Micro-emulsion collecting agent applying jatropha curcas oil to low-rank coal flotation and preparation and application of micro-emulsion collecting agent
By preparing a microemulsion collector for jatropha oil, the problem of poor collector performance in the flotation of low-rank coal was solved, the yield of clean coal and the recovery rate of combustibles were improved, and efficient flotation of low-rank coal was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-01-09
- Publication Date
- 2026-03-06
AI Technical Summary
In the flotation of low-rank coal, the collector has poor collection performance, large reagent dosage, and low flotation efficiency, resulting in low clean coal yield and high ash content.
Using Jatropha curcas oil biodiesel as the main component, and compounded with surfactants and co-surfactants, a microemulsion collector was prepared. Through transesterification and stirring, a microemulsion was formed, which enhanced the adsorption and spreading ability on the surface of low-rank coal.
It significantly improves the clean coal yield and combustible recovery rate of low-rank coal flotation, reduces reagent usage, improves flotation efficiency, and has both environmental and economic benefits.
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Figure CN121607262A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of low-rank coal flotation technology, specifically, it relates to a microemulsion collector for the flotation of low-rank coal using jatropha oil, and its preparation and application. Background Technology
[0003] Efficient separation of low-rank coal is a prerequisite for achieving efficient resource utilization. However, due to its low degree of metamorphism, abundant oxygen-containing functional groups, well-developed pores, and poor hydrophobicity, water molecules readily form a dense hydration film on the surface of low-rank coal. This film hinders the adsorption and spreading of traditional hydrocarbon oil collectors on the coal particle surface, and is detrimental to the effective adhesion of coal particles and air bubbles, resulting in unsatisfactory flotation effects using conventional collectors. High collector dosage and low flotation efficiency are prominent challenges in low-rank coal flotation, becoming a bottleneck restricting the deep washing and beneficiation of low-rank coal.
[0004] Domestic and international researchers have conducted extensive research on the design and development of flotation reagents for low-rank coal. Surfactants, through specific functional groups, interact with the polar functional groups on the surface of low-rank coal, effectively covering the oxygen-containing functional groups on the coal particle surface, increasing the methyl and methylene content on the coal particle surface, thereby enhancing hydrophobicity and improving the flotation effect of low-rank coal. Surfactants are mainly classified into nonionic, ionic, and compound types. Nonionic flotation reagents include oxygen-containing functional group collectors such as polyoxyethylene ether type and polyol type. The polar end of these collectors easily forms hydrogen bonds with the oxygen-containing groups on the coal surface, while the nonpolar end adsorbs onto the nonpolar parts of the coal surface. Ionic collectors mainly include anionic collectors, cationic collectors, and amphoteric collectors. Anionic surfactants include carboxylate, sulfate, sulfonate, and phosphate types; cationic collectors are mainly divided into two categories: amine salt collectors (primary, secondary, and tertiary amine salts) and quaternary ammonium salt collectors; amphoteric surfactants mainly include amino acid and betaine types. Because coal surfaces have polar groups and often carry a weak charge, ionic collectors are mainly adsorbed onto the coal surface through electrostatic interactions, hydrogen bonding, and hydrophobic interactions.
[0005] Low-rank coal surfaces are rich in various polar hydrophilic functional groups. The shielding effect of a single collector on these surface functional groups is limited, while compound collectors, due to their combination of nonionic / cationic / anionic or even amphoteric collector systems, exhibit strong synergistic effects and have great application potential. A common type is the compound collector of nonionic and ionic surfactants. It is generally believed that ionic collectors can adsorb onto low-rank coal surfaces through electrostatic interactions or hydrogen bonding with polar groups on the coal surface, and their hydrophobic tails can promote the adsorption of nonpolar hydrocarbon oils on the low-rank coal surface. Studies have shown that vegetable oils contain a large amount of saturated and unsaturated fatty acids, as well as esters and alcohols. These oxygen-containing components can bind with polar oxygen-containing sites on the low-rank coal surface through hydrogen bonds to form adsorption, thereby enhancing the hydrophobicity of the low-rank coal surface and improving flotation efficiency.
[0006] In view of this, the present invention is proposed. Summary of the Invention
[0007] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a microemulsion collector for the flotation of low-rank coal using jatropha oil, as well as its preparation and application. Using jatropha oil biodiesel as the main component, and compounded with surfactants and co-surfactants, a microemulsion collector with good dispersibility and high low-temperature resistance is prepared to improve the flotation effect of low-rank coal. This solves the problems of poor collection performance, large reagent dosage, and low flotation efficiency of existing collectors, especially the problems of low clean coal yield and low clean coal grade in the flotation of bituminous coal.
