A cementitious material for backfilling of open pit mine and application thereof

Abstract

The application discloses an open-pit mine pit full tailings backfill cementing material, which comprises the following components in percentage by mass: granulated blast furnace slag powder 80-90%, industrial by-product gypsum 5-15%, and composite additive 0.5-5%; and the sum of the mass ratios of the components is 100%. The application also discloses application of the cementing material in open-pit mine pit full tailings filling, wherein the cementing material is used as a binding material, the mine full tailings are used as aggregates, and mixed water is used as water for preparation, so that filling material slurry is prepared. The mass concentration of the filling material slurry is 66-72%, and the ash-sand ratio of the cementing material and the full tailings in the filling material slurry is 1:6-1:20. In the cementing material, the proportion of the granulated blast furnace slag powder is more than 80%, the proportion of the industrial by-product gypsum is more than 10%, and the total proportion of the industrial solid wastes is more than 98%, so that a large amount of cement clinker is not needed, carbon emission and raw material cost in cementing material production are greatly reduced from the source, high-value utilization of blast furnace slag, industrial by-product gypsum and other bulk industrial solid wastes is realized, and the green development concept of mine ecological restoration is met.

Description

Technical Field

[0001] This invention relates to the field of mine ecological restoration and industrial solid waste resource utilization technology, and in particular to a cementitious material for backfilling tailings in open-pit mines and its application. Background Technology

[0002] my country has numerous large, deep open-pit mines, most of which are currently in the middle to late stages of mining and are gradually transitioning to underground mining. Taking Ansteel Mining's Dagushan Iron Mine as an example, its current mining depth has reached 516 meters, and according to its medium- and long-term development plan, it will gradually transition to underground mining. Open-pit mining in metal mines creates large-scale open pits, leading to a series of ecological and environmental problems such as slope instability, soil erosion, water and soil pollution, and damage to land resources. To block the hydraulic channels between the open pit and the underground, avoid the risk of mine flooding, maintain the stability of steep slopes, and protect surrounding waste rock dumps and tailings dams, the core path to achieving coordinated development of ecological restoration and safe mine production is to use tailings cementation and solidification backfilling of the open pit, gradually restoring it to its original pre-mining topography. This solution can simultaneously address the mining needs of approximately 300 million cubic meters of Ansteel Mining's eastern mining area over the next 20 years. 3 Addressing the challenge of surface tailings storage, this approach reduces the cost of managing and maintaining steep slopes in open-pit mines, as well as the risk of landslides.

[0003] However, for mines transitioning from open-pit to underground mining (such as the Ansteel Dagushan Iron Mine), the open-pit backfill not only serves an ecological restoration function but also acts as an overlying protective layer for underground mining. Therefore, there are differentiated requirements for the strength, deformation characteristics, and stability of backfill at different elevations and in different layers. For example, it is necessary to construct backfill with different strength levels, such as a bottom sealing layer, a transition layer, and a normal backfill layer, to meet the multi-layered needs from ecological restoration to safety protection during underground mining.

[0004] Currently, the most commonly used cementing material in mine backfilling projects is ordinary Portland cement (P.O42.5), but it has significant drawbacks in open-pit mine tailings backfilling scenarios: 1. Poor adaptability to high ash-sand ratio: Cement-based materials have insufficient hydration activity when the ratio of cementitious material to tailings is ≤1:10. Especially at an extremely high ash-sand ratio of 1:20, they cannot set and harden normally, and cannot achieve the consolidation and backfilling of large quantities of tailings with extremely low cementitious content.

[0005] 2. Poor performance: The high-ash-sand ratio backfill slurry has poor fluidity and high bleeding rate, which can easily lead to problems such as slurry segregation, uneven strength of the backfill, and large-area water accumulation in the mining area, making it impossible to guarantee the effect of large-area homogeneous backfilling in open pits.

[0006] 3. Complex strength control: In order to meet the gradient requirements of different strength zones such as bottom sealing, transition layer and normal backfill layer, cement-based materials need to frequently change the cementitious material formula or significantly adjust the cementitious dosage to achieve strength control, resulting in a complex on-site construction process and extremely low efficiency.

[0007] 4. Weak adaptability to water-rich environments: Open-pit mines are open construction environments and face water-rich conditions such as rainfall, seepage from surrounding rock, and water accumulation on the top layer of the backfill. The hydration process of cement-based materials is severely hindered in water-rich environments, and the strength cannot develop normally. Problems such as failure to solidify and pulverization may even occur, making it impossible to guarantee the continuity of backfilling construction during the rainy season and under seepage conditions.

[0008] 5. High cost and carbon emissions: Cement production has high carbon emissions and high raw material costs. Using it for large-scale tailings backfilling in open-pit mines will lead to a significant increase in backfilling project costs and does not conform to the concept of green and low-carbon ecological restoration.

[0009] Existing slag-based mine backfill cementitious materials are mostly developed for underground goaf backfilling scenarios. They suffer from low slag content, complex raw material formulations, and high additive costs. Furthermore, they cannot adapt to a wide lime-sand ratio range of 1:6 to 1:20 and a wide concentration range of 66% to 72%, making it difficult to simultaneously meet the requirements for fluidity, low bleeding rate, and gradient strength control. Especially in terms of setting and hardening performance under extremely high lime-sand ratios and adaptability to open-pit water-rich environments, they fail to meet the engineering requirements for open-pit mine backfilling (particularly as an overburden protective layer in open-pit to underground mining scenarios).

[0010] In summary, given the unique technical challenge faced by open-pit mines transitioning to underground mining—that the backfill material must simultaneously fulfill the dual functions of ecological restoration and underground mining overlying protective layer, while also possessing differentiated gradient strength requirements—developing a green, low-carbon, widely adaptable, low-cost, water-rich, and hardenable backfill cementitious material specifically for open-pit mines has become an urgent technical issue for the industry to address. Summary of the Invention

[0011] To address the shortcomings of existing cement-based backfill materials and traditional slag-based cementitious materials in open-pit mine tailings backfilling scenarios, such as inability to harden under high lime-sand ratios, high costs, large carbon emissions, complex construction for gradient strength control, poor adaptability to water-rich environments, and insufficient adaptability to a wide range of working conditions, this invention provides a green and low-carbon cementitious material for open-pit mine ecological restoration tailings backfilling. This material uses bulk industrial solid waste as the main raw material, is green and low-carbon, has a wide lime-sand ratio, meets the requirements for layered backfilling strength control / wide concentration (meeting the underflow concentration requirements of the front-end deep cone thickener, is hardenable in water-rich environments, meets conditions such as rainwater runoff and rock fissure leakage), and achieves controllable strength gradients. The specific technical solution provided is as follows: In a first aspect, the present invention provides a cementitious material for backfilling tailings in open-pit mines, comprising the following components by mass percentage: 80%~90% granulated blast furnace slag powder, 5%-15% industrial by-product gypsum, and 0.5%-5% composite additives; the sum of the mass ratios of the components is 100%.

