Process for comprehensive utilization of cement kiln and rare earth solid waste

Abstract

The invention belongs to the technical field of solid waste comprehensive utilization, and discloses a cement kiln synergistic rare earth solid waste comprehensive utilization process which comprises the following steps: (1) raw material detection; (2) waste evaluation; (3) preparing materials entering a kiln; (4) a sintering test scheme; (5) carrying out a small-scale experiment; (6) pilot test; and (7) tracking and judging. The industrial application of the cement kiln for co-processing the rare earth solid waste is completed, the influence of the comprehensive utilization process on the cement product quality and the environment is researched through raw material monitoring, environment monitoring and the like, and large-scale application of the process is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of solid waste resource utilization, and in particular relates to a process for comprehensive utilization of cement kilns and rare earth solid waste. Background Art

[0002] Solid waste (such as tailings, waste residue, sludge, etc.) generated during the rare earth mining and smelting process contains rare earth elements, heavy metals and radioactive substances. Traditional landfill or storage can easily cause environmental pollution. Cement kiln co-processing technology is a technology that puts solid waste that meets the kiln entry requirements or has been pretreated into the cement kiln and realizes harmless disposal of solid waste through high-temperature calcination. This technology has the advantages of high-temperature calcination (above 1450°C), long residence time, and alkaline environment. It can effectively solidify heavy metals and decompose harmful substances. However, the composition of rare earth waste is complex, and direct addition may affect the quality of cement clinker or the stability of kiln operation. Existing technologies lack systematic pretreatment, compatibility optimization and sintering verification methods for rare earth waste, which limits its large-scale application.

[0003] In summary, how to provide a process for comprehensive utilization of rare earth solid waste in a cement kiln so as to improve the quality of cement while comprehensively utilizing rare earth solid waste is an urgent problem to be solved. Summary of the Invention

[0004] The purpose of the present invention is to overcome the shortcomings of existing technologies and provide a process for the comprehensive utilization of rare earth solid waste in conjunction with cement kilns. The process not only completes the industrial application of cement kilns in the coordinated disposal of rare earth solid waste, but also studies the impact of the comprehensive utilization process on cement product quality and the environment through raw material monitoring and environmental monitoring, thereby ensuring the large-scale application of the process.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A process for comprehensive utilization of rare earth solid waste in a cement kiln, comprising the following steps:

[0007] (1) Raw material testing: The chemical element content of rare earth solid waste and cement plant raw materials is tested and their components are compared and analyzed. The cement plant raw materials include conventional cement raw materials, fuel (coal) and conventional cement clinker produced by cement manufacturers.

[0008] (2) Waste assessment: Analyze the test results of rare earth solid waste and evaluate whether it meets the limit standards for radioactive nuclides, chlorine elements and moisture content.

[0009] (3) Preparation of kiln materials: The kiln materials include cement raw meal and fuel. The cement raw meal includes rare earth solid waste and conventional cement raw meal. The amount of rare earth solid waste added to the kiln materials is 0.01-0.4%.

[0010] (4) Sintering test plan: The kiln materials prepared in step (3) are mixed, crushed, ground and homogenized, and then water (8-10%) is added to disperse them, dried (120°C, about 3 hours), and then placed in a high-temperature furnace at 900-1000°C for calcination for 30-35 minutes (decomposition reaction occurs, preheating stage), then heated to 1400-1500°C, kept warm for 25-35 minutes (solid-liquid phase reaction occurs, calcination stage), and suddenly cooled to obtain cement clinker, which is then mixed with gypsum to make cement.

[0011] (5) Small-scale experiment: A small-scale experiment is carried out according to the sintering test plan of (4), and the quality of the obtained cement clinker is tested to verify whether the addition of rare earth solid waste affects the quality of the cement clinker.

[0012] (6) Pilot test: Continue to conduct pilot tests according to the sintering test plan of (4), conduct quality inspections on the obtained cement clinker, further verify whether the addition of rare earth solid waste affects the quality of cement clinker, and conduct environmental index (environmental gamma radiation dose rate monitoring, air radon concentration, radioactive aerosols, waste gas emissions) inspections to verify whether the rare earth solid waste entering the cement kiln to replace the cement kiln raw material production process has an impact on the surrounding environment and personnel.

[0013] (7) Follow-up judgment: Follow up the results of the aforementioned small-scale and pilot experiments to determine whether rare earth solid waste can enter the cement kiln to replace cement kiln raw materials.

[0014] Preferably, in step (1), the test items include cement chemical analysis items (GB / T176), harmful elements and radioactive elements (GB6566), cement chemical analysis items include loss on ignition, SiO2, CaO, Al2O3, Fe3O4, MgO, K2O, Na2O, TiO2, MnO, P2O3 any one or more, harmful elements include any one or more of heavy metals, chlorine, fluorine, sulfur, radioactive elements include 238 U. 232 Th, 226 Ra, 40 Any one or more of K.

[0015] Preferably, in step (2), the limit standard for radioactive nuclides is specific activity ≤ 50 Bq / g, the limit standard for chlorine element is ≤ 2%, and the limit standard for water content is ≤ 30%.

[0016] Preferably, for rare earth solid waste where chlorine levels fail to meet assessment standards, the process also includes pre-treatment: acid washing and water washing to remove chlorine. Pre-treatment of rare earth solid waste corresponds to sintering testing and is divided into pilot and pilot stages.

[0017] Preferably, the pickling process uses acid to adjust the pH to less than 2, the water washing process uses 3-6 water washing times, and the solid-liquid ratio is 1:(1-6).

[0018] More preferably, the pickling process is specifically as follows: slurrying the rare earth solid waste with water (1:1), adjusting the pH to less than 2 with hydrochloric acid after slurrying, and filtering to obtain the pickling residue.

