Pilot testing of sylvinite ore reserves re-mining method

Introduction

The term “re-mining (re-extraction) of mineral resources” refers to the extraction of reserves that were previously lost (undermined, caved, or left in pillars) or left in the subsoil because their initial extraction was uneconomical at operating or closed mines [1, 2]. Depending on the minerals being extracted and the primary mining method used, different re-stoping processes are employed at different deposits. For instance, the re-mining of ore deposits is primarily based on the additional extraction of caved rock mass from stoping zones [3–5]. To date, this method has found the widest application at coal deposits mined using both breast stoping (longwall system) [6, 7] and the Silesia method [8, 9].

The extraction of mineral using the Silesia / roomand- pillar mining methods opens up broad prospects for the use of methods that allow for the re-extraction of ores left in rib, barrier, and panel pillars [10–12]. There are quite a few technical approaches to implementing re-extraction in such conditions [13, 14], including two-stage stoping [15–17].

In the 1980s and 1990s, attempts were made to carry out planned two-stage mining of sylvinite deposits within the Verkhnekamsk potassium salt deposit (VPSD), the largest of its kind in Europe. The essence of this method is as follows [17]: during the initial stage of layer mining, wide “rigid” rib pillars were left in place, after which the stopes were filled with salt waste, primarily using a hydraulic method. In the second stage, after the filling mass has reached the required strength, the rib pillars were partially extracted. It should be noted that two-stage stoping has not become widespread at the Verkhnekamsk potash salt deposit mines. This is due to a combination of factors: the complexity of coordinating stoping and backfilling operations; the deterioration of the physical and mechanical properties of the rock in the pillars as a result of wetting by backfill brines; the complex nature of deformation in the heterogeneous pillars formed during the second stage; the need to maintain the development workings for an extended period; and so on.

In addition to the extraction of pillars resulting from the commissioning of the second stage, re-mining methods also include the recovery of reserves previously classified as non-commercial due to unfavorable economic conditions and/or technological limitations. Specifically, for the Starobinsky potash salt deposit (Belarus), a study was conducted on the feasibility of extraction of the partially mined out 4th sylvinite layer, which was left behind during the extraction of the underlying layers 2, 2–3, and 3 of the Third Potash Horizon [18–20].

Another approach within the framework of potash deposit re-mining method involves the extraction of reserves from a natural-technogenic layer under VPSD conditions [21]. This layer was formed in areas where room-and-pillar mining system was used to develop the KrII sylvinite layer, with leaving “yielding” rib pillars in the mining field of the BKPRU-2 mine operated by PJSC Uralkali. Under the influence of rock pressure, these pillars deform and fracture, causing the stopes to be filled primarily with their fragments and resulting in the formation of a continuous rock mass with periodically varying physical and mechanical properties. As shown in [21], the secondary natural-technogenic KrII* layer has sufficient recoverable thickness and acceptable strength characteristics that creates the conditions for the re-extraction of sylvinite ore reserves under VPSD conditions. This method is particularly relevant for mines in the final stages of operation, as it is designed to extend the mine's operational life. In addition, its implementation will make it possible to include reserves previously classified as production losses in the extraction process, thereby increasing the overall recovery factor for a mineral from a producing layer.

When selecting process flow sheets for re-mining of the KrII* natural-technogenic layer using room-and-pillar method, the key issues are determining the bearing capacity and deformation behavior of heterogeneous rib pillars composed of rock masses with varying strengths. This is essential for ensuring safe conditions for re-mining of the overlying watertight rock strata and for protecting a mine from flooding. A solution to these problems obtained for the first time on the basis of quantitative estimates using numerical simulation methods [21] provides the scientific basis for demonstrating the feasibility of re-mining of silvinite ore reserves.

The implementation of the proposed method for stoping within the KrII* natural-technogenic layer requires the preparation of a “Safety Justification for a Hazardous Production Facility” and the conduct of pilot tests. This work was carried out between 2022 and 2024 and included scientific support, which involved monitoring the condition of the mine workings. The main objective of the testing was to assess the technical feasibility and safety of driving and stoping within the KrII* natural-technogenic layer rock mass, as well as to clarify the mining-geological and mine engineering conditions for driving and stoping.

In light of this, the following main objectives were set for the testwork:

1) determining the optimal conditions for implementing the method for the re-extraction of reserves from the KrII* natural-technogenic layer;

2) clarification of the geological features and physical and mechanical properties of the KrII* layer;

3) monitoring deformation processes in mine workings and assessing their stability.

Testing conditions and key technical solutions

A 170-by-200-meter area within the mining field of the BKPRU-2 mine was selected for pilot testing of the method for re-mining of the KrII* natural-technogenic layer. The testing area is located in a zone where the initial mining of two gently dipping, closely spaced sylvinite layers was performed in the first half of the 1970s, leaving “yelding” rib pillars in the KrII layer with the parameters shown in Fig. 1, a.