[0012] Furthermore, the granulated blast furnace slag powder is S95 grade or higher granulated blast furnace slag powder with a specific surface area ≥400m². 2 / kg.

[0013] Furthermore, the industrial by-product gypsum is one or more of desulfurized gypsum, phosphogypsum, and fluorogypsum, with a calcium sulfate dihydrate content ≥85% by mass, and after drying and grinding, has a specific surface area ≥300 m². 2 / kg.

[0014] Furthermore, the composite additive is one or more of quicklime, hydrated lime, fly ash, sodium sulfate, and sodium tripolyphosphate.

[0015] A further preferred embodiment of a full tailings backfill cementitious material for ecological restoration of open-pit mines comprises the following components by mass percentage: 85% granulated blast furnace slag powder, 10% industrial by-product gypsum, and 5% composite additives; the sum of the mass percentages of each component is 100%.

[0016] The composite additive, by mass percentage, includes the following components: 25% calcium powder, 45% fly ash, 10% sodium sulfate, 10% dicalcium silicate, and 10% tricalcium silicate; it is used to activate the hydration activity of blast furnace slag, accelerate the generation of early hydration products, and improve the early strength of the slurry. The granulated blast furnace slag powder is S95 grade or above granulated blast furnace slag powder with a specific surface area ≥400m² / kg to ensure sufficient hydration activity; The industrial by-product gypsum is one or more of desulfurized gypsum, phosphogypsum, and fluorogypsum, with a calcium sulfate dihydrate content of ≥85%. After drying and grinding, it has a specific surface area of ​​≥300m² / kg. As a sulfate activator, it reacts with slag hydration products to generate ettringite, thereby improving the early strength and volume stability of the filling body.

[0017] Secondly, the present invention also provides the application of the cementitious material as described in the first aspect in the full tailings backfilling of open-pit mines, especially in the application of open-pit mines transitioning to underground mining.

[0018] Furthermore, a filling slurry is prepared by using cementitious materials as binders, mine tailings as aggregates, and mixing water as preparation water.

[0019] Furthermore, the mass concentration of the filling slurry is 66%~72%; the ash-sand ratio of the cementitious material and the tailings in the filling slurry is 1:6~1:20.

[0020] The concentration control of 66%-72% is limited by the underflow concentration of the deep cone thickener, which is at most 72%. Under normal operating conditions, the underflow concentration is stable at 68-70%. Pursuing a higher underflow concentration would reduce the equipment's throughput and require the addition of a large amount of flocculant, which would affect the thickener's return water and the flotation operation of the concentrator. Therefore, by reducing the slurry concentration, reducing the amount of flocculant used, and increasing the thickener's throughput, the slurry concentration can be controlled at 66-72% while ensuring strength.

[0021] More preferably, the filling slurry is prepared using a multi-component material mixing and feeding device for mine filling, the multi-component material mixing and feeding device for mine filling comprising: The system includes multiple powder silos, a feeding and metering device, a mixing screw feeder, a mixing tank, and a dust collection device. The powder silos store different components of cementitious materials. A feeding and metering device is fixedly installed below each powder silo. The inlet of the feeding and metering device is connected to the powder silo, and the outlet of the feeding and metering device is connected to the powder inlet of the mixing screw feeder. The mixing outlet of the mixing screw feeder is connected to the inlet of the mixing tank. The top of the mixing tank is connected to the dust collection device, and the outlet of the dust collection device is connected to the powder inlet of the mixing screw feeder. The water inlet of the mixing tank is connected to a liquid supply pipe, and the outlet of the mixing tank is connected to a discharge pipe.

[0022] Furthermore, the mixing screw feeder includes a housing, a main shaft, screw blades, and a motor. The screw blades are located inside the housing and are fixedly connected to the main shaft. The main shaft is rotatably connected to the housing, and the motor drives the main shaft to rotate.

[0023] Furthermore, the ratio of the axial length to the diameter of the helical blade is greater than or equal to 10:1, and the housing is a tube sealed at both ends.

[0024] Furthermore, the powder inlet and the mixing outlet are respectively fixedly installed in the shell. The powder inlet includes a raw material powder inlet and a return powder inlet. Each powder bin is connected to the corresponding raw material powder inlet. The outlet of the dust collection device is connected to the return powder inlet, which is located between the raw material powder inlets.

[0025] Furthermore, the first powder silo is connected to the first powder inlet, and the second powder silo is connected to the second powder inlet. The first powder inlet and the mixing outlet are located at opposite ends of the shell, and the second powder inlet is located in the middle of the shell. The return powder inlet is located between the second powder inlet and the first powder inlet, and the discharge rate of the first powder inlet is greater than that of the second powder inlet.

[0026] Furthermore, the feeding and metering device includes a feeder and a micro powder scale. The discharge port at the lower end of the powder silo is fixedly connected to and communicates with the feeder's inlet. The discharge port of the feeder is fixedly connected to and communicates with the inlet of the micro powder scale. The discharge port of the micro powder scale is connected to the powder inlet of the mixing screw feeder.

[0027] Furthermore, it also includes a thickener and a slurry pump, wherein the inlet of the slurry pump is connected to the outlet at the bottom of the thickener, and the outlet of the slurry pump is connected to a feed pipe, the feed pipe being equipped with a flow meter and a flow control valve; and the discharge pipe being equipped with a flow meter and a flow control valve.

[0028] Thirdly, the present invention also provides a layered backfilling process for converting open-pit mines to underground mines, using the cementitious material as described in the first aspect as the binder, the mine tailings as the aggregate, and mixing water as the preparation water to prepare the backfilling slurry; the layered backfilling process is adopted, and the strength gradient of different backfilling layers is controlled by adjusting the ash-sand ratio of the backfilling slurry.

