Environmental and Economic Advantages of Disposal of Phosphoric Industry Waste


 The article presents the types and classification of waste from the phosphorus industry of the Zhambyl region of the Republic of Kazakhstan. Waste is classified by its use as recyclable materials for construction materials. The results of a comparative assessment of the physical, chemical and structural properties of the phosphorus industry waste are presented. The article shows that all studied types of waste have astringent properties and can be used as building materials. In this work, a study of the properties of large-tonnage wastes of the phosphorus industry was carried out: 1) electrothermophosphoric granular slag (granulated slag); 2) phosphogypsum; 3) overburden. A technology has been developed for producing non-fired binders from waste of the phosphorus industry and a methodology for designing the composition of raw mixtures of multicomponent building composites has been proposed.
 Pilot tests and calculation of technical and economic indicators have been carried out, which have shown the economic feasibility of producing a non-firing binder for the construction industry from phosphorus production waste.


INTRODUCTION
World experience shows that construction is one of the most material-intensive industries that require an expansion of the range of binders and building mixtures used, the main components of which can be a variety of industrial waste products. To solve the above problems, it is preferable to use local man-made materials -waste of phosphorus production, such as phosphoric slag, phosphogypsum, as well as overburden from phosphorite mining [1]- [8].
In the development of industrial production, one of the leading places is occupied by the problem of environmental protection and rational use of raw materials. These problems are especially acute at phosphorus production enterprises [9].
The Republic of Kazakhstan has huge reserves of phosphorite ores, concentrated mainly in the bowels of the Karatau basin, located in Zhambyl and partly in South Kazakhstan regions. Here, up to 50 phosphorite deposits have been identified with recorded balance reserves in the amount of 5 billion tonnes of ore and about 1.2 billion tonnes of phosphorus pentoxide (P2O5). Therefore, in recent years, the phosphorus industry has been one of the most promising and financially stable sectors of the chemical complex of the Republic of Kazakhstan. But this development of the industry has negative features in the form of the generated large-tonnage waste of the phosphorus industry.
Currently, more than 40 million tonnes of this waste have been accumulated in the Zhambyl region, which occupy vast areas and have a negative impact on environmental components and affect the ecological and economic state of society. The volumes of technogenic waste in the dumps are: phosphogypsum (stale, dumps of the plant LLP 'Kazphosphate' Mineral fertilizers -MU) ~8.5 million tonnes; granulated phosphoric slag (Zhambyl branch of Kazphosphate LLP, Novodzhambul phosphoric plant -NDFZ) ~10-12 million tonnes; overburden (deposits of phosphorite ores of Karatau) ~20 million tonnes.
To solve the problems associated with the disposal of accumulated and newly formed wastes of the phosphorus industry, a preliminary analysis was carried out for the main types of target products of the phosphorus industry (phosphorus-containing fertilizers, phosphoric acid, yellow phosphorus) associated with the formation of waste. The results of systematization of industrial waste in the Zhambyl region in terms of accumulation volumes made it possible to identify the emerging waste market and the possible volumes of their involvement in economic circulation as raw materials in the production of construction materials.
The importance of using industrial waste is determined by the following factors: solving the problem of environmental protection; release of land plots occupied by dumps; improvement of the ecological situation and public health.
The use of industrial waste in the construction industry will reduce the needs of this industry for natural raw materials and provide production with a source of cheap and already prepared raw materials.
The possibility of using man-made waste in the production of building materials is considered on the basis of a comprehensive study of their composition, physicochemical, mineralogical and toxicological properties [10].
The purpose of this study is to implement scientific and technical solutions for the disposal of industrial solid waste in the production of building materials.

METHODS AND METHODOLOGY
A comprehensive study of their composition, physicochemical, mineralogical and toxicological properties is considered for the development of a technology for the complex processing and use of industrial waste in the production of building materials.
The chemical composition of waste is determined using standard methods of physicochemical and chemical quantitative analysis in accordance with state standard 20851.2-75; state standard 20851.3-93; state standard 20851.4-75 [11], [13].
X-ray phase analysis of phosphogypsum is carried out on an automated DRON-4 diffractometer based on the diffractograms of powder samples using the method of equal portions and artificial mixtures. Interpretation of diffraction patterns is carried out using data from the ASTM Powder diffraction file and diffraction patterns of minerals clean from impurities (gypsum, quartz, calcium phosphates) [15].
The study of the specific effective activity of natural radionuclides in samples of industrial waste is carried out according to the method of measuring the activity of radionuclides using a Progress BG scintillation gamma spectrometer. The measurement results are compared with the level of effective specific activity established by the Sanitary Rules.