[0019] The specific washing process is as follows: the pickling residue is washed three times with clean water. The solid-liquid ratio of the first wash is 1:1, the second wash is 1:2, and the third wash is 1:5. After three washes, the washed residue is filtered and sampled to measure the chlorine content. If the chlorine content meets the standard, it is packaged and shipped. If the chlorine content does not meet the standard, the third washing step is repeated.

[0020] Preferably, the pretreatment of rare earth solid waste also includes a recovery process, specifically including: subjecting the filtrate obtained from the pickling process and the first 2-4 water washing processes to lime water precipitation, followed by filter pressing, and the filter cake is again put into the original rare earth solid waste for the pickling process, and the remaining filtrate and the last 1-2 water washing filtrate are subjected to limestone precipitation, followed by filter pressing. When the U and Th series mononuclear elements in the precipitate produced by the filter pressing do not exceed 1Bq / g, it is treated as general solid waste. Otherwise, it is transferred to the cement kiln for disposal together with the water washing residue.

[0021] Preferably, in step (3), the amount of rare earth solid waste added to the kiln material is 0.3-0.4%.

[0022] Preferably, in step (3), the fuel is coal, and the amount of the fuel added to the kiln material is 1.5-2.2%.

[0023] Preferably, the criteria for determining whether rare earth solid waste can enter the cement kiln to replace cement kiln raw materials in step (7) include:

[0024] a) Cement kiln enterprises have the ability to use rare earth solid waste as a substitute for cement kiln raw materials in cement production, and the risks to human health and environmental safety during the raw material substitution process can be effectively controlled;

[0025] b) The use of rare earth solid waste as a substitute for cement kiln raw materials will not have an adverse impact on the stable production of cement and the quality of cement products.

[0026] The beneficial effects of the present invention are:

[0027] (1) The present invention can achieve comprehensive utilization of rare earth solid waste by co-disposing rare earth solid waste in cement kilns, and achieves the industrial application of co-disposing rare earth solid waste in cement kilns.

[0028] (2) The present invention studies the impact of the comprehensive utilization process on the environment through raw material monitoring, environmental monitoring, etc., to ensure the environmental protection requirements of the process.

[0029] (3) The present invention pre-treats rare earth solid waste by using specific acid washing and water washing processes, which can reduce the chlorine content in the waste to meet the admission standards of cement kilns. Wastewater testing has also verified that the pre-treatment process does not pollute the environment.

[0030] (4) The present invention sequentially uses a small-scale sintering test and a pilot-scale sintering test to test the use of rare earth solid waste in cement kilns to replace cement kiln raw materials. The feasibility of the process of the present invention is proved by cement clinker quality testing and environmental index testing, and is suitable for large-scale application. In addition, the present invention can improve the quality of cement products by strictly controlling the limit standard of chlorine element in rare earth solid waste to ≤2% and the limit standard of water content to ≤30%. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a flow chart of the pre-treatment process of rare earth waste residue (i.e., rare earth acid slag) of the present invention;

[0032] Figure 2 The SEM characterization of the four different doping ratios of the present invention as well as rare earth waste residue (i.e. rare earth acid leached residue) and raw material;

[0033] Figure 3 EDS characterization of the four different doping ratios of the present invention as well as rare earth waste residue (i.e. rare earth acid leached residue) and raw material;

[0034] Figure 4 This is the XRD characterization of the four different doping ratios of the present invention as well as rare earth waste residue (i.e. rare earth acid soluble residue) and raw material. DETAILED DESCRIPTION

[0035] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts are within the scope of protection of the present invention.

[0036] Unless otherwise specified, the reagents involved in the embodiments of the present invention are all commercially available products and can be purchased through commercial channels. The methods involved in the embodiments of the present invention are all conventional processes and can be obtained through the corresponding implementation specifications (such as HJ 662), implementation standards or existing literature in this field.

[0037] Example 1

[0038] This embodiment provides a process for comprehensive utilization of rare earth solid waste in a cement kiln, which mainly includes raw material testing, waste pretreatment, waste access assessment, kiln material matching, sintering test plan, small-scale test and pilot test.

[0039] The rare earth solid waste used in this example is the acid-insoluble material (also known as rare earth slag) produced during the rare earth acid dissolution process at rare earth separation companies. The mechanism of rare earth slag production and analytical data from the Experimental Testing Team of the Jiangxi Provincial Geological Bureau indicate that chlorine is the primary element exceeding the standard in rare earth slag, significantly impacting cement sintering stability. Chlorine in rare earth slag exists primarily in the form of chlorides and hydrogen chloride, and most of it is water-soluble. The pretreatment process for rare earth slag produced by slag production companies primarily utilizes a multi-stage water washing experiment.

[0040] 1. Raw material testing

[0041] The rare earth waste residue of the present invention comes from a rare earth company in Dingnan County, Ganzhou City, Jiangxi Province. The cement plant raw materials (conventional cement raw materials, fuel and conventional cement clinker) are directly purchased from a cement plant in Yudu County, Ganzhou City, Jiangxi Province. The rare earth waste residue and the cement plant raw materials are tested for chemical element content. The element content test results of each raw material are shown in Table 1.

[0042] Table 1 Comparison of element contents in rare earth solid waste and cement plant raw materials

[0043]

[0044]

[0045] The results in Table 1 show that rare earth waste residue contains elements beneficial to cement sintering, such as silicon oxide, iron oxide, and aluminum oxide, as well as significant amounts of harmful elements, such as heavy metals, radioactive elements, chlorine, and sulfur. The rare earth waste residue of the present invention contains relatively high proportions of SiO2, Al2O3, and Fe3O4, with concentrations similar to those found in clinker. Therefore, in the comprehensive utilization of rare earth waste residue in cement kilns, theoretically, rare earth waste residue can replace some mineral components in cement raw materials, meeting kiln entry standards.