[0008] To solve the above-mentioned technical problems, the basic concept of the technical solution adopted by the present invention is as follows:
[0009] This invention provides a microemulsion collector for the flotation of low-rank coal using jatropha oil, comprising the following components by mass fraction: 35% jatropha oil biodiesel, 2% surfactant, 5% co-surfactant, and 58% water;
[0010] The surfactant is one or any combination of rhamnolipin, sophorolipid, and soybean lecithin, and the co-surfactant is one or any combination of n-butanol, n-pentanol, n-hexanol, or n-octanol.
[0011] Another aspect of the present invention provides a method for preparing a microemulsion collector for the flotation of low-rank coal using jatropha oil, comprising the following steps:
[0012] (1) Jatropha curcas oil crude oil and methanol were subjected to transesterification reaction in the presence of an alkaline catalyst. After the reaction was completed, the upper liquid was separated to obtain Jatropha curcas oil biodiesel.
[0013] (2) Dissolve at least one of rhamnolipin / sophorolipid / soybean lecithin in water to prepare a surfactant solution;
[0014] (3) Mix the jatropha oil biodiesel obtained in step (1) with the surfactant solution obtained in step (2), add a co-surfactant, and stir at room temperature to form a microemulsion;
[0015] (4) The obtained microemulsion is filtered to obtain a microemulsion collector.
[0016] Furthermore, the method for preparing jatropha oil biodiesel via transesterification in step (1) includes:
[0017] (I) First, acid-catalyzed pre-esterification is performed to lower the acid value and avoid saponification side reactions in the subsequent alkali-catalyzed stage. The specific operation is as follows:
[0018] (11) Add 100 g of jatropha oil to the three-necked flask, insert the stirrer, thermometer and condenser, place it in the oil bath and start stirring slowly.
[0019] (12) Add 20-24 ml of anhydrous methanol to a beaker, place it in an ice-water bath, and slowly add 0.55 ml of concentrated sulfuric acid using a glass rod. After mixing, a transparent solution is obtained.
[0020] (13) Add the above transparent solution to the three-necked flask all at once; continue to heat and stabilize at 50-60 ± 1 ℃, introduce a weak N2, and start the reaction for 1-1.5 h.
[0021] (14) Turn off the heating, remove the heat source, and continue stirring until the temperature drops below 40 °C.
[0022] (II) The transesterification reaction of the acid-reduced oil is carried out as follows:
[0023] (15) Add 100 g of acid-lowering oil to a three-necked flask, place it in an oil bath at 50-60 ℃, and stir at 150-200 r·min⁻¹.
[0024] (16) Add 20-24 ml of anhydrous methanol to a beaker, then add 1 g of KOH and shake gently until completely dissolved to obtain a clear solution.
[0025] (17) Pour the alcohol-alkali solution into the three-necked flask quickly at once, and continue the reaction at 50-60℃ and 150-200 r·min⁻¹ for 0.5-1h.
[0026] Furthermore, the main components of the jatropha oil are:
[0027] .
[0028] The equation for the reaction that occurs in this step is:
[0029]
[0030] It should be noted that this reaction is a three-step reversible reaction, with each step releasing one fatty acid methyl ester molecule, ultimately producing a glycerol byproduct.
[0031] Furthermore, rhamnolipin, sophorolipid, or soybean lecithin are dissolved in water to prepare a surfactant solution with a mass concentration of 0.1-1%.
[0032] Furthermore, the co-surfactant is at least one of n-butanol, n-pentanol, n-hexanol, or n-octanol, and the amount of co-surfactant used is 5-20% of the mass of Jatropha curcas oil biodiesel.
[0033] Furthermore, the mass ratio of jatropha oil biodiesel to surfactant solution is 1:1-1:5, the stirring speed is 450-550 rpm, and the stirring time is 20-30 minutes.
[0034] Furthermore, in step (3), a microporous filter membrane with a diameter of 0.22-0.45 μm is used for filtration.
[0035] Furthermore, in step (4), the microemulsion particle size D50 ≤ 1 μm, the Zeta potential -30 mV to -45 mV, and the centrifugal stability index K < 5%.