Investigation of the physicochemical, mineralogical and toxicological properties of the phosphorus industry waste in the Zhambyl region
Waste from the production of phosphorus and phosphoric acid (phosphoric slags and phosphogypsum) is a large-tonnage waste of the chemical industrial complex.
When mining phosphorus ores, huge masses of overburden are also formed, which are limestones, dolomites, sands, clays, shales with admixtures of sulphur and phosphorus, which enter the overburden dumps and are currently practically not used.
Based on the analysis of the experimental results, it was shown that the main wastes of phosphorus production (granulated slag and phosphogypsum) have an almost constant composition regardless of the place and time of sampling at the dumps.
In all the wastes selected for the study, the main components are calcium and silicon oxides, in some wastes there are aluminium and iron oxides, which play an important role in the technology of producing building mixtures (Portland cement, alumina cement, glass, fine ceramics, etc.).
The results of X-ray phase analysis showed that these wastes are mainly represented by minerals containing calcium and magnesium oxides, which will make it possible to use the waste of the phosphorus industry in the production of building materials and mixtures.
When choosing wastes from the phosphorus industry as a raw material for the production of building materials, their compliance with the standards for the content of radionuclides was investigated [16]. The sanitary-epidemiological conclusion confirmed the possibility of using this waste as a mineral raw material for all types of building materials without restrictions, since the total specific activity of radionuclides for each type of waste did not exceed 370 Bq/kg, which meets the requirements of the Sanitary Rules and Regulations.

Development of a design methodology for the composition of raw mixes of building materials
The work carried out research on the mathematical modelling of the compositions of new building composites containing such raw materials as phosphogypsum dihydrate, granular phosphorus slag, limestone, phosphate-clay shale, and phosphate-siliceous shale [17], [18]. The presence of oxides CaO, SiO2, Al2O3 in a certain ratio determines the hydration properties of raw materials that can be used as binding mixtures.
To determine the optimal composition of the basic oxides that make up the binders, it was proposed to use the diagram of the CaO -SiO2 -Al2O3 system (Rankin diagram) as a technical model. The CaO -SiO2 -Al2O3 system plays an important role in the technology for the production of Portland cement, alumina cement, fireclay and high-alumina refractories, glass, fine ceramics, since the diagram gives an idea of the quantitative composition of CaO, SiO2, Al2O3 oxides, which have a major effect on the hydration and hardening processes in building binders.
The design methodology is that the initial composition of the main four oxides CaO, SiO2, Al2O3, Fe2O3, which are part of the components of the designed composites, must be reduced to the composition of the oxides CaO, SiO2, Al2O3, Fe2O3 of Portland cement, as a binder with high physical and mechanical characteristics. The basis also includes iron oxide Fe2O3, which forms calcium hydroferrites in the process of hydration, increasing the density and strength of the resulting binders. Other oxides besides the mentioned four oxides are not considered, since they have the least effect on the physical and mechanical characteristics of the resulting binder and hydration processes [19]. Table 1 presents the results of designing compositions of three new building composites: composite 1: phosphate-clay shale -phosphogypsum -limestone.
As an activator, Portland cement was introduced into the composite in an amount of 5 % of the total mass of the mixture, which today of all binder materials has the best physical, mechanical and technical characteristics. The results of the calculation showed that for this composite, the quantitative composition of the oxides of the raw components (wt. %) is closest to the quantitative composition of the oxides of Portland cement. Analysis of Table 1 showed that the chemical composition of the composite mixture from the waste of the phosphorus industry of the Zhambyl region is as close as possible to the chemical composition of the standard: Portland cement in the CaO, SiO2, Al2O3, Fe2O3 system. Moreover, the program selected such a composition of the components of the projected binder, in which the content of oxides CaO, SiO2, Fe2O3 is 100 % identical to the standard. In general, the composition of the chemical oxides of the raw mixture is close by 95.3 % to the chemical composition of the main oxides of Portland cement.
Based on the decision of the program (table 1), the composition of raw materials for the projected binder was determined in the following proportions: phosphate-clay shale -7.11 %; phosphogypsum -50.49 %; limestone -37.71 %.

Experimental-industrial tests for obtaining building mixtures and study of their physical and mechanical properties
The process of obtaining non-firing mineral binders was carried out according to the developed technology. During the tests, a building composite was obtained in the form of a moulding plaster test, which was poured into special moulds 40x40x160 mm in size to obtain beam samples (Fig. 1). After the beams hardened (after 28 days), studies were carried out on the physical and mechanical characteristics of non-fired binders obtained from the waste of phosphorus production.

