1. A method for removing calcium from fly ash and/or coal gangue by using ultrasonic acid washing is characterized by comprising the following steps:

(1) mixing and stirring fly ash and/or coal gangue to be treated with dilute nitric acid solution uniformly, heating to 30-100 ℃, and fully reacting for 0.2-2h under the action of 20-50KHz ultrasonic field to obtain a first material;

(2) filtering the first material to obtain filter residue and filtrate, wherein the filter residue is the fly ash and/or coal gangue after calcium removal;

(3) and (3) supplementing acid to the filtrate, returning to the step (1) for circular acid washing, enriching calcium and magnesium ions in the filtrate to a certain concentration, and precipitating to remove aluminum for preparing the nitro calcium magnesium fertilizer.

2. The method for removing calcium from fly ash and/or coal gangue by acid washing with ultrasonic waves as claimed in claim 1, wherein the particle size of the fly ash and/or coal gangue in step (1) is 0.02-0.2 mm.

3. The method for removing calcium from fly ash and/or coal gangue by acid washing with ultrasonic waves as claimed in claim 2, wherein the particle size of the fly ash and/or coal gangue in step (1) is 0.02-0.1 mm.

4. The method for removing calcium from fly ash and/or coal gangue by acid washing with ultrasonic waves as claimed in claim 1, wherein the mass concentration of the dilute nitric acid solution in the step (1) is 1-20%.

5. The method for removing calcium from fly ash and/or coal gangue by acid washing with ultrasonic waves as claimed in claim 4, wherein the mass concentration of the dilute nitric acid solution in the step (1) is 3-5%.

6. The method for removing calcium from fly ash and/or coal gangue by acid washing with ultrasonic waves according to claim 1, wherein the liquid-solid ratio of the fly ash and/or coal gangue to the dilute nitric acid solution in the step (1) is 1-10: 1.

7. The method for removing calcium from fly ash and/or coal gangue by acid washing with ultrasonic waves as claimed in claim 6, wherein the liquid-solid ratio of the fly ash and/or coal gangue to the dilute nitric acid solution in the step (1) is 5-8: 1.

8. The method for removing calcium from fly ash and/or coal gangue by acid washing with ultrasonic waves according to claim 1, wherein the temperature of the ultrasonic acid washing in the step (1) is 50-80 ℃.

9. The method for removing calcium from fly ash and/or coal gangue by acid washing with ultrasonic waves according to claim 1, wherein the heat preservation time of the ultrasonic acid washing in the step (1) is 0.3-2 h.

10. The method for removing calcium from fly ash and/or coal gangue by acid washing with ultrasonic waves according to claim 1, wherein calcium ions are enriched to a concentration of 10-20g/L and magnesium ions are precipitated to remove aluminum after the concentration of 3-10g/L in the filtrate in the step (3) and the filtrate is used for preparing the nitro calcium magnesium fertilizer.

Background

Fly ash and coal gangue are industrial solid wastes with the largest production amount of a thermal power plant, and a large amount of stockpiling causes serious environmental pollution. The fly ash is a mixed material formed by burning coal powder at high temperature, and in the process of storing in an ash storage pond or an ash yard, toxic and harmful metal elements can be released due to leaching of rainwater, so that surface water and underground water are polluted. Meanwhile, the random accumulation of the fly ash occupies a large amount of land resources. The coal gangue is solid waste discharged in the coal mining and coal washing processes, and is a black grey rock with low carbon content associated with a coal bed in the coal forming process. The large amount of stacking of the coal gangue not only occupies the land, but also causes the pollution of the surrounding soil and the underground water due to the leaching of the rainwater.

China is a country lacking in high-quality aluminum resources, bauxite needs to be imported from abroad in large quantity, and the fly ash and the coal gangue have high alumina content and can be used as production raw materials of alumina to relieve the shortage of aluminum resources in China, so that the alumina extraction mode with low development cost and high utilization rate has important significance. The method for extracting alumina mainly comprises an acid method and an alkaline method, wherein the acid method is widely used due to the advantages of high dissolution rate of the extracted alumina, simple process, recyclable acid and the like. However, the leachate obtained by extracting the alumina by the acid method is mixed with other metal ions such as iron, calcium, magnesium, sodium and the like, and the subsequent impurity removal process is long and high in cost. Particularly, limestone or lime is added as a desulphurization absorbent when the pulverized coal is combusted, so that the content of calcium and magnesium in the pulverized coal is higher, and the difficulty and the cost of removing the calcium and the magnesium in the pickle liquor are increased.

