A method for estimating the total amount of leaching agent remaining in the soil of a closed rare earth in-situ leaching mine

Abstract

A method for estimating the total amount of residual ammonium nitrogen in the soil of a closed rare earth in-situ leaching mine. Representative sampling points are selected at different slope positions, and fresh soil samples and undisturbed soil samples are collected layer by layer until the bedrock layer; the soil water content is determined by the drying method; potassium chloride solution is added to the fresh soil samples transported and stored at low temperature, and the supernatant is extracted after shaking, and the ammonium nitrogen content is detected using a flow analyzer; the soil samples of known volume are dried in a constant temperature drying oven, and the soil bulk density is calculated according to the volume of the soil sample before drying and the weight of the soil after drying; the slope position demarcation points and altitude are determined, and combined with the high-precision remote sensing image data of the closed mine, the top and bottom areas of each part of the closed mine are calculated; a model for estimating the total amount of residual ammonium nitrogen in the soil is established; parameters such as ammonium nitrogen content, bulk density, soil layer thickness, and top and bottom areas of different levels of the mountain body are substituted into the estimation formula to calculate the total amount of residual ammonium nitrogen in the soil of the closed mine.

Description

Technical Field

[0001] The present invention relates to a method for estimating the total amount of residual pollutants in the soil of a closed mine. Specifically, the residual pollutant content and soil bulk density of a representative soil layer of a mountain are determined through on-site sampling. At the same time, the volume of the mountain soil is estimated through remote sensing images of the mountain and the thickness of the soil layer. A residual pollutant calculation equation is established to estimate the total amount of pollutants such as residual leaching agents in the mountain soil, thereby being able to determine the input amount of leaching agents and the restoration cost when remediating a contaminated site. Background Art

[0002] Rare earth elements are a general term for 17 special elements, including 15 lanthanide elements, scandium (Sc) and yttrium (Y). Adding rare earth elements in the industrial production process will greatly improve the performance of materials and products. Therefore, rare earth elements have become an indispensable material for high-tech products and are widely used in various fields such as industry, agriculture and military. They are recognized as important strategic resources in the world.

[0003] Since the late 1960s, my country has a history of more than 50 years of rare earth mining. At present, the main method of mining ionic rare earth ores is to use in-situ leaching. This mining process does not damage the surface vegetation. The leaching agent (3% to 5% ammonium sulfate) is directly injected into the ore-bearing layer through the injection well. The rare earth ions are exchanged with ammonium roots, desorbed from the soil colloids and enter the soil solution. For every ton of rare earth oxide produced, 5 tons of ammonium sulfate are required. Therefore, after the rare earth in-situ leaching mining is completed, there is an excess of leaching agent in the closed mine, especially an excess of ammonium nitrogen in different forms. Although ammonium is an essential nutrient for plants, the excess ammonium nitrogen remaining in the soil of the in-situ leaching site has already had a serious impact on the local ecological environment, and its long-term harm is still unclear.

[0004] Field surveys and sampling in recent years have found that the soil and surrounding water bodies in the in-situ leaching area are seriously polluted by ammonium nitrogen. The ammonium nitrogen content in the soil of a rare earth closed mine in southern my country is 2-1056 mg kg -1 , the average content reached 263mg·kg -1 The ammonium nitrogen content in the main channel downstream of the mining area is 27-161 mg·L -1 , average value 95mg·L -1 , exceeding the "Rare Earth Industry Pollutant Emission Standard" (GB26451-2011) by 15 mg·L -1 The content of ammonium nitrogen in the soil and water will continue to be converted into nitrate nitrogen and produce H + The nitrate nitrogen content in the water in the mining area is 41-186 mg·L -1, the pH of the soil is 4.0-5.4. It can be seen that high concentrations of ammonia nitrogen will further lead to a series of environmental problems such as soil acidification, soil heavy metal activation, water eutrophication and eutrophication, affecting the production and life of residents near and downstream of the closed mine area. Therefore, the state has introduced a series of rare earth mining restrictions and bans, and ordered relevant departments to control the ammonium nitrogen pollution in the rare earth closed mine sites that have been mined, so that the ammonium nitrogen content of surface water is within the limit specified in the "Rare Earth Industry Pollutant Emission Standard" (GB26451-2011). At present, the treatment of residual leaching agents in closed mines requires the input of a certain amount of leaching agent to exchange the ammonium nitrogen adsorbed in the soil and further leach out the soil. Therefore, the input amount of leaching agent depends on the total amount of ammonium nitrogen in the soil in the closed mine. The input of excessive leaching agent will not only increase the cost of treatment, but may also introduce new sources of pollution. Therefore, while ensuring the treatment effect, the input of leaching agent should be controlled as accurately as possible. However, there is currently a lack of methods to estimate the total amount of ammonium nitrogen in a mountain within a specific closed mining area.

[0005] Although the ammonium nitrogen content in the soil can be obtained through sampling and measurement, the existing measurement data show that even in the same mine after mining, the ammonium nitrogen content in different parts of the mountain or at different depths in the same part is very different, and the highest content and the lowest content can differ by 2 orders of magnitude. In view of such complex changes in the ammonium nitrogen content in the soil, how to measure the residual amount based on the ammonium nitrogen content at different depths is one of the difficulties in current research.