During the initial mining of the KrII layer in the area under consideration, a protective silvinite member approximately 0.7 m thick was left in the roof. Subsequently, the stopes were filled primarily with fragments of this layer and the marginal parts of the pillars, and to a lesser extent with host rocks. Under the influence of moisture and rock pressure, the caved rocks eventually consolidated, forming a continuous and relatively monolithic rock mass. Thus, the naturally and technologically formed KrII* layer 2.4–2.8 m thick composed of alternating of caved rock and remnants of pillars, has a KCl content close to that found in natural potash layers, which makes it commercially viable for re-mining.

The paper [21] examines various options for re-extraction of the KrII* natural-technogenic layer using room-and-pillar method. The calculations showed that, at the testing area, safe conditions for re-mining of the KrII* layer were achieved by driving single-pass stopes with an Ural-61A shearer, coaxially with first-stage rib pillars with a twofold increase in the inter-axis distance. The parameters for implementing this process flow are shown in Fig. 1, b.

Fig. 1. Parameters of the initial mining of the KrII and AB layers (a)
and the re-mining of the natural-technogenic KrII* layer (b) in the testing area

The pilot tests involved the re-extraction of reserves from the testing area – occurring in the firststage pillars – by driving ten testing stopes in the meridian direction. The area preparation involved driving transport and ventilation inclines from the main-direction workings, which had been driven through the underlying rock salt layer, as well as an extraction drift through the KrII* layer. To maintain stability, the extraction drift is located in the rigid pillars area between the starting workings of the primary mining stage. At the end of a pass, the stopes were connected by ventilation connections. In addition, several stopes were connected by research connections in order to conduct field research. In order to experimentally evaluate a stable mine working roof span within the natural-technogenic layer, some connections were driven in two passes across the width, with the passes overlapping. The location of the research workings relative to the previously driven development workings and stopes is shown in Fig. 2.

Fig. 2. Testing area workings layout

The stability of the workings in the natural-technogenic layer was ensured by undercutting all three clay “cakes” in the roof, as well as by using roof bolting at the junctions of the workings and in sections where the working width exceeded one shearer pass, when excavating caved unstable rock in former stopes. To monitor the condition of the workings and the load-bearing elements of the room-and-pillar system for re-mining, underground observation stations (OS) were set up at the testing area to monitor the displacement of the workings surrounding rock mass. In addition, the scientific support for the testing project included a detailed study of the geological setting and physical and mechanical properties of the rocks in the KrII* natural-technogenic layer, as well as an assessment of their gas-bearing capacity.

Features of driving workings in the natural-technogenic layer rock mass

During driving the research workings within the KrII* natural-technogenic layer at the testing area, it was found that the voids in the previous stopes are currently almost entirely filled with blocky material that was cemented to varying degrees (Fig. 3). The worked-out stopes were filled in several stages. First, the protective member of sylvinite layer left in the roof fails. Next, vertical compression of a pillar caused it to expand laterally. As a result, the margin parts were broken down into a system of contour subparallel prismatic blocks. These blocks shifted and tilted into the mined-out space, dislodging cuttings from the floor and causing it to accumulate in the center of a stope.

Fig. 3. Structure of the KrII* Natural-Anthropogenic layer

The formation of a system of subvertical fractures in the central part of a pillar results in the formation of a trapezoidal skeleton bounded by faults, within which the lower part of the KrII layer (sublayers, SL 5–7 in Fig. 3) remains virtually unchanged in thickness. At the same time, the reduction in the height of the pillar is primarily due to creep deformations, which predominate in the upper part of the layer (sublayers, SL 1–4), where more than half of the layer’s thickness is lost. At the same time, the rock filling the stopes contains virtually no material from the overlying layers, indicating that the AB-KrII* technical interlayer has not caved. Thus, the stopes were filled primarily through the redistribution of rocks from the KrII sylvinite layer.

As a result of transverse deformation, the width of the pillars increased to 5.0–6.0 m across the layer floor, while the width of the caved rock area at the site of the former stopes was 3.0–4.0 m. The thickness of the KrII layer within the contour of the former stopes decreased to 2.4–2.6 m, and, in the pillars, to 2.6–2.8 m, which represented 52–57% and 57–61%, respectively, of the initial thickness of 4.6 m (see Fig. 1).

In the course of re-mining at the testing area, field tests were conducted on large-scale prismatic specimens measuring 400 × 200 × 200 mm to determine their uniaxial compressive strength, in accordance with the methodology described in [21].