In recent years, many production units in China mostly adopt an acid leaching method for extracting alumina from fly ash or coal gangue, and because leachate contains elements with extremely close properties such as calcium, iron, magnesium and the like, deep removal is difficult by adopting methods such as extraction, ion exchange, precipitation and the like, and a calcium removal process in a technological process is time-consuming, labor-consuming, complex and tedious, has high investment cost and often introduces new impurities.

There are many methods for removing calcium ion in non-ferrous metal hydrometallurgy, and the neutralization precipitation method is adopted in Yangxi cloud, etc., and Na is used₂CO₃Used as impurity removing agent to remove impurities in copper sulfate at one time to make Ca in the product²⁺<0.0020 percent, but has the defects of large copper loss, difficult application to the requirement of removing high-concentration impurities, incomplete calcium and magnesium removal and the like, and the precipitator removal method often generates huge sludge to cause secondary pollution to the environment. Lanzhou university P₅O₇The method is characterized in that the copper sulfate liquid is subjected to calcium extraction, the calcium content is reduced from 272.5mg/L to 189.2mg/L under the optimal experimental conditions, the deep removal of calcium is not realized, and meanwhile, the solvent extraction method needs to consider an extracting agent and the dosage, so that the problem of high cost exists. Jilin mirror industry company mainly adopts a recrystallization production method to realize deep removal of harmful impurities such as calcium and the like in the crystallization process of copper sulfate for producing copper sulfate products meeting the requirement of electroplating grade, and has the advantages of high production energy consumption, low copper yield and the likeAnd the defects can not be utilized in the deep decalcification process of the copper sulfate solution.

At present, the fly ash achieves certain effect by acid washing and calcium removal. The combined wet permanent magnet-acid washing method is adopted in Heyong et al, so that the CaO content in the fly ash can be reduced to l.36wt%, and the decalcification rate is 62.01%. The Xiaojing waves use ammonium bicarbonate as a calcium sulfate phase conversion agent and nitric acid as a calcium removal agent, the removal rate of calcium in high-calcium fly ash is 88.22%, but new impurities are introduced.

Therefore, the development of a high-efficiency and low-cost calcium removal mode aiming at the fly ash or coal gangue is of great significance.

Disclosure of Invention

Aiming at the technical problems of large investment, high equipment requirement, high energy consumption, poor impurity removal effect and the like of the existing calcium removal process of the fly ash or coal gangue, the invention provides a method for removing calcium from the fly ash and/or coal gangue by using ultrasonic waves, and the method effectively reduces the calcium content in the fly ash and/or coal gangue by controlling various factors such as the granularity of raw materials, the concentration of nitric acid, the liquid-solid ratio, the leaching temperature, the heat preservation time and the like under an ultrasonic field with specific frequency, provides low-calcium fly ash and/or coal gangue for extracting industrial-grade alumina by subsequent nitric acid leaching, further simplifies the extraction process and improves the recovery rate; meanwhile, the invention has the advantages of low energy consumption, low cost, simple equipment and the like.

In order to achieve the purpose, the invention adopts the technical scheme that:

a method for removing calcium from fly ash and/or coal gangue by using ultrasonic waves comprises the following steps:

(1) mixing and stirring fly ash and/or coal gangue to be treated with dilute nitric acid solution uniformly, heating to 30-100 ℃, and fully reacting for 0.2-2h under the action of 20-50KHz ultrasonic field to obtain a first material;

(2) filtering the first material to obtain filter residue and filtrate, wherein the filter residue is the fly ash and/or coal gangue after calcium removal;

(3) and (3) supplementing acid to the filtrate, returning to the step (1) for circular acid washing, enriching calcium and magnesium ions in the filtrate to a certain concentration, and precipitating to remove aluminum for preparing the nitro calcium magnesium fertilizer.

