**1. Introduction**

The growth of the world's population more than twofold, from 3 billion people in 1960 to 7.7 billion people at present, has entailed a forced increase in agricultural production [1,2]. This has led to increased consumption of mineral fertilizers [3–5]. The growing demand for mineral fertilizers has required an increase in the production capacity of enterprises producing potash fertilizers [6]. Increasing the production of this type of fertilizer requires

**Citation:** Ermolovich, E.A.; Ivannikov, A.L.; Khayrutdinov, M.M.; Kongar-Syuryun, C.B.; Tyulyaeva, Y.S. Creation of a Nanomodified Backfill Based on the Waste from Enrichment of Water-Soluble Ores. *Materials* **2022**, *15*, 3689. https://doi.org/10.3390/ ma15103689

Academic Editors: Krzysztof Schabowicz and Saeed Chehreh Chelgani

Received: 20 March 2022 Accepted: 17 May 2022 Published: 21 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

an increase in the extraction of potash salts at mining enterprises, which involves a larger amount of reserves in the development.

The increased consumption of potash fertilizers implies the intensification and growth of potash salt extraction, which in turn predetermines the high rates of geotechnology development in general. All of this is accompanied by industrial and environmental risks and induced disasters.

One of the world's largest deposits of potassium–magnesium salts is located in the Russian Federation, in the Perm Territory (59◦35 36 N 56◦48 36 E) (Figure 1).

The Verkhnekamsk potassium–magnesium salt deposit is the main component of the Solikamsk potassium-bearing basin, located in the left-bank part of the Kama river valley. In the north, this deposit is limited by Lake Nyukhti, located in the Krasnovishersk region; in the south, it extends to the Yayva river basin. The length of the explored part of the deposit from north to south is 140 km, and from west to east, about 60 km. The thickness of the ore-bearing strata is about 80 m, and its depth is 400 m. Potash horizons are represented by alternating red layered sylvinites with rock salt interlayers. The thickness of individual potash strata ranges from 0.75 to 5 m.

The salt stratum with a total thickness of up to 550 m is subdivided (from bottom to top) into underlying rock salt (URS-P1br2), potash deposits (P1br3) consisting of sylvinite (SZ) and carnallite (CZ) zones and mantle rock salt (MRS-P1br4) (Figure 2).

All of the main reserves of the Verkhnekamsk potassium–magnesium salt deposit are located on the left bank of the Kama River. There is a small area on the right bank. The total area of the basin is more than 6.5 thousand square kilometers.

The Verkhnekamsk deposit was discovered in 1925, and development has been carried out by the underground method since 1934. Development centers are concentrated in the area of Solikamsk and Berezniki cities (Figure 3). At present, stope and pillar mining is used for the Verkhnekamsk deposit development.

On the basis of the above, creation of nanomodified backfill based on the tailings from enrichment of water-soluble ores, that allows replacing the traditional technology of water-soluble ore mining with a safer one and obtaining an environmental and economic effect, seems to be a very urgent task.

Stope and pillar mining are characterized by high mineral losses. This technology is most often used in the development of water-soluble ores with low value. Extraction of water-soluble ores is characterized not only by high losses (up to 65%) [7] of minerals left in pillars, but also by the formation of a large amount of waste generated during the extraction and processing of water-soluble ores. The volume of the generated waste is 60–70% of the total volume of the extracted ore mass [7].

**Figure 2.** Stratigraphic section of the halogen formation of the Solikamsk depression.

**Figure 3.** Location of the Verkhnekamsk deposit in the Perm Territory.

The plasticity of natural salt pillars causes deformation changes in them, which leads to their destruction [7]. Destruction of pillars causes deformation disturbances of the overlying rock mass [8]. In some cases, the propagation of these deformation disturbances reaches the daylight surface. This leads to the formation of sinkholes and disruption of the waterproof stratum of the aquifer [9]. The violation of the waterproof stratum leads to the breakthrough of water into the mine, to its flooding and loss of reserves. Due to the destruction of rib and barrier pillars at the Verkhnekamsk deposit, deformation disturbances developed in the underworked mass and caused a breakthrough of the aquifer. As a result, two mines of the Verkhnekamsk deposit were lost. Consequently, the use of development systems that exclude or minimize the likelihood of disturbing the waterproof stratum is one of the main tasks in the development of water-soluble ore deposits.

The incessant induced impact, caused by drilling and blasting [10] and extensive exposed surfaces, causes seismic activity in the mining regions [11]. Vibrations and induced earthquakes of up to magnitude 5 are recorded at Russian and foreign mines developing deposits of water-soluble ores [12–15].

The use of development systems with artificial support reduces the likelihood of disasters and improves the qualitative and quantitative indicators of extraction. An artificial mass based on waste, while maintaining its main purpose of supporting the stoping space, allows minimizing the impact of mining enterprises on the environment [16].

Due to the limited ability of the biosphere for self-regulation and self-reproduction, it is necessary to create gentle technologies that minimize the impact of the enterprise on the environment and maintain the ecological balance [17].

#### **2. Materials and Methods**

Geotechnology with artificial support is impossible without the selection of backfill components that satisfy economic, technological and technical conditions [18]. The backfill is a composite material capable of hardening in mining conditions. This material contains aggregate, binder, mixing water and chemical additives.