The results of the field tests on large-scale specimens at the testing area revealed significant variation in the strength characteristics of the KrII* natural- technogenic layer. For the rock caved into the stopes, the compressive strength ranges from 0.4 to 3.8 MPa, with an average value of 2.0 MPa. The strength of the rocks of rib pillars ranges from 16.2 to 24.0 MPa, with an average of 20.6 MPa, which roughly corresponds to the natural properties of the intact KrII layer as determined during background testing.

Thus, within the re-mining area with newly driven stopes, the strength properties of the KrII* natural-technogenic layer are characterized by an extremely heterogeneous distribution. In the central part of the pillars, the strength is at the background levels for the VPSD and decreases by nearly an order of magnitude in areas of caved rock.

A geological study of core samples from boreholes drilled into the roof of the workings at the testing area revealed zones of stratification and loosening in the rocks of the AB-KrII* parting above the former stopes. These often contain accumulations of condensate brines that have migrated from the floor of the stopes created in the overlying AB layer. In addition to the parting, local brine seepage is observed when the shearer cuts through rock that has caved into the stopes. The nature of the brines is due to the long-term condensation of moisture from the air in the old mine workings.

During the research driving at the testing area, it was determined that various metallic objects may be found in the caved rock; these include fragments of mechanisms, assemblies, and equipment parts left during the initial mining of the panel, as well as other debris (conveyor belts, wood, etc.). This leads to increased wear on the shearer’s teeth and makes driving more difficult. In addition, the increased clay content and wetting by condensation brines contribute to the high viscosity of the rock that caved into the stopes. As a result, the driving rate in the layer decreases and the number of tool-related incidents increases.

The complex hypsometry of the KrII* natural-technogenic layer, which differs significantly from the natural conditions under which the VPSD sylvinite layers occur, also complicates driving and stoping.

Fig. 4. Appearance of the workings driven through the KrII* natural-technogenic layer:
a – an intersection of rock caved into the stope on the right wall of the working;
b – an outcrop of weakly consolidated rock on the left wall of the working

The natural-technogenic layer contains voids ranging from small, irregularly or round-shaped caverns with volumes of several cubic centimeters to voids larger than one cubic meter, which are parts of the first mining stage workings left unfilled with rock mass fragments (Fig. 4). The following factors are likely causes of the void formation:

• uneven tilting of the opposite walls of the primary stopes during the deformation of the yelding rib pillars due to the heterogeneous geologic and mining conditions of the KrII layer (folding, variations in occurring depth, thickness, etc.) and local deviations from the pillar design parameters;
• the location of the primary workings near high-stiffness (rigid) pillars, which thus protect them.

If large voids are reopened or if they are located close to the research workings, there is a risk of accelerated deformation of the contour, leading to a loss of its integrity in the form of fractiring, delamination, and roof breaks. Safe driving in the specified zones of the natural-technogenic layer must include the following measures:

– the creation of a safety zone near the contour of a rigid pillar, within which mining operations are prohibited during re-extraction;

– use of a semi-automatic (remote) control mode for a shearer;

– the use of additional methods to monitor the condition of the workings enclosing rock mass (drilling blast holes and performing video endoscopy of the rock mass, monitoring displacements of the workings contours).

Monitoring the condition of workings in a natural-technogenic layer

Monitoring of the displacement of the workings enclosing rock in the KrII* natural-technogenic layer was carried out at underground observation (monitoring) stations (OS) set up in the testing area. The locations of the stations within the testing area are shown in Fig. 2. A long-term observation station consists of deep and contour benchmarks (mine survey plugs) installed within a working cross-section (Fig. 5). The free end of the benchmarks has a mounting hole for attaching them to a measuring device – a belt extensometer. To assess the impact of various factors (the presence of rock caved into the stopes, the width of the working face, support, etc.) on the magnitude of rock displacements in the surrounding rock mass, the stations were installed under a wide range of geological and mining conditions.

Fig. 5. Scheme of stations positioning for monitoring the displacement of the enclosing rock mass:
a – mine workings roof; b – rib pillar

Monitoring of the rock mass displacement was performed by periodically measuring the distances between benchmarks installed in opposite parts of a working (roof-floor, left-right walls). During the initial stage of the observations, periodic measurements were taken once every 3 months on average. Subsequently, the frequency of measurements decreased depending on the intensity of the rock mass displacement.

Based on the observation results, graphs were plotted showing the displacements of the benchmarks as a function of the time elapsed since a station was established. In addition, the average annual displacement rate of the enclosing rock mass was calculated for all observation stations.

As an example, Fig. 6, a shows graphs of the vertical displacements of the roof rock at observation station 4, located at the junction of the extraction drift and the starting research stopes 1–2 (see Fig. 2). An analysis of the monitoring data revealed that the maximum rate of vertical displacement of the roof strata took place during the initial observation period. Subsequently, the deformation rate gradually decreases, stabilizing at 22–28 mm/year. A relationship was also established between the magnitude of the displacements and the distance to the roof of a working: the minimum values were recorded at a distance of 6.0 m from the roof, while the maximum values were recorded directly at a working contour.