[0046] 2. Waste Assessment

[0047] Analyze the test results of rare earth solid waste and evaluate whether it meets the limit standards for radioactive nuclides, chlorine elements and moisture content.

[0048] The limit standard for radioactive nuclides is specific activity ≤ 50Bq / g, the limit standard for chlorine is ≤ 2%, and the limit standard for water content is ≤ 30%

[0049] Table 1 shows that the total chlorine content in rare earth waste is excessively high. Research into the process flow reveals that this waste is extracted from rare earth ore using hydrochloric acid, resulting in a significant amount of residual chloride ions in the solid waste. Considering that excessive total chlorine content in the raw material can affect normal cement production, pretreatment such as dechlorination is required before entering the kiln.

[0050] 3. Waste Pre-treatment Plan

[0051] The rare earth waste residue to be treated is sequentially subjected to an acid washing process and a water washing process to remove the chlorine element, and the water-washed residue and wastewater are tested, and at the same time, a relevant filter residue and filtrate recovery process is carried out.

[0052] 1. Pickling process

[0053] Before the water washing process, acid washing is required to activate the chlorine in the rare earth waste residue to improve chlorine removal efficiency. The rare earth waste residue to be treated is transported to the acid washing tank and slurried with water in a 1:1 ratio to fully disperse the acid-soluble residue. After slurrying, the pH of the slurry is adjusted to <2 with hydrochloric acid, and the acid-washed residue is filtered to obtain the acid-washed residue.

[0054] 2. Water washing process

[0055] The pickling residue is washed three times with clean water, with a solid-to-liquid ratio of 1:1 for the first wash, 1:2 for the second wash, and 1:5 for the third wash. After these three washes, the washed residue is filtered and sampled for chlorine content. If the chlorine content meets the standard, it is packaged and shipped. If the chlorine content does not meet the standard (≤2%), the third washing step is repeated. If the moisture content of the washed residue does not meet the standard (≤30%), it needs to be dried.

[0056] 3. Recycling process

[0057] The filtrate obtained from the pickling process, the first water wash, and the second water wash is subjected to limewater precipitation, followed by filter pressing. The filter cake is then put into the original rare earth waste slag for the pickling process again. The remaining filtrate is mixed with the filtrate from the third water wash and sent to the regulating tank for limestone precipitation to adsorb and remove harmful elements in the filtrate and adjust the pH of the solution. Then, filter pressing is performed. The filtrate after filtration enters the sewage treatment plant through the main outlet for reprocessing. The precipitate produced by filter pressing is called neutralization slag. Samples are taken for testing. When the U and Th series mononuclear elements in the neutralization slag do not exceed 1Bq / g, it is treated as general solid waste. When the U and Th series mononuclear elements in the neutralization slag are greater than 1Bq / g, it is transferred to the cement kiln for disposal together with the filter residue obtained from the water washing process.

[0058] 4. Preparation of kiln materials

[0059] The materials entering the kiln include cement raw meal and fuel (coal). The cement raw meal includes rare earth solid waste obtained from pretreatment (chlorine content ≤ 2%, moisture ≤ 30%) and conventional cement raw meal. The amount of rare earth solid waste added to the materials entering the kiln is 0.35%, and the amount of coal added to the materials entering the kiln is 1.97%.

[0060] 5. Sintering test plan

[0061] The prepared materials going into the kiln are mixed, crushed, ground and homogenized, then stirred with water (8-10%), pressed into cakes or rolled into balls, dried in an oven (120°C, about 3h), broken into multiple blocks and placed in a corundum crucible (note that the blocks cannot be too small and there must be gaps), and then placed in a 950°C high-temperature furnace for calcination for 35 minutes (decomposition reaction occurs, preheating stage), then heated to 1450°C and kept warm for 30 minutes (solid-liquid phase reaction occurs, calcination stage). After calcination is completed, take out immediately and use a fan to speed up the cooling to obtain cement clinker, which is then mixed with gypsum to make cement.

[0062] 6. Small-scale experimental data

[0063] Considering the uncertainty of sintering experiments, it is necessary to verify the feasibility of sintering. Therefore, small-scale experiments are required in the early stage. The total amount of rare earth waste raw material used in the pilot experiment is 66 tons.

[0064] 1. Results of water washing test

[0065] During the washing process, the residues from the first and second washes, the third wash, and the neutralized residue were collected for analysis of mineral composition, radioactive elements, and heavy metals. The first, second, and third wash solutions were also collected for analysis of heavy metals and radioactivity. The results are shown in Tables 2 and 3. The results were also used to analyze the migration of nuclides in the rare earth waste residue.

[0066] Table 2 Contents of elements in washed slag and neutralized slag

[0067]

[0068]

[0069] Table 3 Content of each element in washing wastewater

[0070]

[0071]

[0072] According to the test results in Tables 2 and 3, the following conclusions were drawn:

[0073] (1) After one acid wash and three water washes, the chlorine content in the final third water-washed slag is only 0.54%, reaching the limit requirement of chlorine content ≤ 2% and meeting the kiln entry standard.

[0074] (2) The moisture content of the third washed slag was 27.2%, which is lower than the 30% moisture content limit and meets the requirements for transportation to cement companies. The third washed slag produced in this experiment was not dried. The moisture content of the rare earth waste slag raw materials that have undergone drying is much lower than this value.

[0075] (3) Neutralization of slag 226 Ra, 238 U. 232 Th, 40 The specific activities of K were 13.4 Bq / kg, 470 Bq / kg, 24.4 Bq / kg, and 22.6 Bq / kg, respectively. These are all lower than the pre-treatment requirement of no more than 1 Bq / g, or 1000 Bq / kg, and can be disposed of as general solid waste.

[0076] (4) Calculations based on nuclide migration and elemental balance revealed that the elemental balance ratios for U were 136%, Th was 102%, and Ra was 74.3%. The radionuclide contents in the wastewater were 0.08% for U, 0.001% for Th, and 0.02% for Ra. Most of the elements remained in the waste residue, indicating that water washing did not remove a significant amount of radioactive elements from the solution, thus causing environmental impact. The intended effect was achieved.