[0006] The ore-bearing layer of the rare earth mine is a semi-weathered body of granite, with a relatively thick soil layer of different development levels covering the upper part and a bedrock layer below. According to actual field surveys, the thickness of the soil layer in different parts of the mountain varies greatly. In view of such a complex mountain structure, how to accurately measure the volume and quality of the mountain soil is the second difficulty in current research. Summary of the invention

[0007] Technical problem to be solved: In view of the above-mentioned defects, the present invention provides a method for estimating the total amount of residual leaching agent (ammonium nitrogen) in the soil of closed mines subjected to in-situ leaching of rare earths. The method is applicable to mountains with relatively regular shapes. The residual ammonium nitrogen content and bulk density of the soil at different points of the closed mine are determined in layers. The total amount of residual ammonium nitrogen in the soil of the closed mine is estimated based on the mine terrain conditions and soil layer thickness, thereby determining the pollution situation of closed mines subjected to rare earth leaching. The method can also be used to estimate the total input amount of leaching agent in the subsequent pollution control process and determine the control cost.

[0008] Technical method: A method for estimating the total amount of residual ammonium nitrogen in the soil of a closed rare earth in-situ leaching mine, comprising the following steps: Step 1, determine the sampling points and vertically sample downward layer by layer to obtain fresh soil samples; Step 2, weigh the fresh soil samples, record the weight and then dry them, and calculate the soil water content according to the soil sample quality before and after drying; Step 3, weigh 10.0 g of fresh soil samples and put them into a 200 mL Erlenmeyer flask, add 50.0 mL of potassium chloride solution with a concentration of 2 mol·L -1 seal it, shake for 30 min and then filter, and use a flow analyzer to detect the content of ammonium nitrogen (calculated as nitrogen); Step 4, dry the soil samples with a known volume in a constant temperature drying oven, and calculate the soil bulk density according to the total volume of the soil samples before drying and the dry soil weight; Step 5, determine the slope position demarcation point positioning and altitude, and combine the high-precision remote sensing image data of the closed mine to calculate the top and bottom areas of each part of the closed mine; Step 6, establish an estimation model for the total amount of residual ammonium nitrogen (calculated as nitrogen) in the rare earth closed mine; Step 7, substitute the soil layer thickness, the ammonium nitrogen content of the corresponding soil layer, the soil bulk density and the bottom area of the terrain part into the estimation formula for calculation to obtain the total amount of residual ammonium nitrogen in the soil of the leaching mine site.

[0009] The sampling points shall be determined according to the injection range of the leaching agent during the rare earth ore mining process and combined with the water flow direction of the mine water, and at least three sampling points shall be arranged along the water flow direction from the top of the mountain to the bottom of the mountain.

[0010] The layered sampling is as follows: Sampling is carried out according to the genetic horizon in the soil from the surface layer to 1 m of the soil body. After mixing evenly every 50 cm below 1 m, a sample is collected once. If obvious changes occur in the observable morphological properties such as soil texture, color and structure within the 50 cm thickness range at a certain depth, sampling is divided into two sections.

[0011] The construction method of the estimation model for the total amount of residual ammonium nitrogen in the rare earth closed mine is as follows: The model approximates the closed mine as a frustum of a cone, divides it into three parts: the top of the slope, the middle of the slope and the bottom of the slope according to the slope position and sampling points, establishes the soil volume formula for different layers, and simplifies it to obtain the soil volume formula (1) for a certain soil layer at a certain slope position; Multiply and sum the ammonium nitrogen content and bulk density of the corresponding soil layer with formula (1), and the obtained formula (2) can estimate the total amount of residual ammonium nitrogen in the closed mine;

[0012] V(m 3 ) = Δh(S2 - S1) (Formula 1) In the formula, V is the total volume of this layer of soil (m 3 ); Δh is the soil layer thickness (m), that is, the interval sampling depth; S1 is the top area of the corresponding terrain part (m 2 ); S2 is the bottom area of the corresponding terrain part (m 2 );

[0013]

[0014] In the formula, m a is the total amount of residual ammonium nitrogen (kg) at slope position a; S 1a is the top surface area (m 2 ) at slope position a; S 2a is the bottom surface area (m 2 ) at slope position a; Δh i is the thickness (m) of the i-th soil layer, that is, the interval sampling depth; b i is the bulk density of the i-th soil layer (g·cm -3 ); c i is the content of residual ammonium nitrogen in the i-th soil layer (mg·kg -3 ); 1 to n respectively represent the first layer (the soil layer closest to the ground surface) to the n-th layer (the soil layer closest to the bedrock).

[0015] Beneficial effects: (1) The present invention only needs to collect a small amount of soil samples at representative points in each slope position to detect the actual soil ammonium nitrogen content and distribution in the closed mine, and the shape and area of the closed mine mountain can be obtained more conveniently and accurately through remote sensing images. This method significantly improves the convenience and accuracy of estimating the total amount of residual ammonium nitrogen in the closed mine.

[0016] (2) The present invention can estimate the total amount of residual leaching agent in the closed mine and clarify the pollution degree of the in-situ leaching rare earth mining area. Combining the occurrence form of residual ammonium nitrogen in the closed mine, the total amount of ammonium nitrogen that the mine can release to the surrounding environment can be estimated to evaluate the potential environmental risks existing in the closed mining area.

[0017] (3) After calculating the total amount of residual ammonium nitrogen in the soil of the closed mining area by the present invention, for different treatment schemes and types of leaching agents, the total amount of leaching agent required in the treatment process can be calculated according to the total amount of residual ammonium nitrogen in the mine to determine the treatment cost, which is beneficial to controlling the total amount of leaching agent to be added during treatment and avoiding secondary pollution to the environment caused by excessive input of leaching agent.

[0018] (4) The method for estimating the total amount of residual ammonium nitrogen in the soil of the closed rare earth in-situ leaching mine proposed by the present invention can provide an estimation basis for research such as soil environmental risk assessment, soil pollution remediation, and mine reclamation.