Further, the granularity of the fly ash and/or coal gangue in the step (1) is 0.02-0.2 mm; preferably, the particle size of the fly ash and/or coal gangue in the step (1) is 0.02-0.1 mm. The inventor finds that the particle size of the crushed raw materials is an important parameter influencing the impurity removal rate through a large amount of researches, the impurity removal effect is poor due to too small or too large particle size, and the calcium removal effect is not obvious due to the fact that the raw materials are easy to agglomerate due to too small particle size; the granularity is too large, impurities cannot fully contact with the nitric acid solution, the reaction is not sufficient, and the calcium removal effect is still not good.

Further, the mass concentration of the dilute nitric acid solution in the step (1) is 1-20%; preferably, the mass concentration of the dilute nitric acid solution in the step (1) is 3-5%, at the concentration, the elution rate of calcium in the raw material is more than 70%, the elution rate of magnesium is higher than 25%, and the loss of aluminum is less than 3%.

Further, the liquid-solid ratio of the fly ash and/or coal gangue to the dilute nitric acid solution in the step (1) is 1-10: 1; preferably, the liquid-solid ratio of the fly ash and/or coal gangue to the dilute nitric acid solution in the step (1) is 5-8: 1. It should be noted that the liquid-solid ratio is also an important process parameter in the method, under the condition of a certain nitric acid concentration, the calcium removal effect is not good due to too small liquid-solid ratio, and although a good technical effect can be obtained due to a large liquid-solid ratio, the cost is increased due to too large liquid-solid ratio, which causes waste of water.

Preferably, the temperature of the ultrasonic pickling in the step (1) is 50-80 ℃. The inventor finds that heating is beneficial to improving the impurity removal rate through exploration, but the temperature is too high, so that the calcium and magnesium removal effect cannot be obviously improved, and the aluminum loss can be greatly caused.

Preferably, the heat preservation time of the ultrasonic pickling in the step (1) is 0.3-2 h. It should be noted that when the time of ultrasonic pickling is less than 0.1h, the pickling calcium removal effect is very poor, but after the heat preservation time is more than 2h, the leaching of calcium is not obviously increased.

Further, the filtrate in the step (3) is enriched with calcium ions to a concentration of 10-20g/L, and after the concentration of magnesium ions is 3-10g/L, aluminum is removed through precipitation, and the filtrate is used for preparing the nitro calcium magnesium fertilizer.

Compared with the prior art, the technical scheme of the invention has the following positive effects:

(1) the method has the advantages of simple process, low equipment requirement and low energy consumption, and effectively reduces the content of calcium or magnesium in the fly ash and/or coal gangue on the premise of ensuring less aluminum loss through the synergistic cooperation of various factors such as the granularity of raw materials, the concentration of nitric acid, the liquid-solid ratio, the leaching temperature, the heat preservation time and the like; under a specific combination of conditions, the method can remove more than 80% of calcium or more than 65% of magnesium in the fly ash and/or the coal gangue, and meanwhile, the washing-out rate of aluminum is less than 20% and is only about 2% at the lowest.

(2) In the acid washing and calcium removing process, the acid washing solution is recycled after acid supplementation, the calcium and magnesium content is continuously enriched, and when the calcium and magnesium are enriched to a higher concentration, the calcium and magnesium are precipitated to remove aluminum in the solution and can be used as a raw material for preparing the nitro calcium magnesium fertilizer, so that the economic benefit is improved; and the whole process flow does not produce industrial wastewater and waste residue, and green production is realized.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in further detail below.

The invention discloses a method for removing calcium from fly ash and/or coal gangue by using ultrasonic wave, which effectively reduces the content of calcium in the fly ash and/or coal gangue by using an ultrasonic field and simultaneously controlling various factors such as the granularity of raw materials, the concentration of nitric acid, the liquid-solid ratio, the leaching temperature, the heat preservation time and the like, and can remove more than 80 percent of calcium or more than 65 percent of magnesium in the fly ash and/or coal gangue under the combination of specific conditions, and the washing rate of aluminum is less than 2-20 percent, thereby providing low-calcium fly ash and/or coal gangue for extracting industrial-grade aluminum oxide by subsequent nitric acid leaching, further simplifying the extraction process and improving the recovery rate; meanwhile, the invention has the advantages of low energy consumption, low cost, simple equipment and the like.