[0029] Furthermore, the open-pit to underground mining backfilling process includes bottom sealing works and remediation works; the bottom sealing works include a direct bottom slab and an indirect bottom slab; the remediation works include a transition layer and a normal backfill layer.

[0030] Furthermore, the direct base slab is backfilled with a filling grout with a 28-day strength of not less than 4 MPa; the indirect base slab is backfilled with a filling grout with a 28-day strength of not less than 1.5 MPa and a concentration of 70%; the transition layer is backfilled with a filling grout with a 28-day strength of not less than 1.0 MPa and a concentration of 70%; and the normal backfill layer is backfilled with a filling grout with a 28-day strength of not less than 0.5-2 MPa and a concentration of 70%.

[0031] Preferably, taking Ansteel Mining's Dagushan Iron Mine as an example, the layered filling process can achieve strength gradient control of different filling layers simply by adjusting the ash-sand ratio of the filling slurry, without changing the raw material formula of the cementitious material, which greatly simplifies the on-site construction process.

[0032] A schematic diagram of the layered filling process is shown below. Figure 2 As shown, the strength control method of the layered filling process is as follows: (1) Bottom sealing project The bottom sealing project is a critical layer for backfilling open pits and should be completed quickly during sunny seasons, excluding winter. The bottom sealing project consists of two parts from bottom to top: the direct bottom slab and the indirect bottom slab, with an elevation range of -414m to -354m.

[0033] 1) Direct base plate After the open-pit mining is completed, loose rocks on the slope are cleared and the site is leveled. After the bottom platform is properly prepared, a steel mesh is laid on the -414m platform. Steel mesh laying parameters: mesh size Φ18@400×400; steel mesh extends to the slope and is laid 2m downhill.

[0034] After the steel mesh was installed, backfilling was carried out using filling grout with a 28-day strength of not less than 4 MPa. The backfill elevation was -414m to -394m.

[0035] 2) Indirect base plate After the direct base slab construction is completed, and after curing for no less than 7 days, backfilling is carried out using filling grout with a 28-day strength of no less than 1.5 MPa and a concentration of 70%. The elevation range is -394m to -354m. After backfilling is completed, curing is carried out for no less than 7 days.

[0036] (2) Remediation Project After the bottom sealing of the open pit is completed and cured for no less than 7 days, the upper area can be backfilled. This design divides the upper area into two parts: a transition layer between -354m and -300m, and a normal backfill layer between -300m and +70m.

[0037] 1) Transition layer The transition layer is backfilled with filling grout with a 28-day strength of not less than 1.0 MPa and a concentration of 70%, with an elevation range of -354m to -300m.

[0038] 2) Normal backfill layer Backfilling between -300m and -102m uses grout with a 28-day strength of not less than 0.5MPa and a concentration of 70%; backfilling between -100m and -98m uses grout with a 28-day strength of not less than 2MPa and a concentration of 70%; and backfilling between -98m and +70m uses grout with a 28-day strength of not less than 0.5MPa and a concentration of 70%.

[0039] The present invention has the following beneficial effects: 1. The cementitious material of this invention contains more than 80% granulated blast furnace slag powder, more than 10% industrial by-product gypsum, and more than 98% total industrial solid waste. It does not require a large amount of cement clinker, which significantly reduces carbon emissions and raw material costs in the production of cementitious materials from the source. It realizes the high-value utilization of bulk industrial solid wastes such as blast furnace slag and industrial by-product gypsum, which is in line with the green development concept of mine ecological restoration.

[0040] 2. The cementitious material of this invention can be adapted to the preparation of tailings backfill slurry with a cement-sand ratio of 1:6 to 1:20 and a slurry mass concentration of 66% to 72%. Only by adjusting the cement-sand ratio can the 28-day uniaxial compressive strength of the backfill body be continuously controlled from 0.5MPa to 7.0MPa without changing the raw material formula of the cementitious material. It can be well adapted to the differentiated strength requirements of different elevations in the layered backfilling (bottom sealing, transition layer, and normal backfill layer) of open-pit mines transitioning to underground mines, greatly simplifying the on-site construction process and avoiding the problems of reduced construction efficiency and difficulty in quality control caused by frequent material changes.

[0041] 3. Within a suitable range of cement-sand ratio and concentration, the slump of the filling slurry prepared by the cementitious material of this invention is ≥210mm, exhibiting excellent fluidity and meeting the requirements of high-flow-rate, long-distance gravity-flow transportation in open-pit mines. The slurry bleeding rate can be stably controlled below 12%, demonstrating good water retention and low shrinkage rate of the filling body. This effectively avoids problems such as slurry segregation, uneven filling body strength, and large-area water accumulation in the mining area caused by high bleeding rates, ensuring the engineering quality of large-area homogeneous filling in open-pit mines. The cementitious material has a 7-day strength >0.5MPa (1:20 cement-sand ratio), with significantly improved early strength at low cement-sand ratios. Specimens can be quickly demolded and cured, shortening the backfilling cycle and meeting the requirements of rapid backfilling in open-pit mines. It also exhibits excellent setting and hardening performance at high cement-sand ratios, resulting in extremely low filling costs.

[0042] The cementitious material of this invention can still achieve normal setting and hardening of the backfill slurry under extremely high cement-sand ratio conditions of 1:20, with a 28-day uniaxial compressive strength ≥0.5MPa, meeting the strength requirements of normal backfill layers in open-pit mines; while ordinary P.O42.5 cement cannot set and harden normally under the same conditions, and there is no effective strength development. This invention achieves safe consolidation backfilling of large quantities of tailings with extremely low cementitious material usage, significantly reducing the overall cost of open-pit mine backfilling projects.

[0043] 4. The cementitious material of this invention, through optimization of the hydration system, can still hydrate, solidify and harden normally in water-rich environments (overlying water, open-pit precipitation, mine seepage), without the problems of cement-based materials being hindered in hydration, unable to develop strength, or pulverizing and failing in water-rich environments. It is perfectly adapted to the complex construction environment of open-pit mines with abundant water, ensuring the continuity of filling construction and the long-term stability of the filling body under rainy season and seepage conditions.