[0019] (5) In addition to being applied to the in-situ leaching rare earth mining area, the present invention can also be applied to the estimation of the total amount of residual pollutants in mining areas of other types of minerals, and can also be applied to the treatment of mine polluted soil and pollutants in the closed mining area, having a broad application market and potential. Description of the drawings

[0020] Figure 1 Overall flow chart for estimating the total amount of residual ammonium nitrogen in the soil of the closed rare earth in-situ leaching mine;

[0021] Figure 2Sampling tools: Luoyang shovel, oilcloth, sealed bag, ice pack, insulated box;

[0022] Figure 3 Mountain terrain parts and soil layer distribution;

[0023] Figure 4 Schematic diagram of different terrain parts of the closed mine;

[0024] Figure 5 Cross-section of the geometric body for auxiliary calculation of soil layer volume;

[0025] Figure 6 Contour map of the closed mine and boundaries of each slope position;

[0026] Figure 7 Soil profile samples at the middle of the slope of the closed mine;

[0027] Figure 8 Profile distribution of ammonium nitrogen content in the soil of the closed mine. Specific implementation plan

[0028] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific examples.

[0029] Fresh soil samples of the in-situ leaching mine site are collected layer by layer on-site. The ammonium nitrogen content of each layer of soil is measured using a flow analyzer, and the soil bulk density of the corresponding soil layer is measured using the drying method (the sample volume is known). The boundary and area of the leaching area are determined according to remote sensing image data. Substitute each parameter into the estimation formula to obtain the total residual amount of ammonium nitrogen in the leaching mine site ( Figure 1 ).

[0030] Example 1:

[0031] First step: According to the terrain and mining situation of the rare earth in-situ leaching mine, determine the sampling points, sample vertically downward layer by layer, and preserve and transport the fresh soil samples at low temperature.

[0032] (1.a): Determine the range and geographical coordinates of the leaching area and the specific locations of mining facilities such as leaching solution ponds and injection wells according to mining data. Determine the surface runoff direction according to the local hydrogeological conditions. Slightly away from the mining facilities, three sampling points are evenly arranged at the top, middle and bottom of the leaching mountain body along the runoff direction. Try to keep the points on a straight line. Use GPS to record the specific longitude and latitude coordinates of each point.

[0033] (1.b): Remove the surface dead branches and leaves, and use a Luoyang shovel with a diameter of 10 cm ( Figure 2)Collect soil samples vertically downward from the ground surface. The soil layers from the ground surface to a depth of 1 m are divided according to the genetic horizons. The soil deeper than 1 m is divided into layers with a boundary of 50 cm. If there are obvious changes in soil texture, compactness, color or significantly different weathering degrees within 50 cm, it needs to be divided into two layers. The soil samples are collected down to the bedrock surface. When approaching the bedrock surface, there will be obvious resistance during the downward hammering of the Luoyang shovel, and large stones with obvious rock structures will appear in the collected soil.

[0034] (1.c): First, place the soil samples of each layer collected by the Luoyang shovel on a 1 m × 1 m white oilcloth, place a scale value beside it, take a photo after arranging the soil samples according to the depth. Wear rubber gloves to mix the soil samples of this layer evenly, use the quartering method to take more than 150 g of soil samples and place them in a plastic self-sealing bag for sealing and preservation. Make marks on the outside of the sample bag with a black marker, including: sampling point, sample depth, sampling time and sampling location. Fill enough ice bags with water and freeze them 1 day in advance. Place the ice bags in the incubator before sampling to keep the temperature in the box below 4 °C. Put the collected and packaged soil samples into the incubator and replace the ice bags in time. The soil samples need to be kept at low temperature until the detection of ammonium nitrogen and water content (try to extract within 48 h after sampling).

[0035] (1.d): While collecting fresh samples, it is necessary to collect undisturbed samples of known volume for each layer to measure the bulk density of the soil in this layer. For the surface soil, use a knife to change and collect samples. Clean the ground with an area of about 30 cm × 30 cm near the sampling point, place the core cutter and the guide rod perpendicular to the ground, hammer the guide rod to drive the core cutter vertically into the soil, and use a pickaxe to take out the core cutter (without damaging the sample inside the core cutter), and use a soil trimming knife to trim the two sides of the core cutter flat. Place the collected soil samples in a self-sealing bag (pre-weighed in advance), and record the sampling depth, sampling quantity, sampling point, sampling time and sampling location. For the deep soil undisturbed samples, use the Luoyang shovel to hammer downward 3 cm (which can be adjusted according to the compactness of the soil in different layers), and directly put the extracted soil samples into a self-sealing bag (pre-weighed in advance), and make the same marks and store them at room temperature.

[0036] Second step: Weigh the aluminum box (with known mass) containing the fresh soil samples on the balance. After recording the weight, place it in a preheated constant temperature drying oven at 105 °C for 6 to 8 h, take it out and put it in a desiccator until it reaches room temperature and then weigh it. Calculate the soil water content according to the mass of the soil samples before and after drying.

[0037] (2.a): Weigh the dry aluminum box (already numbered) with a balance, accurate to 0.01 g. Take out the fresh soil sample stored at low temperature from the refrigerator and put it into the corresponding numbered aluminum box (it is advisable to fill the aluminum box to 1 / 2 - 4 / 5). Weigh it, accurate to 0.01 g. Put the aluminum box containing the soil sample (without the lid) into an oven preheated to 105 ± 2 °C and dry it to a constant weight, which takes about 6 - 8 h. Wear heat-insulating gauze gloves to take out the sample, transfer it into a desiccator and cool it to room temperature (about 30 min), then weigh it immediately, accurate to 0.01 g.

[0038] (2.b): Substitute the data into Formula 3 to calculate the soil water content.