### *2.1. Backfill*

The characteristics of the future artificial mass largely depend on the properties of the starting materials. Therefore, their correct choice is one of the most important factors in the backfill technology. The material must be highly transportable, which ensures that it will be delivered through pipes over long distances without fear of premature hardening [19]. The material must have high plasticity for the most complete filling of the mined-out void. The setting time should not be less than that required to deliver the material to the stope [20]. This is especially important for materials with a large aggregate, since in this case stratification leads to an uneven distribution of the components in the mined-out void, the heterogeneity of the created artificial mass, and its reduced strength. The components of the backfill must be selected in such a way as to exclude their negative impact on the created artificial mass: loss of strength; warming up; shrinkage; expansion, etc.

#### *2.2. Characteristics of Aggregate for Backfill*

Due to the fact that the aggregate makes up 75–90% of the total volume of the backfill, its quality has a significant effect on the material and the artificial mass characteristics. In this regard, especially high requirements are imposed on the quality of the aggregate. In addition, large volumes of aggregate have a significant impact on the cost of the backfill, the cost of mining operations, and as a result, on the cost of the extracted ore.

Therefore, the main, widely developing direction is the replacement of the traditional, specially mined aggregate with waste from mining and processing industries. These wastes meet the following requirements: they are cheap, have stable physical and mechanical properties and a low-change granulometric composition, and are located near enterprises engaged in the extraction of minerals. With the appropriate preparation technology, these wastes will completely replace the traditional, specially mined aggregate, while maintaining

the necessary characteristics of the created fill mass. Consequently, the use of waste as a replacement for traditional aggregates in the backfill composite has the potential to reduce the total cost of mining operations.

Waste from the enrichment of water-soluble ores is a product with the following properties: hygroscopicity; tendency to caking and clumping and having mainly sodium chloride in its composition. Depending on the enrichment method, the waste of watersoluble ores is divided into flotation and halurgic types. The chemical compositions of wastes differ slightly, but the difference lies in the granulometric composition. The particle size of halite waste of halurgic enrichment is 4.5 times higher.

For research and experiments, halite wastes of halurgic enrichment were used as an aggregate. Saturated salt solutions were used as a grout to avoid aggregate dissolution. The waste humidity was 10–12%. The chemical composition is given in Table 1, and granulometric composition in Table 2.

**Table 1.** Chemical composition of waste from the enrichment of water-soluble ores (halurgic).


**Table 2.** Granulometric composition of waste from the enrichment of water-soluble ores (halurgic).


#### *2.3. Binder Selection*

In previous studies, various binders were used to prepare the hardening backfill: lime [21], cement [22], ash and slag waste from the State District Power Plant and Thermal Power Plant [23], blast-furnace granulated slags [24], and gypsum and calcium chloride additives [25]. In addition, in a number of studies, bischofite [26], caustic magnesite [27], magnesian cement [28], and expanded clay [29] were proposed as starting materials for the backfill material preparation. In early studies, the advantages of magnesia binders in the fill mass formation with an increased amount of salt in its composition were proved [30].

At the same time, the magnesian component of the binder increases the hardening speed and the strength of the created mass in comparison with traditional binders. Furthermore, one of the features of the magnesian binder is its ability to bind large aggregate masses with a minimum amount. In addition, magnesia binders reduce the negative effect of salt on cement. In this study, magnesia cement was used as a binder, which contained 75–85% magnesium oxide (MgO), depending on the grade.

Magnesia cement (TR (technical requirements) 5745-001-92534212-2014) is produced by mixing magnesium oxide pre-calcined to 800 ◦C with a 30% aqueous solution of MgCl2 (two weight parts of MgO per one weight part of anhydrous MgCl2). The main advantages of magnesia cement are fast hardening, high achievable strength, and high adhesion.

One of the largest producers of magnesia binding cements in Russia is the Russian Chromium group of companies (in the city of Beloretsk, Republic of Bashkortostan, Russia).

#### *2.4. Activation of the Starting Components and Selection of the Activating Additive*

Analysis of previously conducted studies of the geotechnology with backfill shows that the main cost in the backfill material is binder. Physico-chemical activation of the backfill components can improve the quality properties of the binder and, therefore, reduce its consumption.

One of the most affordable and cheap methods of activation is the mechanical method of activation in disintegrators [31,32]. In addition to mechanical treatment of the backfill material, a fairly effective activation method is the addition of activating additives to the material. Considering previous studies [33], it can be concluded that one of the most affordable and cheap activating additives can be lignosulfonate for the preparation of a backfill based on water-soluble ores. Lignosulfonate is an anionic surfactant that is a waste product of the pulp and paper industry.

Carbon frame structures (fullerenes and nanotubes) are used as additives that increase the strength of the created material. The high strength and high elasticity of nanotubes is a rather successful combination, which makes it possible to improve the mechanical properties of the material [34]. It is possible to create new nanomodified materials using the high strength characteristics and elasticity of nanotubes [35]. In this case, nanotubes act as strengthening additives. One such nanomodifying additive is astralene (TR (technical requirements) 31968474.1319.001-2000), obtained by the discharge-arc method [36,37]. Previous studies [38] have demonstrated positive results of using astralene (fulleroid multilayer synthetic nanomodifier). Its inclusion into the material significantly increases the elastic and strength properties [39]. The use of astralene as an activation additive to improve the properties of concrete-building mixtures showed positive results [40]. Water-soluble ores have hygroscopicity, caking ability, and, with a small amount of moisture, form a sufficiently dense and solid mass. Therefore, the effect of a nanomodifying additive without the use of a binder was initially studied to determine the optimal dose in the backfill.

The preparation of the material with the nanomodifying additive was carried out in the following sequence: astralene, the concentration of which is from 0.001% to 0.02% of the mass of the waste from the enrichment of water-soluble ores, is mixed for 5 min, then gaged with brine and blended for 10 min until a homogeneous mass is formed. The resulting mixtures were placed in cubes with faces of 10 cm.