[0044] 5. The cementitious material of this invention can precisely match the gradient strength requirements of the backfill body for open-pit to underground mining pits by adjusting the cement-sand ratio. A cement-sand ratio of 1:6 meets the high strength requirement of more than 5MPa for the bottom sealing project, serving as a safety layer for the roof of underground mining, blocking the open pit from the underground hydraulic channel, and eliminating the risk of well flooding; a cement-sand ratio of 1:10 to 1:12 meets the strength requirement of 1.5 to 2MPa for the transition layer; and a cement-sand ratio of 1:20 meets the strength requirement of 0.5MPa for the normal backfill layer. This achieves a precise match between the performance of the backfill body and the engineering requirements, taking into account both the safety of backfilling and the economic efficiency of the project. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only one embodiment of the present invention. For those skilled in the art, other embodiments can be derived from the provided drawings without creative effort.

[0046] Figure 1 Tailings particle size distribution curve; Figure 2 : Schematic diagram of layered filling process; Figure 3 Schematic diagram of the main structure of a multi-component material mixing and feeding device for mine backfilling; Figure 4 : Schematic diagram of the front section of the mixing screw feeder. Detailed Implementation

[0047] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. The following embodiments are only for illustrative purposes and are not intended to limit the scope of the present invention in any way. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the present invention are conventional methods. Unless otherwise specified, the materials and reagents used in the present invention are commercially available. Furthermore, other terms used in the present invention, unless otherwise specified, generally have the meanings commonly understood by those skilled in the art.

[0048] Example 1: A cementitious material for backfilling tailings in open-pit mines A whole tailings backfill cementitious material for ecological restoration of open-pit mines, comprising the following components by mass percentage: 85% granulated blast furnace slag powder, 10% industrial by-product gypsum, and 5% composite additives; the sum of the mass percentages of each component is 100%.

[0049] The composite additive consists of 25% calcium powder, 45% fly ash, 10% sodium sulfate, 10% dicalcium silicate, and 10% tricalcium silicate, and is used to activate the hydration activity of blast furnace slag, accelerate the generation of early hydration products, and improve the early strength of the slurry. The granulated blast furnace slag powder is S95 grade or above granulated blast furnace slag powder with a specific surface area ≥400m² / kg to ensure sufficient hydration activity; The industrial by-product gypsum is one or more of desulfurized gypsum, phosphogypsum, and fluorogypsum, with a calcium sulfate dihydrate content of ≥85%. After drying and grinding, it has a specific surface area of ​​≥300m² / kg. As a sulfate activator, it reacts with slag hydration products to generate ettringite, thereby improving the early strength and volume stability of the filling body.

[0050] Example 2: Application of cementitious materials in open-pit mine tailings backfilling The application of cementitious materials in open-pit mine tailings backfilling: The backfill slurry is prepared using the cementitious material of Example 1 as the binder, mine tailings as the aggregate, and mixing water as the preparation water. The mass concentration of the backfill slurry is 66%~72%, and the cement-to-tailings ratio is 1:6~1:20.

[0051] The concentration control of 66%-72% is limited by the underflow concentration of the deep cone thickener, which is at most 72%. Under normal operating conditions, the underflow concentration is stable at 68-70%. Pursuing a higher underflow concentration would reduce the equipment's throughput and require the addition of a large amount of flocculant, which would affect the thickener's return water and the flotation operation of the concentrator. By reducing the slurry concentration, reducing the amount of flocculant, and increasing the thickener's throughput, the slurry concentration can be controlled at 66-72% while ensuring strength.

[0052] Test materials: Multi-element analysis was performed on the mixed-configuration tailings from the eastern region according to GB / T6730-2022. The results are shown in Table 1, and the particle size distribution curves are shown in [Table 1]. Figure 1 The tailings have a D50 of 41.40 μm and a D90 of 136.5 μm, with a uniform particle size distribution, making them suitable as aggregate for whole tailings backfill.

[0053] Table 1: Results of multi-element analysis of tailings (wt%)

[0054] Test method: Slurry preparation: For cubic specimens of the entire tailings backfill, 70.7×70.7×70.7mm mortar molds were used. Tailings, water, and cementitious materials were weighed according to the required concentration and mixed in a mortar mixer according to the programmed procedure. The mixed backfill slurry was poured into a mortar bucket while being manually stirred and cast into the mold. After casting, the specimens were leveled and placed at room temperature (laboratory temperature 17-23℃, relative humidity 40-50%). When the specimens were ready to be demolded, they were removed and transferred to a constant temperature and humidity curing chamber for curing. Appropriate mechanical tests were conducted at the designated curing ages.

[0055] Performance Testing: To ensure smooth delivery of the slurry to the mining area via pipelines, the slurry must possess good fluidity. The fluidity index is spread, which refers to the diameter of the slurry after it flows out of the slump cone and spreads out; a larger spread indicates better fluidity. The test was conducted according to the spread test method in the standard "Standard for Test Methods of Performance of Ordinary Concrete Mixtures GB / T 50080-2016".

[0056] In mine backfilling engineering, the bleeding and shrinkage properties of the slurry are important physical indicators. Due to differences in water content and cementitious material quality, backfill slurries with different proportions exhibit varying degrees of bleeding and shrinkage during free settling. Before the slurry sets, some water rises and precipitates as the coarse aggregate settles, which is characterized by the bleeding property of the backfill. As the cementitious material and aggregate set, particles adhere and fill the slurry, resulting in a corresponding decrease in the volume of the slurry after setting, which is characterized by the shrinkage property of the backfill.

[0057] Testing and studying the bleeding and settling properties of slurry can provide theoretical support for actual mine production. Based on the bleeding and settling parameters of slurries with different proportions and concentrations, the required material statistics can be calculated to clarify the filling volume and material consumption, which has certain guiding significance.

[0058] Mechanical property testing: Cast 70.7mm×70.7mm×70.7mm cube test blocks, cure under standard conditions (20℃, relative humidity ≥90%) for 3d, 7d, and 28d, and test the uniaxial compressive strength according to JGJ / T70-2019 "Standard for Test Methods of Basic Performance of Building Mortar".

[0059] Water-rich environment strength test: After the specimen is demolded 24 hours after casting, it is immersed in water for curing for 28 days. The uniaxial compressive strength is then tested to simulate the conditions of water accumulation and rainfall in an open-pit mine.