[0036] In another aspect, this invention provides the application of the aforementioned microemulsion collector in the flotation of low-rank coal. The flotation process specifically includes the following steps:
[0037] S1. Crush the low-rank coal blocks with a jaw crusher and screen them to a particle size ≤5mm for later use;
[0038] S2. The coal slurry is placed in a flotation machine for pre-stirring, then microemulsion reagents and frothers are added. After stirring, the air valve is opened to allow air to enter and scrape the foam until there is no obvious foam in the clean coal, thus obtaining flotation clean coal and tailings.
[0039] S3. Dry and weigh the obtained flotation clean coal and tailings respectively, and calculate the clean coal yield, ash content and combustible gas recovery rate.
[0040] Further, in step S2, the coal slurry concentration is 50 g / L to 150 g / L, the flotation machine speed is 1800 r / min to 2000 r / min; the microemulsion collector is added at a rate of 2 kg / t to 3 kg / t, and the stirring time is 2 min to 4 min; the frother is 2-octanol or methyl isobutyl methanol, with a dosage of 0.1 kg / t to 1 kg / t, and is stirred for 2 min to 4 min after addition; the aeration rate is 0.15-0.20 m³ / t. 3 / h.
[0041] Furthermore, in step S3, the combustible recovery rate of the clean coal obtained by flotation is 77% to 85%, and the ash content of the clean coal is 10% to 15%.
[0042] By adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art.
[0043] (1) Significantly improved flotation performance: Compared with traditional diesel or kerosene, at the same dosage (200g / t), the yield of clean coal increases from about 55% to 81%~82%, the ash content decreases from 8%~9% to 6.5%~7.5%, and the combustible recovery rate increases from 57%~59% to 89%~91%, which greatly improves the utilization rate of coal resources.
[0044] (2) Environmental protection and sustainability are prominent: using renewable jatropha oil as raw material, combined with biosurfactants (such as rhamnolipin and sophorolipid), it has good degradability and no harmful by-products in the preparation process, which is in line with the direction of green chemical industry.
[0045] (3) Strong compatibility with low-rank coal: The microemulsion system enhances the adsorption and spreading ability on the surface of low-rank coal, effectively improving its hydrophobicity and increasing flotation efficiency.
[0046] (4) Good stability and easy to apply: The microemulsion has small particle size (D50 ≤1μm), moderate Zeta potential (-30~-45mV), stable dispersion, and is not easy to separate into layers. The preparation process is simple and suitable for industrial production.
[0047] (5) Significant comprehensive economic benefits: While improving the quality and yield of clean coal, it reduces tailings loss and overall reagent consumption, and has good market application prospects.
[0048] This invention addresses the characteristics of low-rank coal, such as low metamorphism, abundant oxygen-containing functional groups on the surface, well-developed pores, and poor hydrophobicity. By rationally selecting and compounding different types of reagents, it solves the problems of low selectivity, poor collection performance, large reagent dosage, low clean coal yield, and high ash content in the flotation of low-rank bituminous coal. It can significantly improve flotation efficiency and has promising prospects for industrial application.
[0049] The specific embodiments of the present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description
[0050] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention, but do not constitute an undue limitation of the invention. Obviously, the drawings described below are merely some embodiments, and those skilled in the art can obtain other drawings based on these drawings without creative effort. In the drawings:
[0051] Figure 1 This is a schematic diagram of the process for preparing a composite microemulsion collector using crude jatropha oil as a raw material according to the present invention.
[0052] Figure 2 This is a process flow diagram of the two-step method for preparing biodiesel from jatropha oil using acid pretreatment and alkali catalysis in this invention.
[0053] Figure 3 This is a laboratory implementation plan and operation process for preparing a composite microemulsion collector using Jatropha curcas oil as raw material according to the present invention.
[0054] It should be noted that these accompanying drawings and textual descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art by referring to specific embodiments. Detailed Implementation
[0055] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.
[0056] Example 1
[0057] Combined with appendix Figure 1-3 As shown in this embodiment, a low-rank coal flotation microemulsion collector based on esterified jatropha oil is specifically formed by mixing biodiesel generated from transesterification with a surfactant solution, adding additives, and stirring at room temperature. The microemulsion composition, by mass fraction, includes 35% jatropha oil biodiesel, 2% rhamnolipid, 5% n-pentanol, and 58% water.
[0058] The preparation method of the above-mentioned low-rank coal microemulsion collector specifically includes the following steps:
[0059] (1) Jatropha curcas oil crude oil and methanol were subjected to transesterification reaction in the presence of an alkaline catalyst. The reaction conditions were: 60°C, a molar ratio of methanol to jatropha curcas oil crude oil of 6:1, and a reaction time of 0.5 h. After the reaction was completed, the supernatant was separated to obtain jatropha curcas oil biodiesel.
