A method for reducing, decomposing and separating water-leaching residues by magnetic separation

Abstract

The present invention proposes a method for reduction-decomposition-magnetic separation of water-leached slag, which specifically relates to the field of iron-containing rare earth mineral metallurgy. The steps of the present invention are: mixing the water-leached slag with a carbon-based reducing agent, placing it in a roasting furnace for roasting, and magnetically separating the roasted product under a magnetic strength of 0.1-1.0T to obtain a rare earth iron concentrate with an iron content of 55%-60% and a rare earth content of 8-15%, with a yield of 28-30%, and a magnetic tailings with a calcium phosphate content of 90-98%, with a yield of 70-72%. The present invention only uses a small amount of carbon-based reducing agent to promote the decomposition of calcium sulfate in the water-leached slag into calcium oxide, and further uses the generated calcium oxide to decompose the iron phosphate in the water-leached slag into iron oxides. The roasted product can be subjected to magnetic separation to obtain a high-grade iron concentrate containing rare earths, as well as a product rich in calcium and magnesium phosphates. The rare earth-containing iron concentrate can be returned to the mixed rare earth concentrate sulfuric acid roasting link, realizing partial recycling of the water-leached slag.

Description

Technical Field

[0001] This invention belongs to the field of iron-containing rare earth mineral metallurgy, and specifically relates to a method for water-leached slag reduction-decomposition-magnetic separation. Background Art

[0002] Mixed rare earth concentrate is a type of rare earth concentrate obtained from the Bayan Obo mine in my country through beneficiation and separation, and is one of the important rare earth resources. The main processing method for mixed rare earth concentrate is currently the sulfuric acid roasting method. This method involves mixing the mixed rare earth concentrate with concentrated sulfuric acid and ferric sulfate in a certain proportion, then roasting it in a rotary kiln. The roasting product is then leached with water, allowing rare earth, iron, calcium, and other metal ions, as well as phosphate and sulfate ions, to enter the solution. Simultaneously, the pH value is adjusted so that iron reacts with phosphate to form ferric phosphate precipitate, and calcium reacts with sulfate to form calcium sulfate crystals. Therefore, the ferric phosphate precipitate, calcium sulfate crystals, and undecomposed minerals (rare earth phosphates and silica) become the water leaching residue, while the rare earth ions remain in the solution. After filtration, the rare earth elements are separated from iron, calcium, phosphate, etc. The water leaching residue generated in this process needs to be stored in a designated slag storage area.

[0003] To address the problem of water-leached slag produced by the sulfuric acid process for mixed rare earth concentrates, researchers have conducted corresponding studies on improvements to the sulfuric acid roasting method. In 2003, the Baotou Rare Earth Research Institute proposed a low-temperature decomposition technology using concentrated sulfuric acid. After decomposing the mixed rare earth concentrate with sulfuric acid at 150–300℃, it is leached with water. MgO is added to the leachate to adjust the pH value, precipitating and removing iron and phosphorus. The resulting water-leached slag contains 2–5% rare earth elements and less than 3% thorium. After impurity removal, the leachate is extracted to obtain thorium and rare earth products, with the ThO2 / REO ratio in the rare earth products being less than 10. -5 This method can significantly reduce the amount of waste residue, but it also suffers from a lack of industrial-scale experiments, dynamic roasting, and equipment limitations. In 2005, the Beijing Nonferrous Metals Design Institute and Baoding Rare Earth Materials Plant jointly researched a low-temperature dynamic roasting technology for mixed rare earth concentrate with sulfuric acid. The technical principle is the same as described above, but this method requires a aging process to stabilize the concentrated sulfuric acid, which prolongs the operation time and affects continuous production. In 2006, the Beijing Nonferrous Metals Design Institute proposed a method for the comprehensive recovery of rare earth and thorium. Mixed rare earth concentrate is simultaneously roasted at low temperature with concentrated sulfuric acid and iron-containing additives. Thorium, iron, and phosphorus are enriched in the water-leached residue, and thorium nitrate can be obtained by acid extraction. The problem with this method is that the process conditions are difficult to control.

[0004] In 2015, the Institute of Process Engineering, Chinese Academy of Sciences, proposed a clean smelting process for mixed rare earth concentrates. First, the rare earth concentrate is directly roasted to decompose bastnaesite, followed by sulfuric acid leaching. The leaching residue is washed, dried, and then roasted and decomposed again with sodium carbonate. The decomposition product is washed and then leached with sulfuric acid. The two leaching solutions are combined for separation and recovery of rare earth and thorium. This method does not address the removal of iron from the leaching solution. Iron removal in rare earth solutions typically employs neutralization, which would produce iron-thorium slag with a high thorium content. In 2017, the Baotou Rare Earth Research Institute proposed a method for the combined acid-base decomposition of mixed rare earth concentrates. In this process, the rare earth concentrate is first decomposed at low temperature using concentrated sulfuric acid, followed by water leaching and neutralization to form a water-leached residue containing phosphorus minerals and iron-thorium, as well as a rare earth solution. The water-leached residue is decomposed with NaOH solution, washed, and then leached with hydrochloric acid. The leaching solution is neutralized and slag removed, forming a mixed residue of acid-soluble residue and iron-thorium slag, along with a rare earth solution. This method reduces the volume of water-leached residue, but still produces mixed residue with a composition similar to that of current processes, which still requires the construction of storage silos.