[0060] Environmental safety testing: In accordance with HJ557-2010 "Leaching Toxicity of Solid Waste - Horizontal Oscillation Method", a toxicity leaching test was conducted on the backfilled solidified body after 28 days of curing to detect the heavy metal content in the leachate.

[0061] The experimental results are shown in Table 2: Table 2: Performance of filling slurry with different ash-sand ratios of the cementitious material of the present invention.

[0062]

[0063] As can be seen from the experimental data in Table 2, the cementitious material of this invention exhibits a slump ≥210mm and a bleeding rate consistently controlled within 12% of the slurry concentration, within a wide range of ash-sand ratios of 1:6 to 1:20. This demonstrates both wide adaptability to ash-sand ratios and high ash-sand ratio hardening performance. The formulation of this invention is adaptable to a full ash-sand ratio range of 1:6 to 1:20. Even at an extremely high ash-sand ratio of 1:20, the 28-day standard curing strength still reaches 1.32MPa, meeting the strength requirement of ≥0.5MPa for normal backfill layers in open-pit mines.

[0064] The slump of the filling slurry is ≥210mm and the bleeding rate is ≤12%, which can meet the requirements of high flow rate and long distance pipeline gravity flow transportation. It has excellent water retention and no segregation or stratification. Under the condition of ash-sand ratio of 1:20, the filling slurry can set and harden normally. The uniaxial compressive strength of the filling body is ≥1MPa after 28 days, which meets the strength requirements of normal backfill layer in open-pit mines.

[0065] Comparative Example 1: Ordinary Portland cement (P.O42.5) was used as the cementing material. The commonly used method in the prior art, using ordinary Portland cement (P.O42.5) as the cementitious material, was used to conduct a comparative experiment to verify the following. The methods for slurry preparation and workability testing were the same as in Example 2. The performance test results are shown in Table 3. The differences in Table 3 refer to the comparison with the corresponding cementitious material data in Example 2.

[0066] Table 3: Performance of Filler Grout with Different P.O42.5 Cement Mix Proportions in Comparative Example 1

[0067] As shown in Table 3, ordinary P.O42.5 cement exhibits a significant decrease in slump and a substantial increase in bleeding rate when the cement-sand ratio exceeds 1:10. At a cement-sand ratio of 1:20, it fails to set and harden properly, resulting in no effective strength. Furthermore, in water-rich environments, the strength of cement-based materials is significantly reduced, making them unsuitable for waterlogged conditions in open-pit mines. Therefore, the cementitious material of this invention significantly improves the performance of cement-based cementitious materials.

[0068] Comparative Example 2: Optimal Formulations of Two Typical Bulk Solid Waste-Based Cementitious Materials in Existing Technologies This invention selects two typical bulk solid waste-based cementitious materials with optimal formulations disclosed in the prior art (corresponding to patent 1: CN121107803A and patent 2: CN117486574A respectively) as comparison objects. Under the same experimental conditions (same tailings, standard curing and water-rich environment curing), the working performance and mechanical properties of these materials under the core working conditions of open-pit mines with ash-sand ratios (1:6, 1:10, 1:20) are systematically tested.

[0069] Experimental methods: The cementitious material was prepared according to the optimal proportions disclosed in patent 1 CN121107803A. The mass parts of each component were as follows: 25 parts desulfurization ash, 20 parts steel slag powder, 15 parts slag powder, 10 parts construction waste powder (total solid waste materials: 70 parts), 15 parts compound activator (triethanolamine and cationic starch in a mass ratio of 1:1), 12 parts desulfurization gypsum, and 3 parts triethanolamine. The fineness of all raw materials was controlled to be above 450 mesh. The cementitious material was obtained by mixing and grinding in a vertical roller mill for 15 minutes.

[0070] The cementitious material was prepared according to the optimal proportions disclosed in patent 2 CN117486574A. The mass fractions of each component were as follows: 80 parts of ferromanganese slag powder, 80 parts of blast furnace slag powder, 16 parts of calcium carbide slag powder, 24 parts of desulfurized gypsum powder, and 0.8 parts of polycarboxylate-based water-reducing agent. The specific surface area of ​​the ferromanganese slag and blast furnace slag powder was controlled to be 600 m² / kg, and the specific surface area of ​​the calcium carbide slag and desulfurized gypsum powder was controlled to be 600 m² / kg. The cementitious material was obtained by mixing the components evenly for 15 minutes.

[0071] Slurry preparation: The slurry mass concentration was fixed at 66% and 72% (the mainstream concentration range for open-pit mine backfilling), and the cement-sand ratio was set at 1:6 (bottom sealing project), 1:10 (transition layer), and 1:20 (normal backfill layer), respectively. The cementitious materials, tailings sand and water were weighed according to the proportions and added to the mortar mixer and stirred for 3 minutes to obtain the backfill slurry.

[0072] Table 4: Performance of Filler Slurry with Different Proportions of Two Typical Bulk Solid Waste-Based Cementitious Materials in Comparative Example 2

[0073] The formula of Patent 1 has a strength of only 0.68-0.84 MPa at a 1:20 cement-sand ratio after 28 days, and the early strength development is slow, which cannot meet the requirements of rapid backfilling. Although the strength of the patent 2 formula is slightly higher at a low ash-sand ratio of 1:6, the strength decreases significantly as the ash-sand ratio increases. At a ash-sand ratio of 1:20, the 28-day strength is only 0.85-0.91 MPa, and there is also the problem of excessively long setting time, which prevents demolding and hardening after seven days.

[0074] From the performance analysis, the formula of this invention has a slump of ≥220mm across the entire lime-sand ratio range, and the bleeding rate is stably controlled within 12%, exhibiting excellent water retention and no segregation or stratification. It can meet the requirements of high-flow-rate, long-distance pipeline gravity transport and large-area homogeneous backfilling in open-pit mines. The formula of Patent 1 has poor slurry fluidity, with a slump of only 215mm at a lime-sand ratio of 1:20 and a bleeding rate as high as 19.5%, which easily leads to uneven strength of the backfill and increased water accumulation in large areas, increasing the mine drainage volume and the cost of water return. The bleeding rate of the formula of Patent 2 is generally 10%~30% higher than that of the formula of this invention, and the bleeding problem is particularly prominent at high lime-sand ratios, which will also increase the drainage load of the open-pit mine and the shrinkage rate of the backfill.