[0060] (2) Dissolve rhamnolipin in water to prepare a rhamnolipin solution;
[0061] (3) Mix the jatropha oil biodiesel obtained in the above steps with the obtained rhamnolipin solution, add n-pentanol, stir at room temperature at a stirring speed of 500 rpm for 25 minutes to form a microemulsion;
[0062] (4) The obtained microemulsion is filtered to obtain a microemulsion collector.
[0063] The application of the aforementioned low-rank coal microemulsion collector specifically includes the following steps:
[0064] S1. A low-rank bituminous coal (ash content 15.88%) from Yulin District, Yulin City, Shaanxi Province, was ground to below 0.5 mm. The pulp concentration was 80 g / L, the flotation machine speed was 2000 r / min, and the aeration rate was 0.20 m³ / min. 3 / h;
[0065] S2. First, pre-wet 80 g of dry coal with water for 2 minutes, then add microemulsion collector to the slurry at a rate of 200 g / t. After adjusting the slurry for 2 minutes, add frother (2-octanol) at a rate of 400 g / t. After adjusting the slurry again for 2 seconds, open the air inlet valve to introduce air for foam scraping. Collect the foam product after 3 minutes to obtain flotation concentrate and tailings.
[0066] S3. The obtained flotation clean coal and tailings are dried and weighed respectively. The yield of the obtained flotation clean coal is 81.98%, the ash content is 7.50%, and the combustible recovery rate is 90.15%.
[0067] Example 2
[0068] A microemulsion collector for low-rank coal flotation based on esterified jatropha oil is specifically formulated by mixing biodiesel generated from transesterification with a sophorolipid solution, followed by the addition of an additive and stirring at room temperature to form a microemulsion. The microemulsion comprises, by mass fraction, 35% jatropha oil biodiesel, 2% sophorolipid, 5% n-pentanol, and 58% water.
[0069] The preparation method of the above-mentioned low-rank coal microemulsion collector specifically includes the following steps:
[0070] (1) Jatropha curcas oil crude oil and methanol were subjected to transesterification reaction in the presence of an alkaline catalyst. The reaction conditions were: 60°C, a molar ratio of methanol to jatropha curcas oil crude oil of 6:1, and a reaction time of 0.5 h. After the reaction was completed, the supernatant was separated to obtain jatropha curcas oil biodiesel.
[0071] (2) Dissolve sophoryl alcohol in water to prepare a sophoryl alcohol solution;
[0072] (3) Mix the jatropha oil biodiesel obtained in the above steps with the obtained sophorolipid solution, add n-pentanol, stir at room temperature at a stirring speed of 500 rpm for 25 minutes to form a microemulsion;
[0073] (4) The obtained microemulsion is filtered to obtain a microemulsion collector.
[0074] The application of the aforementioned low-rank coal microemulsion collector specifically includes the following steps:
[0075] S1. A low-rank bituminous coal (ash content 15.88%) from Yulin District, Yulin City, Shaanxi Province, was ground to below -0.5 mm. The pulp concentration was 80 g / L, the flotation machine speed was 2000 r / min, and the aeration rate was 0.20 m³ / min. 3 / h;
[0076] S2. First, pre-wet 80 g of dry coal with water for 2 minutes, then add microemulsion collector to the slurry at a rate of 200 g / t. After adjusting the slurry for 2 minutes, add frother (2-octanol) at a rate of 400 g / t. After adjusting the slurry again for 2 seconds, open the air inlet valve to introduce air for foam scraping. Collect the foam product after 3 minutes to obtain flotation concentrate and tailings.
[0077] S3. The obtained flotation clean coal and tailings are dried and weighed respectively. The yield of the obtained flotation clean coal is 82.51%, the ash content is 7.00%, and the combustible recovery rate is 91.22%.
[0078] Example 3
[0079] The formula in Example 1 above was adjusted to 35% jatropha oil biodiesel, 2% soybean lecithin, 5% n-pentanol, and 58% water, while the other operating steps remained the same.
[0080] The obtained flotation clean coal and tailings were dried and weighed separately. The yield of the obtained flotation clean coal was 80.40%, the ash content was 6.50%, and the combustible recovery rate was 89.37%.