The percentage contents appearing in the following examples are not directly specified as percentages by mass.

[ example 1 ]

Crushing the No. 1 coal gangue to the granularity of 0.1mm, preserving the heat for 0.5h under the conditions that the mass concentration of nitric acid is 10%, the liquid-solid ratio is 3:1 and the temperature is 50 ℃ under the action of a 40KHz ultrasonic field, and filtering to obtain the calcium-removed coal gangue.

The contents of iron, calcium, aluminum and magnesium in the coal gangue before and after impurity removal are tested, and the test results are shown in table 1.

TABLE 1 content of substances in coal gangue before and after impurity removal

Example 1	Iron content (%)	Aluminum content (%)	Magnesium content (%)	Calcium content (%)	Total weight of
						1# coal gangue before impurity removal	2.01	23.66	0.74	7.92	300g
1# coal gangue after impurity removal	2.11	25.19	0.32	3.29	250.8g

Calculated, the Al loss rate is 9.46 percent, the Fe removal rate is 12.24 percent, the Ca removal rate is 65.27 percent, and the Mg removal rate is 63.84 percent.

[ example 2 ]

Crushing the No. 1 coal gangue to the granularity of 0.02mm, preserving the heat for 1h at the temperature of 80 ℃ and the mass concentration of 20 percent of nitric acid and the liquid-solid ratio of 4:1 under the action of a 45KHz ultrasonic field, and filtering to obtain the calcium-removed coal gangue.

The contents of iron, calcium, aluminum and magnesium in the coal gangue before and after the impurity removal in example 2 were tested, and the test results are shown in table 2.

TABLE 2 content of substances in coal gangue before and after impurity removal

Example 2	Iron content (%)	Aluminum content (%)	Magnesium content (%)	Calcium content (%)	Total weight of
						1# coal gangue before impurity removal	2.01	23.66	0.74	7.92	300g
1# coal gangue after impurity removal	2.14	25.68	0.29	2.95	234.3g

Calculated, the Al loss rate is 15.23 percent, the Fe removal rate is 16.85 percent, the Ca removal rate is 70.91 percent, and the Mg removal rate is 69.39 percent.

[ example 3 ]

Crushing the No. 2 fly ash to the granularity of 0.1mm, preserving the heat for 2h under the action of a 20KHz ultrasonic field and at the temperature of 60 ℃ and the mass concentration of 3 percent of nitric acid and the liquid-solid ratio of 5:1, and filtering to obtain the fly ash after calcium removal.

The contents of iron, calcium, aluminum and magnesium in the fly ash before and after the impurity removal in example 3 were tested, and the test results are shown in table 3.

TABLE 3 content of substances in fly ash before and after impurity removal

Example 3	Iron content (%)	Aluminum content (%)	Magnesium content (%)	Calcium content (%)	Total weight of
						No. 2 fly ash before impurity removal	1.11	23.83	0.16	2.26	300g
2# fly ash after impurity removal	1.01	24.71	0.12	0.65	282.6g

Calculated, the Al loss rate is 2.32%, the Fe removal rate is 14.28%, the Ca removal rate is 72.90%, and the Mg removal rate is 25.64%.

[ example 4 ]

Crushing the No. 2 fly ash to the granularity of 0.03mm, preserving the heat for 1h under the conditions that the mass concentration of nitric acid is 5%, the liquid-solid ratio is 7:1 and the temperature is 80 ℃ under the action of a 25KHz ultrasonic field, and filtering to obtain the fly ash after calcium removal.

The contents of iron, calcium, aluminum and magnesium in the fly ash before and after the impurity removal in example 4 were tested, and the test results are shown in table 4.

TABLE 3 content of substances in fly ash before and after impurity removal

Example 4	Iron content (%)	Aluminum content (%)	Magnesium content (%)	Calcium content (%)	Total weight of
						No. 2 fly ash before impurity removal	1.11	23.83	0.15	2.26	300g
2# fly ash after impurity removal	1.05	25.17	0.11	0.48	277.5g

Calculated, the Al loss rate is 2.29 percent, the Fe removal rate is 12.50 percent, the Ca removal rate is 80.35 percent, and the Mg removal rate is 30 percent.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.