[0039]

[0040] In the formula, m0 is the mass of the dried empty aluminum box (g); m1 is the mass of the aluminum box and the soil sample before drying (g); m2 is the mass of the aluminum box and the soil sample after drying (g).

[0041] Step 3: Weigh 10.0 g (accurate to 0.1 g) of fresh soil sample and put it into a 200 mL Erlenmeyer flask. Add 50.0 mL of KCl solution with a concentration of 2 mol·L -1 Seal it, shake it for 30 min and then filter it. Use a flow analyzer to detect the ammonium nitrogen content.

[0042] (3.a): Prepare 2 mol·L -1 KCl solution. First, weigh 149.1 g of KCl (analytical pure), then dissolve it with distilled water, dilute it to nearly 1 L and make it up to 1 L with distilled water.

[0043] (3.b): Weigh 10.0 g of fresh soil sample, accurate to 0.01 g. Put the sample into a 200 ml Erlenmeyer flask, avoiding touching the bottle wall. Measure 50 mL of 2 mol·L -1 KCl solution with a 50 mL measuring cylinder and slowly add it along the bottle wall into the Erlenmeyer flask. Tighten the rubber stopper and shake well. At the same time, set up parallel samples and blank samples.

[0044] (3.c): Set the vibration amplitude of the shaker to 180 ± 20 r·min -1 , and the temperature to 20 °C. Put the Erlenmeyer flask into a centrifuge and shake it for 30 min. After shaking well, filter it with medium-speed qualitative filter paper into a 50 mL Erlenmeyer flask.

[0045] (3.d): Use a flow analyzer to measure the ammonium nitrogen content in the leaching solution;

[0046] (3.e): According to the ammonium nitrogen concentration of the leaching solution and the measured soil water content, substitute them into Formula 4 to calculate the ammonium nitrogen content of the soil sample.

[0047]

[0048] In the formula, ρ is the mass concentration of ammonium nitrogen in the extraction solution (mg·L -1 ); V is the volume of the extraction solution (mL); m is the sample mass (g); 10 3 is for converting mL to L; 1000 is for converting to the content of ammonium nitrogen per kg of soil.

[0049] Step 4: Dry the soil sample with a known volume in a constant-temperature drying oven, and calculate the soil bulk density based on the total volume of the soil sample before drying and the total dry soil weight.

[0050] (4.a): Weigh the total weight of the collected soil sample and the self-sealing bag with a balance, and subtract the weight of the self-sealing bag (pre-weighed before sampling) to obtain the total wet soil weight.

[0051] (4.b): Determine the water content of the soil sample according to the operation steps in Step 2.

[0052] (4.c): Calculate the total volume of the soil sample. The volume of the sample collected by the core sampler is the core sampler volume, and the volume of the sample collected by the Luoyang shovel is the collection depth × (inner radius of the Luoyang shovel) 2 ×π.

[0053] (4.d): Substitute the above parameters into Formula 5 to calculate the soil bulk density of the corresponding sample.

[0054]

[0055] In the formula, b is the soil bulk density (g·cm -3 ); m is the total wet soil weight (g); w is the soil water content (%); V is the total sample volume (cm 3 ), here is the inner radius of the Luoyang shovel, which is 5 cm.

[0056] Step 5: Determine the location of the demarcation points and the altitude of each slope position, and calculate the top and bottom areas of each part of the closed mine in combination with the high-precision remote sensing image data of the closed mine.

[0057] (5.a): Determine the longitude and latitude of the closed mine, and download the mine DEM image with appropriate accuracy according to the size of the closed mine range.

[0058] (5.b): Determine the altitude of the demarcation points of each mountain body part, use the DEM to extract the contour lines to ensure that the elevation differences of each divided part of the mountain body are consistent. Calculate the elevation differences of each demarcation point.

[0059] (5.c): Take the closed contour line with the same altitude as the demarcation point as the slope position demarcation line, and calculate the area within the demarcation line, which is the bottom area at the corresponding slope position. If the contour line is not closed, it needs to be processed by software into a closed line for area calculation.

[0060] Step 6: Establish an estimation model for the total amount of residual ammonium nitrogen.

[0061] (6.a): Since the ammonium nitrogen content in the soil at different topographic positions and different depths of the closed mine varies greatly, the enrichment amount of ammonium nitrogen in the soil at each position should be calculated layer by layer, and the total amount of residual ammonium nitrogen in the soil of the closed mine can be obtained by accumulation. Therefore, it is first necessary to estimate the volume of the soil at each layer for each part, where the position of each sample point and the depth of each soil layer are determined on-site during field sampling.

[0062] (6.b): Divide the mined mountain body into three topographic parts: the top of the slope, the middle of the slope, and the bottom of the slope. Since the interior of the mountain body is deep bedrock and the soil adheres to the surface of the bedrock, forming a "shell" thinner than the rock. Soil samples are collected along the vertical direction (perpendicular to the horizontal plane). Therefore, considering the soil layer thickness, assume that each topographic part is Figure 3 、 4 the geometric body shown. Calculating the volume according to this assumption can ensure that the upper and lower surface areas of each layer of the "soil shell" at the same position are the same, and the same formula and the same area data can be used to calculate the volume of different soil layers at the same position, reducing variables to simplify the calculation.

[0063] Among them, the top of the slope is approximately a cone + a bottom cylinder, and the middle and bottom of the slope are approximately a frustum of a cone + a large bottom cylinder - a small top cylinder (which is the bottom cylinder of the higher-level topographic part). Therefore, the total volume (soil + bedrock) of each part can be approximately calculated using Formula 6.