[0005] Although numerous studies have been conducted on decomposition methods for mixed rare earth concentrates, currently 90% of mixed rare earth concentrates still employ the method of high-temperature sulfuric acid roasting, water leaching, neutralization and impurity removal, extraction transformation, and extraction separation of rare earth elements. According to statistics, processing 1 ton of rare earth concentrate generates over 0.8 tons of water-leached residue, meaning that processing 200,000 tons of rare earth concentrate annually would generate over 160,000 tons of water-leached residue. In 2013, the Baotou Rare Earth Research Institute proposed a process for recovering rare earth, thorium, and iron from waste residue in rare earth acid processes. This method involves acid leaching of the waste residue, extraction and recovery of thorium from the leachate, precipitation and recovery of ferric hydroxide from the raffinate, and the filtration residue as a rare earth solution product. This process reduces waste residue by more than half, but the treatment process still presents the problem of "three wastes" (waste gas, wastewater, and solid waste). Summary of the Invention

[0006] To address the problems of the existing technology, this invention provides a method for reducing, decomposing, and separating water-leached residue using magnetic separation. A small amount of added carbon-based reducing agent is used to promote the decomposition of calcium sulfate in the water-leached residue into calcium oxide. The generated calcium oxide is then used to decompose ferric phosphate in the water-leached residue into iron oxides (ferric oxide or magnetite). The roasted product, after magnetic separation, yields a high-grade iron concentrate containing rare earth elements, as well as a product rich in calcium phosphate. The rare earth-containing iron concentrate can be returned to the sulfuric acid roasting stage of the mixed rare earth concentrate, thus solving the current problem of lacking a treatment method for water-leached residue and achieving partial recycling and volume reduction of the residue.

[0007] The technical solution of the water-leached residue reduction-decomposition-magnetic separation method of the present invention includes the following steps:

[0008] (1) Introduce air or nitrogen into the roasting furnace, mix the water-impregnated residue and carbon-based reducing agent evenly in the roasting furnace, and roast to obtain roasting product;

[0009] (2) The tail gas from the roasting furnace is absorbed by circulating water, which converts sulfur trioxide in the tail gas into sulfuric acid;

[0010] (3) The roasting product in step (1) above is separated by magnetic separation to obtain rare earth iron concentrate and magnetic tailings.

[0011] The water-leached residue mentioned in step (1) above is the water-leached residue obtained in the neutralization and impurity removal stage of the sulfuric acid roasting process of mixed rare earth concentrate;

[0012] The carbon-based reducing agent mentioned in step (1) above includes one of graphite powder, activated carbon powder, and biomass fuel powder (straw type);

[0013] The mixing described in step (1) above specifically involves mixing the water-leached residue with carbon-based materials at a weight ratio of 100:(3-20);

[0014] The roasting described in step (1) above specifically refers to a roasting temperature of 500-1000℃ and a roasting time of 30-180min;

[0015] When air is introduced into the roasting furnace in step (1) above, iron in the roasting product exists in the form of ferric oxide and rare earth exists in the form of oxides; when nitrogen is introduced, iron in the roasting product exists in the form of iron tetroxide and rare earth exists in the form of oxides.

[0016] The magnetic separation process described in step (3) above is carried out under a magnetic strength of 0.1 to 1.0 T;

[0017] The rare earth iron concentrate obtained in step (3) above has an iron content (calculated as iron element content) of 55% to 60%, a rare earth content (calculated as rare earth oxide content) of 8% to 15%, and a yield of 28% to 30%; the magnetic separation tailings obtained above have a calcium phosphate content of 90% to 98% and a yield of 70% to 72%.

[0018] The sulfuric acid described in step (3) above is used for recycling in mixed rare earth concentrate.

[0019] Compared with the prior art, the water-leached residue reduction-decomposition-magnetic separation method of the present invention has the following advantages:

[0020] 1. The method of this invention can effectively solve the problem of unusable water-leached slag. Through the roasting process, the corresponding phases of iron, rare earth, calcium, magnesium, phosphorus and sulfur are transformed, so that iron and rare earth form oxides, sulfur is recovered to prepare sulfuric acid, and it is reused in the sulfuric acid roasting process of mixed rare earth concentrate; phosphorus, calcium and magnesium form products of specific phases, realizing their productization.