[0075] Adaptability to water-rich environments is a key indicator for open-pit mines. The formula of this invention, through optimization of the hydration system, achieves a 28-day strength retention rate of >90% in water-rich environments, with no pulverization or solidification, perfectly adapting to complex conditions such as rainfall, seepage, and surface water accumulation in open-pit mines. Patent 1's formula, when cured in water, has a 28-day strength retention rate of only 47.1%~73.9%, almost losing effective strength under high ash-sand ratios, making it unsuitable for construction during the rainy season. Patent 2's formula, when cured in water, has a strength retention rate of 53.7%~76.1%, mainly due to the slow hydration rate of the ferrosilicon slag, which severely hinders the hydration process in water-rich environments, failing to guarantee the long-term stability of the backfill.

[0076] This invention's formula achieves continuous gradient control of 28-day strength from 1.32 MPa to 6.58 MPa simply by adjusting the ash-sand ratio, without requiring changes to the raw material formula. It can precisely match the differentiated strength requirements of open-pit to underground mining pit sealing projects, transition layers, and normal backfill layers. Patents 1 and 2 have narrow strength control ranges and significant performance fluctuations under different ash-sand ratios, requiring frequent adjustments to the raw material ratio to meet layered filling requirements, significantly increasing on-site construction complexity. Furthermore, this invention's formula has a total industrial solid waste content of >98% and uses only two major solid wastes (blast furnace slag and industrial by-product gypsum), eliminating the need for regionally specific and unstable-supply solid wastes such as steel slag, ferrosilicon slag, and calcium carbide slag. This results in extremely low raw material procurement costs, controllable below 110 yuan / ton. Patent 1 requires a compounding of four solid wastes, and Patent 2 relies on ferrosilicon slag and calcium carbide slag, leading to a complex raw material supply chain and higher grinding energy consumption, resulting in higher overall production costs than this invention's formula.

[0077] Through systematic comparative studies of various solid waste-based cementitious materials, this invention demonstrates that it has been specifically optimized for the unique requirements of open-pit mine tailings backfilling (wide ash-sand ratio adaptability, water-rich hardening capability, high flow rate transport, and gradient strength control). It significantly outperforms existing publicly available technologies in terms of workability, mechanical properties, environmental adaptability, and economic efficiency. After extensive formula screening and performance verification, the final formula determined by this invention—85% blast furnace slag powder + 10% industrial by-product gypsum + 5% composite additives—is currently the most suitable cementitious material solution for open-pit mine ecological restoration and layered backfilling of open-pit mines transitioning to underground mining.

[0078] Example 3: Environmental Safety Assessment The toxicity leaching test was conducted on the filling solidified body prepared by the cementitious material of the present invention, and the test results are shown in Table 5.

[0079] The toxicity leaching test and hazardous substance detection of the backfill solidified body were conducted according to the "Solid Waste Leaching Toxicity Leaching Method - Horizontal Oscillation Method" (HJ557-2010). The backfill solidified body, cured for 28 days, was removed, crushed, and sieved. A leaching agent was added for leaching, and the leachate was sent to a testing unit for hazardous substance detection. The slurry bleeding water and the leachate from the backfill block were tested separately.

[0080] Table 5. Test results of bleeding and leachate samples (tap water) of cementitious materials with cement-sand ratios of 1:6 and 1:10.

[0081] The samples of cementitious materials with a cement-sand ratio of 1:6 and 1:10, including both the leaching water and the leachate, were alkaline. Their heavy metal content was very low, far below the heavy metal standard values ​​required by the drinking water hygiene standards, surface water environmental quality standards, and integrated wastewater discharge standards.

[0082] Example 4: A layered backfilling process for converting open-pit mines to underground mines Taking Ansteel Mining's Dagushan Iron Mine as an example, the layered filling process can achieve strength gradient control of different filling layers simply by adjusting the ash-sand ratio of the filling slurry, without having to change the raw material formula of the cementitious material, which greatly simplifies the on-site construction process.

[0083] The open-pit mine is a transitional mine from open-pit to underground mining. The backfilling process includes: tailings produced by the ore dressing plant → new backfilling station → high-concentration cemented tailings → pipeline transportation → open-pit backfilling. Backfilling creates conditions for future underground mining, eliminates the risk of water hazard from the open-pit runoff flowing into the mine, provides a direct roof for the subsequent recovery of pillars directly connected to the pit bottom, provides a disposal site for tailings storage, and restores the ecological environment of the open-pit mine, creating a large area of ​​green space (or parkland, agricultural land) – achieving multiple benefits.

[0084] A schematic diagram of the layered filling process is shown below. Figure 2 As shown, the strength control method of the layered filling process is as follows: (1) Bottom sealing project The bottom sealing project is a critical layer for backfilling open pits and should be completed quickly during sunny seasons, excluding winter. The bottom sealing project consists of two parts from bottom to top: the direct bottom slab and the indirect bottom slab, with an elevation range of -414m to -354m.

[0085] 1) Direct base plate After the open-pit mining is completed, loose rocks on the slope are cleared and the site is leveled. After the bottom platform is properly prepared, a steel mesh is laid on the -414m platform. Steel mesh laying parameters: mesh size Φ18@400×400; steel mesh extends to the slope and is laid 2m downhill.

[0086] After the steel mesh was installed, backfilling was carried out using filling grout with a 28-day strength of not less than 4 MPa. The backfill elevation was -414m to -394m.

[0087] 2) Indirect base plate After the direct base slab construction is completed, and after curing for no less than 7 days, backfilling is carried out using filling grout with a 28-day strength of no less than 1.5 MPa and a concentration of 70%. The elevation range is -394m to -354m. After backfilling is completed, curing is carried out for no less than 7 days.

[0088] (2) Remediation Project After the bottom sealing of the open pit is completed and cured for no less than 7 days, the upper area can be backfilled. This design divides the upper area into two parts: a transition layer between -354m and -300m, and a normal backfill layer between -300m and +70m.

[0089] 1) Transition layer The transition layer is backfilled with filling grout with a 28-day strength of not less than 1.0 MPa and a concentration of 70%, with an elevation range of -354m to -300m.