A qualitatively similar pattern of deformation in the roof rock, despite quantitative differences in the absolute values of the displacements, was observed at all observation stations equipped with vertically oriented deep benchmarks according to scheme shown in Fig. 5, a. At the same time, an analysis of the monitoring results showed that the displacement rates of the contours of the research workings at the points where they intersect caved rock are generally lower than in the previously left pillars in the KrII* layer. This observed pattern is due to the vastly different deformation properties of these structural elements within the natural-anthropogenic layer, which cause a heterogeneous stress field to form within it. As a result, stress concentrations in the primary mining pillars lead to increased rates of displacement of the workings contours relative to parts of former stopes filled with caved rock.

This same reason explains the nonuniform nature of the horizontal displacements of the benchmarks shown in Fig. 6, b, which were installed in the re-formed rib pillar according to the scheme in Fig. 5, b. Thus, the greatest rock displacements – 39.4 mm – over the observation period were recorded at the pillar contour. As the distance from the contour exceeds 0.75 m, the displacements decrease sharply, stabilizing in the range of 14.5 to 23.8 mm. At the same time, the measured displacement value at a distance of 1.5 m is lower than the values observed in the 3.0–4.5 m distance range. This deviation from the general pattern is due to the fact that the benchmark P1.5 is located in the caved rock mass with different deformation characteristics.

Fig. 6. Rock mass displacement as a function of time:
a – vertical roof displacements at the junctions of workings (OS 4); b – horizontal displacements of the rib pillar (OS 3)

Based on the results of workings condition observations in the testing area, the following conclusions can be drawn. When exposed, weakly consolidated caved rock exhibits low stability. The most dangerous areas in this regard are sections where old workings have been undercut by inclines, as well as areas near abandoned pillars with increased rigidity. For the most part, the observed workings contour displacements fall within the range of values typical of the standard mining and geological conditions at VPSD. The stability of the workings enclosing rock mass in the KrII* natural-technogenic layer is assessed as satisfactory.

Conclusion

In order to evaluate the new mining method for the Verkhnekamsk potash salt deposit, which involves the re-extraction of reserves from the KrII sylvinite layer, pilot tests of this method were conducted. The tests were carried out within the mining field of the BKPRU-2 mine operated by Uralkali PJSC in the area where room-and-pillar mining method with “yielding” pillars was previously used.

The tests conducted confirmed the practical feasibility and specific characteristics of driving workings within the KrII* natural-technogenic layer. The technical solutions adopted for the conditions of the testing area ensure the stability of the mine workings throughout the entire period of mining.

The main requirements governing safe mining conditions during the re-extraction of reserves from the KrII* natural-technogenic layer include the following:

• a sufficient degree of compactness and continuity of the formed rock mass, which results from complete backfilling of stopes when using a primary mining method with “yielding” pillars;
• selection of re-mining parameters that ensure rigid support of the overlying watertight strata;
• driving stopes coaxially with the rib pillars of the primary mining system;
• the creation of a safety zone near the contour of a rigid pillar, within which mining operations are prohibited during re-extraction;
• use of the semi-automatic (remote) control mode for a shearer.

Based on the results of a study of the geological structure of the KrII* natural-technogenic layer, sporadic voids were discovered, consisting of areas of the first stage of mining that were not filled with rock fragments. Cyclic variability in the physical and mechanical properties of the KrII* layer has also been established. Specifically, the rock strength ranges from 0.4–3.8 MPa in the zone of caved rock to 16.2–24.0 MPa in pillars.

An analysis of the monitoring data on deformation processes in the mine workings driven in the KrII* natural-technogenic layer showed that the rates of displacement of their contours are generally 22–28 mm/year and fall within the range typical of the standard mining-geological conditions of the VPSD. The stability of the workings enclosing (surrounding) rock mass is assessed as satisfactory.

Despite the broad prospects for applying the re-mining method in the BKPRU-2 mine field, most of which has been mined-out with leaving “yielding” pillars in the KrII layer, additional studies are needed to calculate recoverable reserves to be involved, with a focus on assessing the feasibility of implementing this method at specific stoping areas. This is due to the variability of geological and mining conditions, which are determined by factors such as the parameters of primary mining, the degree of compactness and continuity of the formed rock mass of the natural-technogenic layer, the presence of geological anomalies in the structure of the watertight strata, proximity to the permanent boundaries of mining operations, the presence of protected sites on the Earth’s surface, and so on.

Thus, the implementation of this method will make it possible to involve previously written-off reserves into extraction and increase the overall sylvinite ore recovery factor from the KrII layer. Under conditions similar to those at the testing area, the expected increase in the recovery factor averages 16% [21].