[0021] 2. The principle of this invention is that calcium sulfate decomposes to produce calcium oxide, which then reacts with ferric phosphate to ultimately produce calcium phosphate and iron oxide. Without carbon-based additives, the calcination product at 900°C still consists mainly of ferric phosphate and calcium sulfate; however, with the addition of carbon-based additives, the calcination product at 500°C is dominated by ferric oxide and calcium magnesium phosphate phases, indicating that the method of this invention can significantly reduce the reaction temperature using carbon-based additives.

[0022] 4FePO4+6CaSO4+3C+3O2(g)=2Fe2O3+2Ca3(PO4)2+6SO3(g)+3CO2(g)

[0023] CaSO4 + C + O2 = CaO + SO3 + CO2

[0024] 2FePO4+2.89CaO+0.11MgO=Fe2O3+Ca 2.89 Mg 0.11 (PO4)2

[0025] 3. The process of the present invention enables the decomposition products iron oxide and calcium magnesium phosphate to have a degree of dissociation of more than 98%. Based on the difference in the specific magnetic susceptibility of iron oxide and calcium magnesium phosphate, magnetic separation can be used to recover iron oxide and calcium magnesium phosphate separately, avoiding the acid and alkali consumption of chemical separation.

[0026] 4. The method of this invention utilizes magnetic separation to obtain calcium magnesium phosphate, which can be directly applied as a multi-element phosphate fertilizer product. Attached Figure Description

[0027] Figure 1 This is a process flow diagram of a water-leached residue reduction-decomposition-magnetic separation method according to the present invention;

[0028] Figure 2 Phase analysis diagram of the product of water-leached residue roasted at 900℃ without carbon-based additives;

[0029] Figure 3 The image shows the phase analysis of the product from the water-leached residue roasted at 500°C in Example 1. Detailed Implementation

[0030] The calcining furnace used in this embodiment of the invention is a YZ-1433 rotary kiln, and the magnetic separation equipment is a CTB6012 magnetic separator;

[0031] The carbon-based additives, graphite powder and activated carbon powder, used in the embodiments of this invention are commercially available chemical reagents.

[0032] In this embodiment of the invention, the analytical method for obtaining refined iron ore is as follows: the total iron content in the iron concentrate obtained by magnetic separation is detected and defined as the grade of the iron concentrate; the rare earth oxides in the iron concentrate are expressed as REO, and their content is determined by testing the rare earth element content in the sample using an ICPS-8100 ICP emission spectrometer, thereby calculating the REO content.

[0033] The composition of the water-leached residue in this embodiment of the invention is shown in the table below:

[0034] element TFe Ca RE P Mg S O other content / % 11.07 13.08 6.22 6.75 5.8 11.4 42.3 3.35

[0035] The present invention will be described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0036] Example 1

[0037] Water-leached residue and carbon-based reducing agent were mixed at a weight ratio of 100:3. The mixture was then thoroughly mixed and heated to 500℃ in a roasting furnace. The mixture was placed in the furnace and roasted for 180 minutes, with air introduced into the furnace. The roasted product was then magnetically separated at a magnetic strength of 1.0T to obtain a rare-earth iron concentrate with an iron content (based on elemental iron) of 60% and a rare-earth content (based on rare-earth oxide content) of 8%, with a yield of 30%, and magnetic tailings with a calcium magnesium phosphate content of 98%, with a yield of 70%. The process flow diagram is shown below. Figure 1 As shown, the phase analysis diagram of the calcination product is as follows. Figure 3 As shown.

[0038] Example 2

[0039] Water-leached residue and carbon-based reducing agent were mixed at a weight ratio of 100:20. The mixture was then heated to 1000℃ in a roasting furnace and roasted for 180 minutes with nitrogen gas introduced into the furnace. The roasting product was then magnetically separated at a magnetic strength of 0.1T to obtain a rare earth iron concentrate with an iron content (based on elemental iron content) of 55% and a rare earth content (based on rare earth oxide content) of 15%, with a yield of 28%, and magnetic tailings with a calcium magnesium phosphate content of 90%, with a yield of 72%.

[0040] Example 3

[0041] The water-leached residue was mixed with a carbon-based reducing agent at a weight ratio of 100:10. The mixture was then thoroughly mixed and heated to 800°C. The mixture was placed in the roasting furnace and roasted for 90 minutes, with air or nitrogen introduced into the furnace. The roasted product was then magnetically separated at a magnetic strength of 0.5T to obtain a rare-earth iron concentrate with an iron content (based on elemental iron) of 58% and a rare-earth content (based on rare-earth oxide content) of 13%, yielding 29%, and magnetic tailings with a calcium magnesium phosphate content of 97%, yielding 71%.