[0064]

[0065]

[0066] In the formula, h1 is the height of the frustum of a cone or cone (m); h2 is the thickness of the soil layer overlying the bedrock (m); S1 is the top surface area of the frustum of a cone (m 2 ), and here it can be considered that the top surface area of the cone is 0; S2 is the bottom surface area of the frustum of a cone or cone (m 2 ).

[0067] The volume of the bedrock in each part can be considered as a cone (at the top of the slope) or a frustum of a cone (in the middle and bottom of the slope) embedded in the composite body, and the volume of the bedrock is calculated according to Formula 7.

[0068]

[0069]

[0070] In the formula, h1 is the height of the frustum or cone (m); S1 is the top surface area of the frustum (m 2 ), and here the top surface area of the cone can be considered as 0; S2 is the bottom surface area of the frustum or cone (m 2 ).

[0071] The overall volume of the soil layer in each part can be regarded as a shell covering the surface of the cone or frustum, and the overall soil volume is calculated according to formula 8.

[0072] Vtotal soil layer at the slope top (m 3 ) = Vsoil layer at the slope top - Vbedrock at the slope top = h ＝ (S2 - S1)

[0073] Vtotal soil layer at the middle or bottom of the slope (m 3 ) = Vsoil layer at the middle or 坡底 - V 坡中 or 坡底基岩 = h2(S2 - S1) (Formula 8) In the formula, h2 is the thickness of the soil layer overlying the bedrock (m); S1 is the top surface area of the frustum or cone (m 2 ), and here the top surface area of the cone can be considered as 0; S2 is the bottom surface area of the frustum or cone (m 2 ).

[0074] In the above formulas 6, 7, and 8, h1 (the height of the frustum or cone) corresponds to the elevation difference at the demarcation points of different parts in the actual mountain body. S1 and S2 correspond to the areas of the upper and lower demarcation surfaces at different terrain parts. The elevation difference and the demarcation surface area can both be obtained through step five.

[0075] (6.c): Divide the soil layers according to the collection depth. The total volume of the soil in different layers depends on the soil layer thickness. The volume calculation of each soil layer (with a thickness of Δh) can also be carried out according to the above overall soil volume calculation idea (assuming h2 = Δh). Taking the bottom of the slope (frustum + cylinder) as an example to derive the volume of the soil in different layers, the soil layer with a thickness of Δh can be regarded as the remaining part after removing the small cylinder of the frustum from the large cylinder combination of the frustum ( Figure 5 ), where the two combinations are auxiliary geometric bodies used to calculate and simplify the soil layer volume, only for calculation reference.

[0076]

[0077]

[0078] V i (m 3 ) = V 2i - V 1i = Δh i S2 - Δh i S1 = Δh i × (S2 - S1) (Formula 9) In the formula, V 1iis the volume of the frustum-small cylinder composite at the i-th soil layer ( Figure 5 medium dark gray part); V 2i is the volume of the frustum-large cylinder composite at the i-th soil layer ( Figure 5 medium light gray part); V i is the volume of the i-th layer of soil ( Figure 5 medium black gray part); S1 is the top surface area of the frustum (m 2 ); S2 is the bottom surface area of the frustum (m 2 ); h is the height of the frustum (m); Δh i is the thickness of the i-th layer of soil (m).

[0079] The derivation and calculation process of the soil layer volume formula in the middle and bottom of the slope are exactly the same; at the top of the slope, it is approximately a cone + cylinder. Therefore, it can be considered that the top of the slope is a frustum + cylinder with a top surface area of 0, and the volume calculation of each soil layer at the top of the slope can also use formula 9, just let S1 = 0.

[0080] (6.d): Multiply the volume of each soil layer by the soil bulk density and ammonium nitrogen content of that layer, and then sum them up to obtain the total amount of residual ammonium nitrogen at the corresponding slope position. Then, add the residual ammonium nitrogen at each slope position to obtain the total amount of residual ammonium nitrogen in the soil of the rare earth closed mine.

[0081]

[0082] In the formula, m a is the total amount of residual ammonium nitrogen at slope position a (kg); S 1a is the top surface area at slope position a (m 2 ); S 2a is the bottom surface area at slope position a (m 2 ).; Δh i is the thickness of the i-th layer of soil at slope position a (m), that is, the interval sampling depth; b i is the soil bulk density of the i-th layer of soil at slope position a (g·cm -3 ); c i is the residual ammonium nitrogen content of the i-th layer of soil at slope position a (mg·kg -3 ). 1 to n represent the first layer (the soil layer closest to the surface) to the n-th layer (the soil layer closest to the bedrock) respectively.

[0083] m 闭矿山 (kg) = m 坡顶 + m 坡中 + m 坡底 (Formula 10) In the formula, m 闭矿山 is the total amount of residual ammonium nitrogen in the soil of the rare earth closed mine (kg); m 坡顶 , m 坡中 , m 坡顶 are the total amounts of residual ammonium nitrogen in the soil at the top, middle and bottom of the closed mine slope (kg) respectively.

[0084] Step 7: Substitute the soil layer thickness, the ammonium nitrogen content in the corresponding soil layer, the soil bulk density, and the bottom area of the terrain part into the estimation model to obtain the total amount of residual ammonium nitrogen in the soil of the in-situ leaching site.

[0085] Note: 1. During the treatment of pollutants in Weikuanshan, it is necessary to calculate the input amount of the leaching agent according to the total amount of pollutants. For the convenience of calculation, it is necessary to convert the total amount of pollutants from mass to amount of substance. 2. To facilitate understanding of the pollution status of the closed mine, the volume content of pollutants in the mountain body (g·cm -3 ) can also be calculated from the total amount of pollutants.