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With the obtained non-fired binders, which are crushed samples of beams, a number of tests were carried out according to the methods of state standard 23789-79 'Gypsum binders' [20] and the grade of gypsum binder was determined according to state standard 125-79 [21].
The value of the ratio of gypsum: water (G:W) = 1:0.065. The preparation of the gypsum dough of standard consistency at the above values of the ratio G:W and the determination of the setting time was carried out in accordance with state standard 23789-79 'Plaster binders' clause 4.4-6.
On the Vika device, experiments were carried out to determine the time of the beginning and end of the setting of a plaster dough of standard consistency.
The obtained values of the time of the beginning and the end of the setting of the gypsum dough obtained from non-fired binders (composites 1) were compared with the requirements of state standard 125-79 (Table 2). Tests of binder samples to determine the compressive and flexural strength were carried out on standard samples of beams 40×40×160 mm in accordance with state standard 23789-79 'Gypsum binders' (Table 3). Thus, a sample of a non-firing binder obtained from phosphorus production waste should be classified as a slowly hardening gypsum binder, and the strength of standard beams in bending and compression corresponds to the G-4 gypsum binder [22].

Technical and economic efficiency of obtaining non-fired binders from waste of phosphorus production.
This section presents the comparative performance indicators of two sites for the production of binders from various raw materials: site No. 1 from natural gypsum raw materials and site No. 2 from phosphorus industry waste.
The main performance indicators of conditionally operating industries (No. 1 and No. 2) and operating costs are presented in Table 4 and Table 5.  Table 6.
Calculation of the production cost of 1 tonne of non-fired binder produced at site No. 2 from the waste of phosphorus production: The production cost is calculated as the sum of the unit costs in monetary terms for the purchase and transportation of raw materials, the cost of energy resources when processing raw materials, as well as salary costs.
When summing up the above listed costs, we get the average profit -the commercial cost (Сcom) of the product (non-fired binder from phosphorus production waste) equal to 14000-2352.9 = 11 647 tenge/tonne. For comparison, the market value of natural gypsum is 14 000 tenge/tonne.  The production cost and the forecast of income and expenses for the year are presented in Table 7 and Table 8.

Name Unit measurement Sales of binder
Due to systematic fluctuations in prices and rates for services, the calculations presented in this section cannot be considered final. However, they can serve as a basis for the development of an algorithm for calculating the production cost of a non-fired binder from the waste of the phosphorus industry.  Thus, the performed technical and economic calculations have shown the economic feasibility of producing a non-fired binder for the construction industry from the waste of phosphorus production.