[0042] Example 4

[0043] Water-leached residue and carbon-based reducing agent were mixed at a weight ratio of 100:15. The mixture was then heated to 700°C and placed in the roasting furnace for 90 minutes, with air introduced into the furnace. The roasting product was then magnetically separated at a magnetic strength of 0.5T to obtain a rare earth iron concentrate with an iron content (based on elemental iron content) of 57% and a rare earth content (based on rare earth oxide content) of 14%, with a yield of 28%, and magnetic tailings with a calcium magnesium phosphate content of 95%, with a yield of 72%.

[0044] Example 5

[0045] Water-leached residue and carbon-based reducing agent were mixed at a weight ratio of 100:20. The mixture was then heated to 900°C and placed in the roasting furnace for 90 minutes, with nitrogen gas introduced into the furnace. The roasting product was then magnetically separated at a magnetic strength of 1T to obtain a rare earth iron concentrate with an iron content (based on elemental iron content) of 60% and a rare earth content (based on rare earth oxide content) of 8%, with a yield of 29%, and magnetic tailings with a calcium magnesium phosphate content of 96%, with a yield of 71%.

[0046] Example 6

[0047] Water-leached residue and carbon-based reducing agent were mixed at a weight ratio of 100:5. The mixture was then heated to 700°C and placed in the roasting furnace for 180 minutes, with nitrogen gas introduced into the furnace. The roasting product was then magnetically separated at a magnetic strength of 0.5T to obtain a rare earth iron concentrate with an iron content (based on elemental iron content) of 57% and a rare earth content (based on rare earth oxide content) of 12%, with a yield of 29%, and magnetic tailings with a calcium magnesium phosphate content of 94%, with a yield of 71%.

[0048] Comparative Example 7

[0049] Without adding a carbon-based reducing agent, the calcination furnace was heated to 900℃, and the water-leached residue was placed in the furnace and calcined for 180 minutes, with air introduced into the furnace. The main phases of the calcined product remained ferric phosphate and calcium sulfate. The phase analysis diagram of the calcined product is shown below. Figure 2 As shown.

Claims

1. A method for reduction-decomposition-magnetic separation of water-leached residue, characterized in that: The steps include: (1) introducing air or nitrogen into the roasting furnace, mixing the water-leached slag and the carbon-based reducing agent evenly, placing the mixture in the roasting furnace for roasting, and obtaining a roasted product; (2) The exhaust gas from the roasting furnace is absorbed by circulating water, converting the sulfur trioxide in the exhaust gas into sulfuric acid; (3) The roasted product in the above step (1) is separated by magnetic separation to obtain rare earth iron concentrate and magnetic separation tailings; The water-leached residue in step (1) is the water-leached residue obtained from the neutralization and impurity removal process in the sulfuric acid roasting process of the mixed rare earth concentrate, the carbon-based reducing agent includes one of graphite powder, activated carbon powder, and biomass fuel powder (straw), and the roasting temperature is 500-1000°C.

2. The method for reduction-decomposition-magnetic separation of water-leached residue according to claim 1, characterized in that: The mixing described in the above step (1) is specifically mixing the water-leached residue and the carbon base in a weight ratio of 100:(3-20).

3. The method for reduction-decomposition-magnetic separation of water-leached residue according to claim 1, characterized in that: The calcination in the above step (1) is specifically performed at a calcination temperature of 500-1000° C. and a calcination time of 30-180 min.

4. The method for reduction-decomposition-magnetic separation of water-leached residue according to claim 1, characterized in that: When air is introduced into the calcination furnace in the above step (1), iron in the calcination product exists in the form of ferric oxide and rare earth exists in the form of oxides; when nitrogen is introduced, iron in the calcination product exists in the form of ferroferric oxide and rare earth exists in the form of oxides.

5. The method for reduction-decomposition-magnetic separation of water-leached residue according to claim 1, characterized in that: The sulfuric acid described in the above step (2) can be used for recycling in the mixed rare earth concentrate.

6. The method for reduction-decomposition-magnetic separation of water-leached residue according to claim 1, characterized in that: The magnetic separation process described in the above step (3) is carried out at a magnetic strength of 0.1~1.0T.

7. The method for reduction-decomposition-magnetic separation of water-leached residue according to claim 1, characterized in that: The rare earth iron concentrate obtained in step (3) has an iron content (calculated as iron element content) of 55% to 60%, a rare earth content (calculated as rare earth oxide content) of 8% to 15%, and a yield of 28% to 30%.

8. The method for reduction-decomposition-magnetic separation of water-leached residue according to claim 1, characterized in that: The calcium phosphate content of the magnetic separation tailings obtained in the above step (3) is 90-98%, and the yield is 70-72%.

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