[0086] Taking a certain rare earth in-situ leaching area in southern Jiangxi as an example.

[0087] The study area is located in a rare earth mining area in Jiangxi Province, in the subtropical monsoon climate zone. The parent rock is acidic granite, the main soil type is humid ferralic soil, and the surface vegetation is a natural forest mainly composed of Masson pine and Chinese fir. The selected mine in this study is a typical ionic rare earth mine. After in-situ leaching mining with ammonium sulfate as the leaching agent, the mine has been closed for 4 years, and there is a large amount of ammonium nitrogen in the mountain body.

[0088] (1) Field sampling and investigation

[0089] The sampling points were selected at the positions between the injection wells, far from the collecting ditch. Sampling points were arranged at the top, middle, and bottom of the slope respectively. The specific positions are shown in Figure 6 . Among them, the sampling depth at the top of the slope is 9.7 m, and a total of 25 samples were collected; the sampling depth in the middle of the slope is 8.0 m, and a total of 18 samples were collected; the sampling depth at the bottom of the slope is 5.5 m, and a total of 13 samples were collected.

[0090] Taking the middle of the slope as an example, the layering rule in the specific sampling process is described as follows. First, the soil within 1 m deep from the surface has a higher degree of development. Therefore, according to the soil genetic horizons, sampling is carried out in sections. A total of 3 soil samples were collected within 1 m deep at the sampling point in the middle of the slope, which are 0 - 8 cm, 8 - 50 cm, and 50 - 100 cm respectively. The soil at 0 - 8 cm deep is dark brown, and there is some dead branches and leaves in the soil, which is looser and fluffier than the underlying soil ( Figure 7 ), because the humus content in this layer of soil is relatively high, which can be called the A layer in soil science; the soil at 8 - 100 cm deep is overall dark brown, darker in color but lighter than the surface soil. At the same time, this layer of soil is more compact and sticky, and it can be classified as the B layer. Since there is no obvious change in the morphological characteristics of the soil within the range of 8 - 100 cm, sampling is carried out in layers with 50 cm as the boundary, divided into two parts: 8 - 50 cm and 50 - 100 cm.

[0091] Samples were collected once every 50 cm when the depth was less than 1 m after thorough mixing. If obvious changes occurred in observable morphological properties such as soil texture, color, and structure within a 50-cm thickness range, sampling was carried out in two sections. There were no significant differences in the soil morphology at a depth of 100 - 150 cm in the middle of the slope compared to the upper layer, and it was still the B-layer soil, so it was classified as one layer; within the range of 150 - 200 cm, the soil transitioned from the B-layer to the C-layer, the soil color became lighter, and the granular feeling when rubbed by hand increased, but there was no obvious boundary within this depth range, so only one sample was collected after mixing; below a depth of 200 cm, the soil entered the C-layer, the degree of weathering decreased, and the semi-weathered bodies increased. The soil at depths of 200 - 250 cm and 250 - 300 cm could not be clearly stratified, and one sample was taken from each; within the range of 300 - 350 cm, the soil above and below the depth of 320 cm was significantly different. The soil color changed suddenly from brown to light brown with gray without any transition, and the soil suddenly became loose. The soil from 320 cm to 350 cm was mainly in a granular structure, so sampling was carried out in two parts; the soil layers below a depth of 350 cm were divided according to this principle. (2) Comparison of ammonium nitrogen in soils at different mountain positions and different depths

[0092] The ammonium nitrogen content in the soil at the top of the closed mine slope ranged from 7 to 1056 mg·kg -1 , and that in the middle of the slope was 2 - 178 mg·kg -1 , and that at the bottom of the slope was 3 - 177 mg·kg -1 . It can be seen that in-situ leaching of rare earth with ammonium sulfate caused a large amount of residual ammonium nitrogen in the soil of the closed mine, and there were significant differences in the ammonium nitrogen content among different topographic positions ( Figure 8 ).

[0093] From the stratified comparison, the average value of the ammonium nitrogen content in the surface soil of the closed mine was 14.86 mg·kg -1 , the ammonium nitrogen in the shallow soil (the soil layer below the surface and above the ore-bearing layer) was 158.07 mg·kg -1 , and the ammonium nitrogen in the ore-bearing layer soil was 359.46 mg·kg -1 . It can be seen that the direct injection of a large amount of leaching agent ammonium sulfate into the ore-bearing layer during mine exploitation caused a large amount of residual ammonium nitrogen in the ore-bearing layer soil, and the ammonium nitrogen content fluctuated greatly among different soil layers.

[0094] The average value of the ammonium nitrogen content in the soil of the closed mine cannot represent the overall situation of the mine, and directly calculating using the overall average value will lead to a serious deviation in the total amount of residual ammonium nitrogen in the mine. Therefore, it is necessary to calculate the ammonium nitrogen accumulation amount by slope position and layer to ensure the accuracy of the data on the total amount of ammonium nitrogen enrichment in the closed mine.

[0095] (3) Indoor image information extraction

[0096] Download the DEM image with a precision of 12.5 m. Open the image in Arcgis, set the cropping range according to the closed mine boundary (obtained from on-site sampling survey positioning), and use the "Raster - Raster Processing - Clip" in the toolbar to crop the large-scale DEM image into an appropriate size for subsequent data processing and display; use the "3D Analyst Tools - Raster Surface - Contour" tool to process the cropped DEM image and extract the contour lines of the closed mine; import the sampling points, and determine the demarcation elevations of each part in the contour map to be 330 m, 334 m, and 339 m respectively according to the principle that "the sampling points are located at the average elevation of each part of the mine", and extract the corresponding contour lines to a new line file for preservation( Figure 6 ); close the extracted contour lines into polygons, use the "Feature - Feature to Polygon" tool to convert the lines into a reference plane, and then use the "Calculate Geometry" tool in the ArcGIS attribute table to calculate the area of the reference plane. Obtain the area of the conical bottom at the top of the slope as 4486.67 m 2 , the top and bottom areas of the frustum of a cone in the middle of the slope are 4486.67 m 2 and 6101.94 m 2 , the top and bottom areas of the top of the slope at the bottom of the slope are 6101.94 m 2 and 8953.88 m 2 .