[0077] 2. Results of sintering test

[0078] In this pilot sintering test, the rare earth solid waste was added to the kiln feed at a ratio of 0.35%. The pilot sintering test was conducted according to the aforementioned sintering test protocol, and the quality of the cement raw meal and clinker was tested to verify whether the addition of rare earth solid waste affected the quality of the cement clinker. The blank data represents the test results of the cement raw meal or clinker without the addition of rare earth solid waste.

[0079] (1) Element content in raw materials and clinker

[0080] Table 4 Element contents in raw materials and clinker

[0081]

[0082] The analysis and comparison of the test results in Table 4 show that the heavy metal element content of cement raw materials or clinkers doped with rare earth waste slag is not much different from that of cement raw materials or clinkers without rare earth waste slag, and the content of some elements is slightly higher. The radioactive nuclides in the raw materials from the mill to the kiln raw materials and then to the clinker show an increasing trend. This may be due to (1) the presence of some radioactive elements in the fuel coal; (2) the reduction in the total mass of the raw materials leads to the enrichment of nuclides; (3) the dust in the dust collector at the kiln head and kiln tail is returned to the high-temperature rotary kiln, resulting in an increase in the concentration of nuclides.

[0083] pass 232 Th, 226 Ra, 40 The internal irradiation index I of clinker is obtained by calculating the K specific activity Ra The value is 0.285, the external radiation index I r The value was 0.399, both below the limit of 0.6. This meets the requirements for Class A building materials in the "Limits of Radioactive Nuclides in Building Materials" and also meets the quality requirements for cement products. The expected results have been achieved, and subsequent pilot tests are possible.

[0084] Clinker 238 The radioactive nuclide content of U is 51.8Bq / kg, which is lower than the natural radioactive nuclide content of 60.6Bq / kg in the local background of Ganzhou.

[0085] In addition, some abnormal data were found in the elemental content. For example, hexavalent chromium was found in both the raw meal leaving the mill and the raw meal entering the kiln, while hexavalent chromium was also found in the clinker. This is likely due to high-temperature oxidation of metallic chromium and the re-entry of dust from the kiln's dust recovery device into the kiln. The decrease in mercury and thallium content is due to the fact that mercury is easily vaporized at high temperatures, thereby reducing the mercury content in the clinker.

[0086] (2) Leachable heavy metal content of cement clinker

[0087] Table 5 Leachable heavy metal content in cement clinker

[0088] index <![CDATA[ 226 Day]]> 238U <![CDATA[ 232 Th]]> <![CDATA[ 40 K]]> Total α Total beta Cd unit Bq / L Bq / L Bq / L Bq / L Bq / L Bq / L mg / L clinker 0.009 0.00879 0.00067 2.73 0.179 2.31 0.00393 Clinker (blank) 0.018 0.0291 0.0008 2.56 0.286 1.84 0.00435 index Cr Cu Mn Ni Pb Zn As unit mg / L mg / L mg / L mg / L mg / L mg / L mg / L clinker 0.25 ND ND ND 0.008 ND 0.00422 Clinker (blank) 0.32 ND ND ND 0.00726 ND 0.00613

[0089] Table 5 shows that the leachable heavy metal content of cement clinker is below the limits specified in Section 8.2, Table 3 of the Technical Specification for Co-processing of Solid Waste in Cement Kilns (GB30760-2014). The radioactivity content in water is also below the limits specified in Table 1 of the Integrated Wastewater Discharge Standard (GB8978-1996).

[0090] The leaching results show that cement clinker doped with rare earth waste will not be released into the environment due to leaching in natural water bodies.

[0091] (3) Conventional analysis of cement clinker

[0092] Among them, the blank sample is the test result of cement clinker without adding rare earth waste slag, and the test sample is the test result of cement clinker with adding rare earth waste slag.

[0093] Table 6 Conventional test data of cement clinker

[0094]

[0095] Table 6 shows that rare earth waste slag had a minor impact on the operation of this test kiln, resulting in fluctuating kiln conditions, a decrease in hourly output, and frequent crusting. Clinker quality also changed: key indicators included a decrease in 3-day compressive strength of 2.6 MPa and a decrease in 28-day compressive strength of 0.7 MPa, an increase in fc of 0.36%, and an increase in chloride ion of 0.002. These quality changes are still within controllable limits and can be corrected later based on the gypsum addition rate.

[0096] Radionuclides in clinker test samples 238 U specific activity 51.8Bq / Kg, blank sample 238 U specific activity <26.1Bq / Kg, increase 25.7Bq / Kg or more, refer to the national standard GB6566 "Limits of Radioactive Nuclides in Building Materials" 5.2 requirements, its radioactive specific activity is not greater than the average background level of the corresponding natural radioactive nuclides in the local surface soil, and can be used only in this area. According to relevant data (Overview of the National Survey on Natural Radioactive Levels in the Environment (1983-1990): Survey on the Content of Natural Radioactive Nuclides in the Soil of Jiangxi Province), the average background level in Jiangxi Province: the weighted average values ​​by area and grid points are respectively 238 U: 55.9 and 57.4 Bq / Kg, Ganzhou average background level: weighted average values ​​by area and grid points are 238 U: 64.3 and 60.6 Bq / Kg. Clinker test sample 238 The U specific activity is 51.8Bq / Kg, which is at the same level as the average background level in Jiangxi Province and slightly lower than the average background level in Ganzhou City. It is within an acceptable range, but there is basically no room for adjustment.

[0097] 7. Pilot test data

[0098] The results of the pilot test confirmed the feasibility of using rare earth waste slag as a raw material in cement kilns. The project team then conducted a pilot test using 500 tons of rare earth waste slag as raw material.