[0090] 2) Normal backfill layer Backfilling between -300m and -102m uses grout with a 28-day strength of not less than 0.5MPa and a concentration of 70%; backfilling between -100m and -98m uses grout with a 28-day strength of not less than 2MPa and a concentration of 70%; and backfilling between -98m and +70m uses grout with a 28-day strength of not less than 0.5MPa and a concentration of 70%.

[0091] The backfilling of open-pit to underground mine pits includes: S1: Conduct on-site investigation and geological and hydrological surveys to ascertain the pit size, slope stability, water inflow, underground pipelines, and rock characteristics. S2: Prepare the preliminary design and safety plan for backfilling, determine the backfilling materials, mix ratio, layer thickness, drainage and slope protection measures, and complete the approval and safety pre-evaluation; S3: Make preliminary preparations, clear dangerous rocks from the pit, seal leaks and abandoned tunnels, improve the drainage system and build temporary facilities for backfilling, water supply, power supply and transportation. S4: Backfill in layers and zones according to the design. First, make a bottom sealing and solidification layer, then fill in layers and drain and maintain at the same time. Control the backfilling sequence and rate to ensure slope stability. S5: After the backfill meets the standards, carry out ecological restoration, including soil covering and shaping, greening, and improving drainage and monitoring facilities; finally, establish a long-term monitoring and operation and maintenance system to monitor settlement, displacement, water level, seepage, etc., to ensure the long-term safety and stability of the backfill and slope.

[0092] The filling process is as follows: The open pit is filled with a full tailings cemented slurry filling process. The slurry is pumped through pipelines or transported by gravity, and discharged at multiple points in the open pit. The slurry is gradually increased in height in the open pit. The water collected in the open pit and the water released from the filling body are discharged to the water purification station outside the open pit for treatment by the floating pump station in the open pit and the existing fixed pump station.

[0093] Compared to the limited filling space in underground mining areas, open-pit pits have a much larger filling space, thus requiring the filling system to achieve continuous filling with a large flow rate. On the other hand, the area to be filled in a single open-pit pit is also larger, so the filling slurry must have good fluidity to fill the entire filling area before the slurry solidifies.

[0094] In response to its characteristics, the open-pit filling adopts a high-flow-rate, high-concentration cemented filling process. The tailings are transported to the tailings concentration facility of the filling station via the tailings distribution pump station for concentration and storage. The concentrated tailings are then transported to the mixing tank by the underflow pump. Cementing powder and water are added as needed, and the mixture is stirred to prepare a high-concentration filling slurry that meets the requirements. The slurry is then transported to the open-pit by gravity (or pumping) through pipelines.

[0095] The filling station is designed to meet the tailings volume processing requirements, with the main equipment being a deep cone thickener. It is equipped with one set of flocculant addition device, three underflow pumps at the bottom of each deep cone, and three DN250 underflow conveying pipelines. Each pipeline is equipped with a regulating valve, flow meter, concentration meter, and several electric valves. Each filling station has a cementitious powder silo with internal partitions and one discharge port. Each discharge port is equipped with a micro-powder scale for quantitative feeding and weighing of the powder. The medium to be mixed is a high-concentration mixture of tailings slurry, cementitious powder, and water concentrated by the thickener. Due to its high flow rate and high concentration, a vertical mixing tank is used for the filling station. The slurry does not require pumping and can be conveyed by gravity. The filling pipeline is laid along the open pit road, and after entering the open pit, it descends along the slope to the open pit backfill working face. The open pit needs to be drained. The drainage process involves creating a water collection area at the lowest point of the pit, then using a floating pump station to pump the water to a secondary pump station, and finally to a tertiary pump station to pump it to a purification station outside the pit.

[0096] Example 5: Mixing and Feeding Device for Multi-Component Materials Used in Mine Backfilling like Figure 3 and Figure 4 As shown, this embodiment provides a multi-component material mixing and feeding device for mine backfilling, including: multiple powder silos, a feeding and metering device, a mixing screw feeder 1, a mixing tank 4, and a dust collection device 7. The powder silos store different components of cementitious materials respectively. A feeding and metering device is fixedly installed below each powder silo. The feeding and metering device is used to weigh the powder and output the powder. The inlet of the feeding and metering device is connected to the powder silo, and the outlet of the feeding and metering device is connected to the powder inlet of the mixing screw feeder 1. The mixing outlet 18 of the mixing screw feeder 1 is connected to the inlet of the mixing tank 4. The top of the mixing tank 4 is connected to the dust collection device 7. The outlet of the dust collection device 7 is connected to the powder inlet of the mixing screw feeder 1. The water inlet of the mixing tank 4 is connected to the liquid supply pipe 61, and the outlet of the mixing tank 4 is connected to the discharge pipe 62.

[0097] like Figure 4 As shown, the mixing screw feeder 1 includes a housing 11, a main shaft 12, a screw blade 13 and a motor 14. The screw blade 13 is located inside the housing 11 and is fixedly connected to the main shaft 12. The main shaft 12 is rotatably connected to the housing 11 and the motor 14 drives the main shaft 12 to rotate.

[0098] During operation, the feeding and metering devices corresponding to each powder silo accurately output various gelling components (powders) of specified weight into the mixing chamber 10 inside the mixing screw feeder 1 according to the proportion. The mixing screw feeder 1 utilizes the synergistic effect of forced shearing, radial tumbling and axial mixing generated by the continuously rotating screw blades 13 during the screw conveying process to ensure that all fresh gelling components and dust-collected return materials are fully and uniformly mixed before entering the mixing tank 4 (the coefficient of variation of mixing uniformity is less than 5%).

[0099] Furthermore, the ratio of the axial length to the diameter of the spiral blade 13 is greater than or equal to 10:1 to ensure that the various cementitious components and the dust collection return material can be fully and uniformly mixed. The shell 11 is a tube sealed at both ends to reduce the outward leakage of dust during mixing.

[0100] Furthermore, the powder inlet and the mixing outlet 18 are respectively fixedly installed on the shell 11. The powder inlet includes the raw material powder inlet and the return powder inlet 17. Each powder bin is connected to the corresponding raw material powder inlet. The outlet of the dust collection device 7 is connected to the return powder inlet 17, which is located between the raw material powder inlets.