[0097] Effect Analysis of Estimating Ammonium Nitrogen Accumulation by Remote Sensing Images and Soil Stratified Sampling

[0098] Substitute the ammonium nitrogen content, bulk density, soil layer thickness, and the top and bottom areas of the composite body of different soil layers into Formula 1 and Formula 8 for calculation, and obtain that the total amount of residual ammonium nitrogen in the closed mine of ion-type rare earth by in-situ leaching of ammonium sulfate is about 35 tons. The corresponding volume content of ammonium nitrogen in the mountain soil is about 486 g·m -3 . Among them, the ammonium nitrogen accumulation at the top of the slope is about 32.4 tons, about 1.0 ton in the middle of the slope, and about 1.7 tons at the bottom of the slope. Since the soil layer thickness of the closed mining area is the top of the slope > the middle of the slope > the bottom of the slope, the enrichment amount of rare earth ore at the top of the mountain is the largest. During the mining process of rare earth ore, liquid is preferentially injected into the top ore body, and the injection volume is the largest; at the same time, the leaching solution pool at the top of the mountain stores all the leaching solution ammonium sulfate required during the mining period. Therefore, the total amount of ammonium nitrogen at the top of the slope is extremely high. The difference in the total residual amount of ammonium nitrogen in the soil of different topographic parts is extremely large. The average value of ammonium nitrogen in the soil of this rare earth closed mine is 263.12 mg·kg -1 . If calculated according to the average value of ammonium nitrogen in the whole mine without dividing soil layers and parts, the total amount of ammonium nitrogen in the soil of the closed mine is 26.6 tons, which is significantly lower than the calculated result of 35.1 tons by stratifying and dividing parts. If the subsequent treatment effect is estimated and the leaching agent is put in according to this calculation result, it will be poor.

[0099] Application Example 1: Long-term Harmfulness of Ammonium Nitrogen Release

[0100] The total amount estimation of the residual leaching agent in a closed mine can be used for the environmental risk assessment of the closed mine. Although the ammonium ion is positively charged and the soil has a certain adsorption capacity for ammonium nitrogen, the ammonium nitrogen remaining in the closed mine is mainly in the free state and is easily lost with water. The average annual rainfall in the study area is 1.61 m, and the precipitation from April to June accounts for nearly 50% of the annual rainfall. The rainfall in the rainy season is large and rapid, and the ammonium nitrogen remaining in the closed mine migrates into the mine water under the action of rainfall leaching. Some studies have found that the content of ammonium nitrogen in the surface runoff of rare earth closed mines is as high as 133.49 mg·L -1 (Li Yu et al., 2021), and the ammonia nitrogen in the mine water seriously exceeds the standard. The investigation found that the average annual evaporation in the closed mine area is 1.21 m, the occupied area is about 8953.88 m 2 , the slope is 37°, the hillside is relatively steep, and the average annual runoff coefficient of mountainous areas with a relatively steep slope under medium rainfall intensity is about 0.6 (Li Yanqiu et al., 2019). From this, it is estimated that the average annual underground runoff generated by this closed mine is about 2149 m 3 , so the total amount of ammonium nitrogen migrating from the closed mine to the surrounding water bodies every year is about 287 kg.

[0101] The total amount of residual ammonium nitrogen in the in-situ leaching closed mine with ammonium sulfate is about 35.1 tons. According to the current annual leaching loss of ammonium nitrogen, if there is no environmental treatment and only natural precipitation leaching is considered, it will take about 122 years for all the residual ammonium nitrogen in the closed mine to be completely released. However, as the ammonium nitrogen content in the mine decreases, the amount of ammonium nitrogen migrating into the water body every year will also gradually decrease. Therefore, considering only precipitation leaching, the time for the closed mine to release ammonium nitrogen into the environment is much longer than 122 years, and the closed mine area will cause continuous harm to the surrounding and downstream ecological environment for a long time.

[0102] Application Example 2: Total amount estimation of the eluent

[0103] The total amount estimation of the residual leaching agent in a closed mine is the basis for accurately estimating the total amount of eluent input in the closed mine area. Convert the total amount of residual ammonium nitrogen from mass to amount of substance (Formula 11), and calculate the theoretical total amount of eluent required to completely elute the ammonium nitrogen according to the chemical equation of the reaction between different eluents and ammonium nitrogen (Formula 12). Due to the relatively complex internal structure of the mine and uncontrollable underground seepage, if the amount of eluent is insufficient during the in-situ leaching process, it will not be able to fully contact and react with the soil particles. Therefore, the actual input amount of the eluent during mine treatment needs to be greater than the theoretical input amount, and the ratio between the two is jointly determined by the type, concentration, pH, and injection liquid intensity of the eluent, etc. It is necessary to determine the optimal parameters through indoor simulation experiments in advance to improve the elution efficiency of the eluent and estimate in advance the total amount of eluent input required during the actual ammonium nitrogen pollution treatment process (Formula 13).