[0099] The pilot test process was designed based on the experience gained from the early stage small-scale experiments. This included the three-stage water washing process of the rare earth waste slag and the monitoring and testing before and after the washing process; radioactivity monitoring of the waste slag before and after entering the plant; and radionuclide testing of the waste slag before and after entering the kiln.

[0100] 1. Water washing pilot results

[0101] The mineral composition, radioactive elements, and heavy metals of the incoming slag pile (i.e., rare earth waste), washed slag, and neutralized slag during the water washing pilot test were tested. Heavy metal and radioactivity levels were also tested in the tertiary water washing liquid, the regulating tank, and the main outlet (wastewater discharged to the sewage treatment plant). The results are shown in Tables 7 and 8. The content of each component was analyzed based on the results.

[0102] Table 7 Contents of elements in washed slag and neutralized slag

[0103]

[0104]

[0105] Table 8 Content of each element in washing wastewater

[0106]

[0107]

[0108] According to the test results in Tables 7 and 8, the following conclusions were drawn:

[0109] (1) After one acid wash and three water washes, the chlorine content in the final third water-washed slag is only 0.247%, which is lower than the chlorine content limit of 2% required for entering the kiln and meets the kiln entry standard.

[0110] (2) The moisture content of the third washed slag is 36%, which is higher than the 30% moisture content limit requirement. However, considering that the washed slag has not yet undergone the drying process, the moisture content of the acid-soluble slag after drying is 29%, which meets the 30% limit requirement. This meets the requirements for transportation to cement companies.

[0111] (3) Neutralization of slag 226 Ra, 238 U. 232 Th, 40 The specific activities of K were 71.4 Bq / kg, 127 Bq / kg, 23.1 Bq / kg, and <21.7 Bq / kg, respectively. These are all lower than the pre-treatment requirement of no more than 1 Bq / g, or 1000 Bq / kg, and can be disposed of as general solid waste.

[0112] (4) The results of water washing show that the content of each element decreased from the raw slag (rare earth waste slag) to the first and second water washing slags. This is because the acid washing process was added before the water washing, and the strong acid eluted some elements in the waste slag. This was confirmed again in the first and second water washing liquids. The content of each element in the third water-based slag did not change significantly from the first and second slags. The simple water washing process does not wash out the elements in the waste slag.

[0113] 2. Environmental monitoring during water washing pilot test

[0114] Referring to the "Technical Specifications for Environmental Gamma Radiation Dose Rate Monitoring" (HJ 1157-2021), the environmental gamma radiation dose rate before and after water washing was measured and recorded. The results are shown in Table 9.

[0115] Table 9 Comparison of γ dose rate before and after water washing

[0116]

[0117]

[0118] Table 9 shows that gamma dose rates increased significantly at the rare earth waste slag, filter press, and mixing tank before and after water washing. These sites are all associated with direct exposure to rare earth waste slag. In areas like the pickling and water washing tanks, the rare earth waste slag is deposited at the bottom, partially isolated by a thick layer of water, resulting in minimal changes. The gamma dose rates at the main outlet, office building, and cafeteria remained virtually unchanged. This suggests that the rare earth waste slag had limited impact on the surrounding radiation environment during the water washing process and did not affect the normal work and life of nearby residents. A comparison of the gamma dose rates at the incoming slag pile and an open area 2 meters away from the pile shows that radiation levels drop sharply 2 meters away from the incoming slag pile, reaching a safe level.

[0119] Table 10 Changes in air radon concentration before and after water washing

[0120]

[0121] Table 10 compares the air radon concentrations before and after water washing, showing significant changes in radon concentrations in the incoming slag yard and raw material warehouse. This is because both the incoming slag yard and the raw material warehouse are relatively closed and unventilated. The long-term accumulation of rare earth waste slag leads to the decay of radioactive nuclides, producing radon and accumulating it. The changes in air radon concentrations indicate that the rare earth waste slag has little impact on the office area, and does not affect normal office activities in the building.

[0122] Table 11 External radiation dose values ​​for workers

[0123] Name Report results (mSv) Annual effective dose (mSv) XXX1 0.11 1.30 XXX2 0.10 1.22 XXX3 0.08 0.98 XXX4 0.10 1.20 XXX5 0.11 1.29 XXX6 <MDL <MDL XXX7 0.04 0.53 XXX8 <MDL <MDL XXX9 0.12 1.48 XXX10 0.09 1.02 XXX11 0.10 1.21 XXX12 0.17 2.01 XXX13 0.13 1.57 XXX14 0.11 1.35 XXX15 0.08 0.92 XXX16 0.15 1.84 XXX17 0.11 1.32 XXX18 0.06 0.66 XXX19 0.06 0.68 XXX20 0.07 0.89

[0124] According to the "Basic Standard for Ionizing Radiation Protection and Safety of Radiation Sources" (GB18871-2002), the annual exposure dose limit for occupational personnel is 5mSv. Through one month of tracking and monitoring of employee external exposure doses during the test period, according to the results, the annual exposure dose of all workers did not exceed this limit. This shows that the radiation from rare earth waste slag has no significant impact on the workers in the water washing plant. However, it is still necessary to pay attention to the high values ​​in some cases. This may be because the workers' workstations are extremely close to rare earth waste slag, such as incoming slag piles, slurry mixing tanks, mixing tanks, etc. Considering the employee exposure dose and the gamma radiation dose rate results at this point, workers cannot stay in this working environment for a long time. It is recommended to adopt a rotation system to regulate the time workers stay at this workstation.

[0125] Summary: A comparison of the effects of rare earth waste residue before and after washing shows that they are limited to a relatively small area. During the washing process, workers should take precautions to protect themselves and avoid prolonged contact with acid-dissolved residue. They should also maintain a safe distance of 2 meters from the residue.

