the additives of chalk and dolomite powder. Due to their availability, they are the most competitive in the technical and economic sense.

Conclusion

The obtained model and theoretical regularities characterize the studied process of inhibition of the sulphide dust and satisfy the solution of the set problems of the research. The performed calculation demonstrated the reasonability of the industrial implementation of the efficient inhibiting additives (dolomite powder and chalk) as a preventive means of coping with the sulphur dioxide gas in the explosion chamber. The introduction of the recommended additives to the explosives to inhibit the oxidation reactions with the sulphur in the explosion chamber is a preventive measure directed to the elimination of the reason of the dangerous dust and gas emissions.

References

1. Baum, R.A., Orlenko, L.P. and Stanyukovich, K.P. 1975. Physics of Explosion. Moscow: Nauka.
2. Kaganovich, B.M., Keyko, A.V. and Shamansky, V.A. 2007. Equilibrium thermodynamic simulation of dissipative macroscopic systems. Irkutsk: MESI SB RAS. 76.
3. Menzhulin, M.G. and Paramonov, G.P. 1998. Model of explosive destruction of rock and formation of dust fraction on its base. Gornyizhurnal. 10 : 23-25.