[0104] n 闭矿山 (kmol) = m 闭矿山÷M N (Equation 11) In the formula, n 闭矿山 is the total amount of substance of residual ammonium nitrogen in the soil of the rare earth closed mine (kmol); m 闭矿山 is the total mass of residual ammonium nitrogen in the soil of the closed mine (kg); M N is the molar mass of nitrogen element, which is 14 g·mol -1 .

[0105] n 理论 (kmol) = n 闭矿山 ÷v 淋洗剂 (Equation 12) In the formula, n 闭矿山 is the total amount of substance of residual ammonium nitrogen in the soil of the rare earth closed mine (kmol); n 理论 is the total amount of eluent ions (for exchanging ammonium ions) (kmol); v 淋洗剂 is the valence state of the eluent ions (for exchanging ammonium ions).

[0106] n 实际 (kmol) = n 理论 ÷a 淋洗剂 (Equation 13) In the formula, n 实际 is the total amount of eluent ions that actually need to be input (kmol); n 理论 is the total amount of eluent ions (for exchanging ammonium ions) (kmol); a 淋洗剂 is the elution rate of the eluent, which is jointly determined by the type, concentration, pH, and liquid injection intensity, etc., and needs to be determined through indoor simulation experiments.

[0107] According to Equation 11, it is calculated that the total amount of ammonium nitrogen in the closed mine of this study is about 2500 kmol. Using magnesium sulfate as the eluent for ammonium removal elution, theoretically 1250 kmol of magnesium sulfate is required to undergo an exchange desorption reaction with ammonium nitrogen. The simulation experiment study found that when a magnesium sulfate solution with a pH of 5 and a mass percentage concentration of 2.5% is injected into the mountain body at a speed of 0.3 mL·min -1 the elution effect is the best, and when the elution rate is about 0.14, 96.7% of the ammonium nitrogen will be eluted (Xiao Xinjin et al., 2022). Therefore, according to Equation 13, it is calculated that about 8929 kmol of solid magnesium sulfate, about 1232 tons, is actually required for the closed mine of this study. When configured as a solution at a 2.5% concentration, about 50,000 tons of magnesium sulfate solution is required.

Claims

1. A method for estimating the total amount of residual ammonium nitrogen in the soil of a closed rare earth in-situ leaching mine, characterized in that Including the following steps: Step 1: Determine the sampling points and sample vertically downward in layers to obtain fresh soil samples; Step 2: Weigh the fresh soil samples, record the weight, dry them, and calculate the soil water content based on the soil mass before and after drying; Step 3: Weigh 10.0 g of fresh soil samples and put them into a 200 mL Erlenmeyer flask, add 50.0 mL of potassium chloride solution with a concentration of 2 mol·L -1 -1, seal it, shake for 30 min and then filter, and use a flow analyzer to detect the ammonium nitrogen content; Step 4: Dry the soil samples with a known volume using a constant temperature drying oven, and calculate the soil bulk density based on the total volume of the soil samples before drying and the dry soil weight; Step 5: Determine the location of the slope position demarcation point and the altitude, and combine the high-precision remote sensing image data of the closed mine to calculate the top and bottom areas of each part of the closed mine; Step 6: Establish an estimation model for the total amount of residual ammonium nitrogen in the rare earth closed mine; Step 7: Substitute the soil layer thickness, the ammonium nitrogen content of the corresponding soil layer, the soil bulk density, and the bottom area of the terrain part into the estimation formula for calculation to obtain the total amount of residual ammonium nitrogen in the soil of the leaching mine site.

2. The total amount estimation method of the residual leaching agent in the soil of the in-situ leaching closed mine according to claim 1, wherein The sampling points shall be determined according to the injection range of the leaching agent during the rare earth ore mining process and in combination with the water flow direction of the mine water. At least three sampling points shall be arranged along the water flow direction from the top of the mountain to the bottom of the mountain.

3. The method for estimating the total amount of in-situ leaching agent remaining in the soil of a closed mine according to claim 1, characterized in that, The layered sampling is as follows: Sampling is carried out according to the genetic horizons in the soil from the surface layer to the soil layer of 1 m. After mixing evenly every 50 cm below 1 m, a sample is collected. If obvious changes occur in the observable morphological properties such as soil texture, color and structure within the 50-cm thickness range at a certain depth, sampling is carried out in two sections.

4. The method for estimating the total amount of in-situ leaching agent remaining in the soil of a closed mine according to claim 1, wherein The construction method of the total ammonium nitrogen content estimation model for the rare earth closed mine residues is as follows: The closed mine is approximated as a frustum of a cone in the model and divided into three parts: the top of the slope, the middle of the slope and the bottom of the slope according to the slope position and the sampling points. The soil volume formula at different levels is established and simplified to obtain the soil volume formula (1) of a certain soil layer at a certain slope position; Multiply the ammonium nitrogen content and bulk density of the corresponding soil layer by formula (1) and sum them up, and the obtained formula (2) can estimate the total ammonium nitrogen content of the closed mine residues; V(m 3 ) = Δh(S2 - S1) (Formula 1) Where V is the total volume of the soil layer, m 3 ; Δh is the soil layer thickness, m, that is, the interval sampling depth; S1 is the top surface area of the corresponding topographic part, m 2 ; S2 is the bottom surface area of the corresponding topographic part, m 2 ; where m a is the total amount of residual ammonium nitrogen at slope position a in kg; S 1a is the top surface area at slope position a in m 2 ; S 2a is the bottom surface area at slope position a in m 2 ; Δh i is the thickness of the i-th soil layer in m, i.e., the interval sampling depth; b i is the bulk density of the i-th soil layer in g·cm -3 ; c i is the residual ammonium nitrogen content of the i-th soil layer in mg·kg -3 ; 1 to n represent the soil layer closest to the ground surface to the soil layer closest to the bedrock respectively.

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