[0126] 3. Sintering pilot results

[0127] Based on the results of the small-scale test, the pilot test's doping ratio was analyzed and it was concluded that a 0.35% doping ratio for this rare earth waste residue was more reasonable. A pilot sintering test was conducted according to the above sintering test plan, and the quality of the cement raw meal and clinker was tested to further verify whether the addition of rare earth solid waste affected the quality of the cement clinker.

[0128] (1) Element content in raw materials, clinker and kiln dust

[0129] Table 12 shows the element contents in raw meal, clinker and kiln tail dust. The blank is the test results of cement raw meal, clinker and kiln tail dust without adding rare earth waste slag.

[0130] Table 12 Element contents in raw materials, clinker and dust

[0131]

[0132] Through the analysis and comparison of the test results in Table 12, the difference in heavy metal element content between cement raw materials or clinker doped with rare earth waste slag and cement raw materials or clinker without rare earth waste slag is not significant, and the content of some elements is slightly higher. The radioactive nuclides in the raw materials from the mill to the raw materials entering the kiln and then to the clinker show an increasing trend. This may be due to (1) the presence of some radioactive elements in the fuel coal; (2) the reduction in the total mass of the raw materials leading to the enrichment of nuclides; (3) the dust in the dust collector at the kiln head and kiln tail returns to the high-temperature rotary kiln, resulting in an increase in the concentration of nuclides. The content of each element in the test results of the kiln tail dust has increased significantly, proving that each element will be enriched at the kiln tail.

[0133] pass 232 Th, 226Ra, 40 The internal irradiation index I of clinker is obtained by calculating the K specific activity Ra The value is 0.03, the external exposure index I r The value is 0.26, which is lower than the limit of 0.6. It meets the requirements of Class A building materials in the "Limits of Radioactive Nuclides in Building Materials" and the quality requirements of cement products, achieving the expected results.

[0134] Clinker 238 The radioactive nuclide content of U is 60Bq / kg, which is lower than the natural radioactive nuclide content of 60.6Bq / kg in the local background of Ganzhou.

[0135] In addition, some abnormal data were found in the element content values. For example, hexavalent chromium was not found in the raw meal leaving the mill, the raw meal entering the kiln, or the dust, but it was found in the clinker. This is likely due to the high-temperature oxidation of metallic chromium, which is not easily volatile. The mercury and thallium contents in the clinker decreased, while those in the dust increased. This is because at high temperatures, mercury and thallium are easily vaporized and enter the dust bag at the kiln tail.

[0136] (2) Leachable heavy metal content of cement clinker

[0137] Table 13 Leachable heavy metal content in cement clinker

[0138] index <![CDATA[ 226 Day]]> 238U <![CDATA[ 232 Th]]> <![CDATA[ 40 K]]> Total α Total beta Cd unit Bq / L Bq / L Bq / L Bq / L Bq / L Bq / L mg / L Clinker (blank) 0.065 0.001305 0.00051 9.73 0.4405 12 0.004325 clinker 0.205 0.00222 0.00222 9.27 0.783 21.7 0.0112 index Cr Cu Mn Ni Pb Zn As unit mg / L mg / L mg / L mg / L mg / L mg / L mg / L Clinker (blank) 0.085 ND ND ND 0.08 0.12 0.00111 clinker 0.1693 ND ND ND 0.037 0.01 0.0088

[0139] Table 13 shows that the leachable heavy metal content of cement clinker is below the limits specified in "8.2, Table 3" of the "Technical Specification for Co-processing of Solid Waste in Cement Kilns" (GB30760-2014). The radioactivity and heavy metal content in water is below the limits specified in "Table 1" of the "Integrated Wastewater Discharge Standard" (GB8978-1996). The differences from the blank sample are minimal, indicating that cement clinker doped with rare earth waste does not significantly affect cement leaching. Leachable results indicate that cement clinker doped with rare earth waste is not released into the environment through leaching in natural water bodies.

[0140] (3) Conventional analysis of cement clinker

[0141] Among them, the blank sample is the test result of cement clinker without adding rare earth waste slag, and the test sample is the test result of cement clinker with adding rare earth waste slag.

[0142] Table 14 Conventional test data of cement clinker

[0143]

[0144]

[0145] Table 14 shows that the chemical and mineral composition of the clinker doped with rare earth waste slag fluctuated slightly compared to the blank sample, but the fluctuations were not significant. Chloride and alkali contents decreased, demonstrating that water washing was effective, removing a significant number of harmful components from the rare earth waste slag. Key indicators: 3-day compressive strength increased by 2.8 MPa, 28-day compressive strength increased by 0.5 MPa, fc decreased by 0.04%, and chloride ion decreased by 0.011. The addition of rare earth waste slag improves the quality of cement clinker.

[0146] (4) Material characterization

[0147] The present invention prepares cement clinker with different rare earth waste residue doping ratios (0.5%, 1%, 2%, 3%) through laboratory pilot test, and characterizes the morphology of the material through SEM, EDS, XRD and other means.

[0148] Figure 2 The SEM images of four different doping ratios, as well as rare earth slag and raw meal, show that the morphology of cement doped with rare earth slag remains unchanged compared to the raw meal. Furthermore, the morphology of clinker doped with different ratios of rare earth slag is similar. This demonstrates that the addition of rare earth slag does not alter the microscopic morphology of the cement itself.

[0149] Figure 3 EDS characterization of four different doping ratios, as well as rare earth waste residue and raw meal, revealed that the main components of cement raw meal and cement clinker at different ratios are O, Ca, Si, C, Mg, and Al. Rare earth waste residue also contains a large amount of S. This may be due to the use of S-containing acids and organic solvents in the extraction process of the rare earth raw materials.