Environmental and economic assessment of environmental protection measures for the disposal of phosphorus production waste
The assessment of the economic efficiency of environmental protection measures is determined by comparing environmental costs with the amount of prevented environmental damage.
The work calculates the costs of environmental protection measures associated with the use of waste from the phosphorus industry as raw materials for the production of building materials.
The introduction of this measure will significantly reduce the volume of waste disposed of on dumps and having a negative impact on environmental components due to dusting and blowing off pollutants from the surface of the dumps. Phosphogypsum, which is a finely dispersed material, is especially dangerous in this regard.
Dusting from the dump increases under unfavourable weather conditions, especially when the wind speed increases to 10-15 m/s, which is typical for almost the entire territory of South Kazakhstan.
To calculate the effectiveness of environmental protection measures, the following main economic indicators are determined: one-time costs associated with the implementation of a technical measure (capital investments), changes in current operating costs.
Calculation of prevented environmental damage: For calculations, we take the dusting of phosphogypsum from the dump of the mineral fertilizers plant Kazphosphate LLP ( Table 9).
The calculation of the amount of harmful substances emitted during dusting is determined by Eq. (1): where K a dimensionless coefficient that considers the gravitational settling of harmful substances (for solid particles it is taken equal to 0.16; for gases -1.0); в specific blowing off of dust from the surface of open dumps (mg/m 2 s) at wind speed (m/s); A the amount of pollutants escaping from the emission source, t/year; ƞ ′ coefficient of efficiency of the means of dust and gas suppression, dol. units, which was taken equal to 0.6. To calculate the prevented damage according to Table 9, we select open dumps with a dusty surface at an average wind speed of 6-8 m/s, then the specific dust blowing will be equal to 25 mg/m 2 s. According to Table 10, we determine the amount of pollutants A escaping from the source of emission -we select inorganic dust (111.7605 t/year -the permitted regulatory emission). The amount of inorganic dust blown off from the surface of the phosphogypsum dump is calculated according to Eq.
Payment for emissions into the environment carried out by users of natural resources within the limits of the standards specified in the environmental permit is charged according to the list of pollutants and waste types approved by the Government of the Republic of Kazakhstan.
The amount of prevented damage from the emission of pollutants into the atmosphere is determined by Eq. (3): where Р rate of payment for emissions of pollutants into the atmosphere, MCI/tonne; K1 multiplicity factor equal to 10; K2 multiplicity factor, taking into account the environmental hazard of pollution; Mave is the averted mass of the emission of the i-th pollutant as a result of the implementation of an environmental measure, equal to the difference in the mass of emissions before and after the implementation of the environmental measure. The results of calculating the prevented damage are presented in Table 11. Calculation of the indicator of the absolute economic efficiency of environmental solutions. Considering the magnitude of the prevented economic damage, it is possible to determine the absolute efficiency of capital investments in the implementation of environmental protection measures -utilization of waste from the phosphorus industry.
The absolute efficiency of capital investments in environmental protection measures is determined by Eq. (4): where Ep indicator of the overall economic efficiency of capital investments in environmental protection measures; Эij result (effect) of nature protection measures of the i-th type at the j-th object. When solving a single-purpose task to prevent or reduce the negative impact of the object on the natural sphere, Eij is equal to the value of the annual prevented damage to the environment (∆Уij,), which amounted to 10 583.75 thousand tenge/year, considering the impact of only one of the wastes of the phosphorus industry -phosphogypsum; С annual operating costs for maintenance of fixed assets of environmental protection, i.e. the cost of production of building materials from phosphorus waste at site No. 2 is 20 555.28 thousand tenge/year; K capital expenditures for environmental protection measures were taken on the basis of the construction of similar production sites and were estimated at 600 000 thousand tenge. When solving a multipurpose task in the process of implementing environmental measures based on the use of waste as a secondary raw material in the production of building materials, an increase in income Eij can be obtained, which will be equal to the sum of ∆Уij and ∆D. To assess the feasibility of introducing measures, it is proposed to compare Еp with the standard coefficient of the efficiency of capital investments -Еn, which for environmental protection measures is taken equal to 0.12. If Еp is greater than or equal to Еn, the environmental measure is recognized as cost-effective: Ep = (103 962.99 -20 555.28) / 600 000.0 = 0.14 Since Ep is greater than 0.12, therefore, the use of phosphorus production waste as a raw material for the production of building materials is cost-effective (Table 12). Absolute economic efficiency ratio, Ер 0.14 Thus, the introduction of environmental protection measures will significantly improve the environmental situation in the dumping area at the enterprises of Kazphosphate LLP, in particular, by reducing the area of dusty surfaces [10].

CONCLUSIONS
Summary of the results: − Carried out systematization of wastes from the phosphorus industry in the Zhambyl region of the Republic of Kazakhstan in terms of possible areas of their use, as well as a comprehensive study of their composition, physicochemical, mineralogical and toxicological properties, which made it possible to establish the expediency of using these wastes in the production of building materials. − A systematic approach to designing the composition of a multicomponent mineral binder made it possible to obtain composite materials of complex structure (composites 1), consisting of mineral materials (waste) with different properties and acquiring a complex of new properties as a result of their combination. − Investigation of the physical, mechanical and strength characteristics of the phosphogypsum binder obtained from the waste of phosphorus production, showed that the samples of the non-fired binder can be classified as slowly hardening gypsum binder. The results of strength tests (for compression and bending) of standard beams made of binder showed their compliance with the requirements of state standard for gypsum binder grade G-4. − Calculation of technical and economic indicators for obtaining non-fired binders from phosphorus production wastes has been carried out. The calculation results showed that the production of a non-fired binder for the construction industry from the waste of phosphorus production is economically feasible. − An ecological and economic assessment of environmental protection measures for the disposal of phosphorus production wastes was carried out by comparing environmental costs with the amount of prevented damage to the environment. It is shown that the introduction of environmental protection measures is cost-effective, since it will significantly improve the environmental situation in the area of dumps at the enterprises of Kazphosphate LLP, in particular, by reducing the area of dusty dump surfaces.

ACKNOWLEDGMENT
The main sections of the work were carried out within the framework of a scientific project 'Disposal of waste from the phosphorus industry to obtain multipurpose products for the construction industry' (state registration number: 0115RK01932) approved by the State Institution 'Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan'.