[0101] Furthermore, the first powder hopper 2 is connected to the first powder inlet 15, and the second powder hopper 3 is connected to the second powder inlet 16. The first powder inlet 15 and the mixing outlet 18 are located at both ends of the shell 11, the second powder inlet 16 is located in the middle of the shell 11, and the return powder inlet 17 is located between the second powder inlet 16 and the first powder inlet 15. The discharge amount of the first powder inlet 15 is greater than the discharge amount of the second powder inlet 16.

[0102] In another embodiment, all powder silos and dust collection device 7 use a single powder inlet for their discharge ports.

[0103] Furthermore, the feeding and metering device includes a feeder and a micro powder scale. The discharge port at the lower end of the powder silo is fixedly connected to and communicates with the feeder's inlet. The discharge port of the feeder is fixedly connected to and communicates with the inlet of the micro powder scale. The discharge port of the micro powder scale is connected to the powder inlet of the mixing screw feeder 1. During operation, the feeder delivers the gelling component to the micro powder scale. The micro powder scale weighs the gelling component passing through the micro powder scale and controls the discharge speed of the feeder based on the weighed weight to ensure that a specified weight of gelling component is delivered to the mixing screw feeder 1 per unit time.

[0104] like Figure 4As shown, the powder silo, feeder, and micro powder scale are connected in a one-to-one manner. The first powder silo 2 is connected and fixedly connected to the first feeder 21, and the first feeder 21 is connected and fixedly connected to the first micro powder scale 22. The second powder silo 3 is connected and fixedly connected to the second feeder 31, and the second feeder 321 is connected and fixedly connected to the second micro powder scale 32. It may also include multiple distribution silos. like Figure 3 As shown, it also includes a thickener 5 and a slurry pump 6. The inlet of the slurry pump 6 is connected to the outlet at the bottom of the thickener 5, and the outlet of the slurry pump 6 is connected to a feed pipe 61. The feed pipe 61 is equipped with a flow meter and a flow control valve; the discharge pipe 62 is equipped with a flow meter and a flow control valve. The uniformly mixed cementitious material from the mixing screw feeder 1 and the tailings slurry, whose flow rate is precisely controlled by the electromagnetic flow meter and electric regulating valve via underflow conveying from the thickener 5, undergo efficient liquid-solid two-phase mixing in the mixing tank 4 to finally prepare a filling slurry. The thickener 5 separates the liquid from the tailings slurry and outputs it to the slurry pump 6. In another embodiment, the feed pipe 61 is directly connected to a water source.

[0105] It is understood that those skilled in the art can make equivalent substitutions or modifications to the technical solutions and concepts of this invention, and all such substitutions or modifications should fall within the protection scope of the appended claims.

Claims

1. A cementitious material for backfilling tailings in open-pit mines, characterized in that, By mass percentage, it includes the following components: 80%~90% granulated blast furnace slag powder, 5%-15% industrial by-product gypsum, and 0.5%-5% composite additives; the sum of the mass ratios of all components is 100%.

2. The cementitious material for backfilling tailings in open-pit mines according to claim 1, characterized in that, The granulated blast furnace slag powder is S95 grade or higher granulated blast furnace slag powder with a specific surface area ≥400 m². 2 / kg.

3. The cementitious material for backfilling tailings in open-pit mines according to claim 1, characterized in that, The industrial by-product gypsum is one or more of desulfurized gypsum, phosphogypsum, and fluorogypsum, with a calcium sulfate dihydrate content ≥85% by mass, and after drying and grinding, has a specific surface area ≥300 m². 2 / kg.

4. The cementitious material for backfilling tailings in open-pit mines according to claim 1, characterized in that, The composite additive is one or more of quicklime, hydrated lime, fly ash, sodium sulfate, and sodium tripolyphosphate.

5. The application of the cementitious material as described in any one of claims 1-4 in the backfilling of tailings in open-pit mines.

6. The application according to claim 5, characterized in that, The filling slurry is prepared by using cementitious materials as binders, mine tailings as aggregates, and mixing water as preparation water; the mass concentration of the filling slurry is 66%~72%; the ash-sand ratio of cementitious materials and mine tailings in the filling slurry is 1:6~1:

20.

7. The application according to claim 6, characterized in that, The filling slurry is prepared using a multi-component material mixing and feeding device for mine filling. This multi-component material mixing and feeding device for mine filling includes: The system includes multiple powder silos, a feeding and metering device, a mixing screw feeder (1), a mixing tank (4), and a dust collection device (7). The powder silos store different components of cementitious materials. Each powder silo is fixedly equipped with a feeding and metering device below it. The inlet of the feeding and metering device is connected to the powder silo. The outlet of the feeding and metering device is connected to the powder inlet of the mixing screw feeder (1). The mixing outlet (18) of the mixing screw feeder (1) is connected to the inlet of the mixing tank (4). The top of the mixing tank (4) is connected to the dust collection device (7). The outlet of the dust collection device (7) is connected to the powder inlet of the mixing screw feeder (1). The water inlet of the mixing tank (4) is connected to the liquid supply pipe (61). The outlet of the mixing tank (4) is connected to the discharge pipe (62).

8. A layered backfilling process for converting open-pit mines to underground mines, characterized in that, Using the cementitious material as described in any one of claims 1-4 as the binder, using mine tailings as aggregate, and using mixing water as preparation water, a filling slurry is prepared; a layered filling process is adopted, and the strength gradient of different filling layers is controlled by adjusting the ash-sand ratio of the filling slurry.

9. The layered backfilling process for converting an open-pit mine to an underground mine according to claim 8, characterized in that, The open-pit to underground mining backfilling process includes bottom sealing works and treatment works; the bottom sealing works include a direct bottom slab and an indirect bottom slab; the treatment works include a transition layer and a normal backfill layer.

10. The layered backfilling process for converting an open-pit mine to an underground mine according to claim 9, characterized in that, The direct base slab is backfilled with filling grout with a 28-day strength of not less than 4 MPa; the indirect base slab is backfilled with filling grout with a 28-day strength of not less than 1.5 MPa and a concentration of 70%; the transition layer is backfilled with filling grout with a 28-day strength of not less than 1.0 MPa and a concentration of 70%; and the normal backfill layer is backfilled with filling grout with a 28-day strength of not less than 0.5-2 MPa and a concentration of 70%.

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