[0150] Figure 4 XRD characterization of four different doping ratios, as well as rare earth waste slag and raw meal, reveals no significant change in the main mineral composition of the clinker across the various gradient experiments. While the content of tricalcium silicate increases with the addition of rare earth waste slag, the increase is limited. Comparison of the contents of tricalcium silicate and dicalcium silicate in the clinker reveals that their contents tend to be stable, with no significant fluctuations depending on the gradient of rare earth waste slag addition.

[0151] Summary: Full-process monitoring of radioactive elements in rare earth waste residue revealed that the radioactive elements are primarily retained within the residue and are not released into the environment due to environmental changes such as pre-washing, high temperatures, crushing, and transportation. This significantly ensures the safety and feasibility of comprehensive utilization of rare earth waste residue in cement kilns.

[0152] 4. Environmental monitoring during sintering pilot test

[0153] During the sintering pilot process, environmental indicators (environmental gamma radiation dose rate monitoring, air radon concentration, radioactive aerosols, and waste gas emissions) are tested to verify whether rare earth waste slag entering the cement kiln to replace cement kiln raw materials will have an impact on the surrounding environment and personnel during the production process.

[0154] Referring to the "Technical Specifications for Environmental Gamma Radiation Dose Rate Monitoring" (HJ 1157-2021), the environmental gamma radiation dose rate before and after sintering was measured and recorded.

[0155] Table 15 Comparison of γ dose rate before and after entering the kiln

[0156]

[0157]

[0158] Table 15 shows that significant increases before and after kiln entry were only observed in the slag storage, weighing belt, and exhaust fan behind the slag storage. These areas are in close contact with the rare earth waste slag. No significant changes were observed in surrounding sensitive areas, the factory boundary, and residential areas. This is due to the distance from the acid-dissolved slag. This indicates that maintaining a certain distance from the rare earth waste slag can minimize its radiation impact.

[0159] Table 16 Changes in air radon concentration before and after entering the kiln

[0160]

[0161]

[0162] Table 16 shows that the radon concentration in the slag storage facility fluctuated significantly. This is because the environments in these two locations are relatively closed, with slow air circulation, which significantly affects radon concentrations. Radon concentrations in open areas remained almost unchanged.

[0163] The content of radioactive elements in radioactive aerosols before and after entering the kiln was studied, the concentration of radionuclides released into the environment during the cement kiln sintering process before and after the rare earth waste slag entered the kiln was analyzed, and the impact of rare earth waste slag on the environment during the comprehensive utilization of cement kilns was studied. The results are shown in Table 17.

[0164] Table 17 Changes in radioactive aerosols before and after entering the kiln

[0165]

[0166]

[0167] The results in Table 17 show that there is no significant change in the nuclide content in the radioactive aerosol before and after the rare earth waste is put into the kiln, and the nuclide content in the air at the surrounding sensitive points and the factory boundary area is basically the same, with no place being too high or too low.

[0168] The comprehensive utilization of rare earth waste residue in cement kilns not only poses the risk of radioactive contamination but also generates atmospheric pollutants. Emission data from cement kiln plants and surrounding maintenance organizations were collected through gas sampling and testing. The results are shown in Tables 18 and 19.

[0169] Table 18 Changes in organized exhaust gas emissions before and after entering the kiln

[0170] element Cd Ni Co As Sb Hg Sn Cu unit <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> Organized discharge before entering the kiln 0.016 3.6 0.122 2.7 0.21 0.0047 0.8 0.4 Organized emission after entering the kiln-1 0.026 0.7 0.057 5.9 0.14 0.0034 0.6 0.6 Organized emission after entering the kiln-2 0.017 0.3 0.037 5.6 0.14 0.0025 0.7 0.4 element Pb Tl Mn Be V chloride Fluoride unit <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[μg / m 3 ]]> <![CDATA[mg / m 3 ]]> <![CDATA[mg / m 3 ]]> Organized emission before entering the kiln-1 1.6 0.014 1.79 ND 5.9 2.54 0.76 Organized emission after entering the kiln-1 ND 0.216 1.7 0.018 8.85 3.13 0.32 Organized emission after entering the kiln-2 ND 0.066 0.8 ND 7.83 7.8 0.24

[0171] Table 19 Changes in unorganized exhaust gas emissions before and after entering the kiln

[0172]

[0173]

[0174] The results in Tables 18 and 19 show that atmospheric pollutant emissions before and after rare earth waste slag was introduced into the kiln were below the limits specified in the "Pollution Control Standard for Co-processing Solid Wastes in Cement Kilns" (GB30485-2013) and the "Emission Standard for Air Pollutants from the Cement Industry" (GB4915-2013). Comparison of atmospheric pollutant concentrations before and after the introduction revealed no significant change. This indicates that the comprehensive utilization of rare earth waste slag in cement kilns has not yet generated significant atmospheric pollution. Rare earth waste slag can be introduced into the kiln normally, with little difference from normal Portland cement production.

[0175] 8. Tracking and Judgment

[0176] The criteria for determining whether rare earth waste residue can enter cement kilns to replace cement kiln raw materials include:

[0177] a) Cement kiln enterprises have the ability to use rare earth waste residue as a substitute for cement kiln raw materials in cement production, and the risks to human health and environmental safety during the raw material substitution process can be effectively controlled;

[0178] b) The use of rare earth waste slag as a substitute for cement kiln raw materials will not have an adverse impact on the stable production of cement and the quality of cement products.

[0179] According to the above-mentioned small-scale experiments and pilot experiments, the rare earth waste slag of the present invention does not bring obvious safety risks to human health and the environment during the replacement process, nor does it have obvious adverse effects on the stable production of cement and the quality of cement products. It can enter the cement kiln to replace the cement kiln raw materials and carry out comprehensive utilization of cement kilns and rare earth wastes.

[0180] Comparative Example 1

[0181] This comparative example is the same as the pilot experiment scheme in Example 1, except that the moisture content of the rare earth waste slag in this comparative example is 35.3% when entering the kiln.

[0182] Pilot sintering test results

[0183] (1) Construction changes: The rare earth waste slag doped in this comparative ratio has a high moisture content and is powdery, so the material cannot be discharged smoothly and needs to be manually poked.

[0184] (2) Kiln conditions: The operation of the test kiln was affected, the kiln conditions fluctuated, the hourly output (feed amount) dropped by about 10 tons, and crusting occurred frequently.

[0185] (3) Conventional analysis of cement clinker

[0186] Among them, the blank sample is the test result of cement clinker without adding rare earth waste slag, and the test sample is the test result of cement clinker with adding rare earth waste slag.

[0187] Table 20 Conventional test data of cement clinker of comparative example 1

[0188]

[0189] Table 20 shows that the test sample, compared to the blank sample, required 0.6% less water at standard consistency, had an initial setting time extended by 23 minutes, and a final setting time extended by 28 minutes. The 3-day flexural strength decreased by 0.3 MPa, and the 3-day compressive strength decreased by 2.6 MPa. The 28-day flexural strength decreased by 0.1 MPa, and the 28-day compressive strength decreased by 0.7 MPa. This indicates that the addition of rare earth waste residue in this experiment degraded cement quality. Rare earth waste residue with substandard moisture content compromised both stable cement production and cement quality, and therefore did not meet the criteria for replacing raw materials in cement kilns.

Claims

1. A process for comprehensive utilization of rare earth solid waste in a cement kiln, characterized by: The following steps are involved: (1) Raw material testing: Detect the chemical element content of rare earth solid waste and cement plant raw materials, and compare and analyze their components; (2) Waste assessment: Analyze the test results of rare earth solid waste to assess whether it meets the limit standards for radionuclides, chlorine elements and moisture content; (3) Preparation of kiln materials: The kiln materials include cement raw meal and fuel. The cement raw meal includes rare earth solid waste and conventional cement raw meal. The amount of rare earth solid waste added to the kiln materials is 0.01-0.4%; (4) Sintering test plan: The kiln materials prepared in step (3) are mixed, crushed, ground and homogenized, then dispersed with water, dried, and then calcined in a high-temperature furnace and quenched to obtain cement clinker. The cement clinker is mixed with gypsum to make cement; (5) Small-scale test: Carry out small-scale test according to the sintering test plan of (4), and conduct quality inspection on the obtained cement clinker; (6) Pilot test: Continue to conduct pilot test according to the sintering test plan of (4), conduct quality test on the obtained cement clinker, and conduct environmental index test at the same time; (7) Follow-up judgment: Follow up the results of the aforementioned small-scale and pilot experiments to determine whether rare earth solid waste can enter the cement kiln to replace cement kiln raw materials.

2. The process according to claim 1, characterized in that: In step (1), the test items include cement chemical analysis items, harmful elements and radioactive elements. The cement chemical analysis items include loss on ignition, SiO 2、 CaO, Al2O 3、 Fe3O 4、 MgO, K2O, Na2O, TiO 2、 Any one or more of MnO, P2O3, harmful elements include any one or more of heavy metals, chlorine, fluorine, sulfur, radioactive elements include 238 U. 232 Th, 226 Ra, 40 Any one or more of K.

3. The process according to claim 1, characterized in that: In step (2), the limit standard for radioactive nuclides is specific activity ≤ 50 Bq / g, the limit standard for chlorine element is ≤ 2%, and the limit standard for water content is ≤ 30%.

4. The process according to claim 1, wherein: It also includes pre-treatment of rare earth solid waste: the rare earth solid waste to be treated is subjected to an acid washing process and a water washing process in sequence. The acid washing process uses acid to adjust the pH to less than 2. The water washing process is washed 3-6 times, and the solid-liquid ratio is 1: (1-6).

5. The process according to claim 1, characterized in that: In step (3), the amount of rare earth solid waste added to the kiln material is 0.3-0.4%.

6. The process according to claim 1, characterized in that: In step (3), the fuel is coal, and the amount of the fuel added to the kiln material is 1.5-2.2%.

7. The process according to claim 1, characterized in that: The calcination process of step (4) includes: calcining in a high-temperature furnace at 900-1000°C for 30-35 minutes, then heating to 1400-1500°C and keeping warm for 25-35 minutes.

8. The process according to claim 1, characterized in that: Step (7) The criteria for determining whether rare earth solid waste can enter cement kilns to replace cement kiln raw materials include: a) Cement kiln enterprises have the ability to use rare earth solid waste as a substitute for cement kiln raw materials in cement production, and the risks to human health and environmental safety during the raw material substitution process can be effectively controlled; b) Substituting rare earth solid waste for cement kiln raw materials will not have an adverse impact on the stable production of cement and the quality of cement products.

9. The process according to claim 4, characterized in that: The pretreatment of rare earth solid waste also includes a recovery process, specifically including: lime water precipitation of the filtrate obtained from the pickling process and the first 2-4 water washing processes, followed by filter pressing, and the filter cake is put into the original rare earth solid waste for the pickling process again. The remaining filtrate and the filtrate from the last 1-2 water washing processes are subjected to limestone precipitation, followed by filter pressing. If the U and Th series mononuclear elements in the precipitate produced by the filter pressing do not exceed 1Bq / g, it will be treated as general solid waste. Otherwise, it will be transferred to the cement kiln for disposal together with the water washing residue.

10. The process according to claim 4, characterized in that: The specific pickling process is as follows: slurrying the rare earth solid waste with water, adjusting the pH to less than 2 with hydrochloric acid, and filtering to obtain the pickling residue; The specific washing process is as follows: the pickling residue is washed three times with clean water, the solid-liquid ratio of the first washing is 1:1, the solid-liquid ratio of the second washing is 1:2, and the solid-liquid ratio of the third washing is 1:

5. After three washings, the washed residue is filtered to obtain the washed residue.

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