[0081] Comparison Case 1
[0082] In Example 1 above, the collector for low-rank coal flotation was replaced with diesel oil instead of jatropha oil microemulsion collector, while all other conditions remained unchanged. The flotation test was conducted as follows:
[0083] First, pre-wet 80 g of dry coal with water for 2 minutes. Then, add diesel oil to the slurry at a rate of 200 g / t. After adjusting the slurry for 2 minutes, add a frother (2-octanol) at a rate of 400 g / t. After adjusting the slurry again for 2 seconds, open the air inlet valve to introduce air for foam scraping. Collect the foam product after 3 minutes to obtain flotation concentrate and tailings.
[0084] S3. The obtained flotation clean coal and tailings were dried and weighed respectively. The yield of the obtained flotation clean coal was 55.78%, the ash content was 8.56%, and the combustible recovery rate was 59.47%.
[0085] Comparison Case 2
[0086] In Example 1 above, the collector for low-rank coal flotation was replaced with kerosene instead of jatropha oil microemulsion collector, while all other conditions remained unchanged. The flotation test was conducted as follows:
[0087] First, pre-wet 80 g of dry coal with water for 2 minutes. Then, add kerosene to the slurry at a rate of 200 g / t. After adjusting the slurry for 2 minutes, add a frother (2-octanol) at a rate of 400 g / t. After adjusting the slurry again for 2 seconds, open the air inlet valve to introduce air for foam scraping. Collect the foam product after 3 minutes to obtain flotation concentrate and tailings.
[0088] The obtained flotation clean coal and tailings were dried and weighed separately. The yield of the obtained flotation clean coal was 54.06%, the ash content was 8.87%, and the combustible recovery rate was 57.53%.
[0089] Analysis of implementation cases 1, 2, and 3 and comparative cases 1 and 2 shows that the prepared jatropha oil microemulsion collector is significantly more effective than traditional hydrocarbon oil collectors such as diesel and kerosene in the flotation of low-rank coal.
[0090] In summary, addressing the problems of low selectivity, poor collection performance, large reagent dosage, low clean coal yield, and high ash content in existing technologies, this invention designs a microemulsion collector for the flotation of low-rank coal using jatropha oil and its preparation method. The microemulsion collector exhibits stronger adsorption on the coal surface than ordinary diesel oil, reducing reagent dosage and significantly improving the adsorption strength and spreading rate of the collector. Furthermore, jatropha oil is a biodiesel, reducing dependence on traditional diesel oil, thus possessing certain economic advantages and significant environmental benefits.
[0091] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.
Claims
1. A microemulsion collector for the application of jatropha oil to low-rank coal flotation, characterized by, Consists of the following mass fraction groups: 35% Jatropha oil biodiesel, 2% surfactant, 5% co-surfactant, 58% water; The surfactant is one or any combination of rhamnolipid, sophorolipid, and soybean lecithin, and the co-surfactant is one or any combination of n-butanol, n-pentanol, n-hexanol, or n-octanol.
2. A process for the preparation of the microemulsion collector as claimed in claim 1, characterized in that, The method comprises the following steps: Step (1), ester exchange reaction of Jatropha oil crude oil and methanol in the presence of an alkaline catalyst, after the reaction is completed, the upper liquid is separated to obtain Jatropha oil biodiesel; Step (2), the surfactant is dissolved in water to prepare a surfactant solution; Step (3), the Jatropha oil biodiesel obtained in step (1) is mixed with the surfactant solution obtained in step (2), and a co-surfactant is added, stirred at room temperature, and a microemulsion is formed; Step (4), the obtained microemulsion is filtered to obtain a microemulsion collector.
3. The preparation method according to claim 1, characterized in that, In step (1), acid catalysis pre-esterification is performed before ester exchange reaction to reduce acid value and avoid saponification side reaction in the subsequent alkaline catalysis stage; the specific operation steps of acid catalysis pre-esterification are as follows: Step (11), add Jatropha oil into a three-necked flask, insert a stirring paddle, a thermometer, and a condenser tube, and place it in an oil bath, and start slow stirring; Step (12), first add anhydrous methanol in a beaker, place it in an ice water bath, slowly add concentrated sulfuric acid by using a glass rod as a flow guide, and mix well to obtain a transparent solution; Step (13), add the transparent solution in step (12) into the three-necked flask at one time; continue to heat and stabilize at 50-60℃, and pass in weak N2, and time reaction for 1-1.5h; Step (14), turn off the heating, remove the heat source, continue to stir, and reduce to below 40℃ to obtain acid-reduced oil.
4. The production method according to claim 3, characterized by, The specific steps of ester exchange reaction of acid-reduced oil are as follows: Step (15), add acid-reduced oil into a three-necked flask, place it in a 50-60℃ oil bath, and stir at 150-200 r·min⁻¹; Step (16), first add anhydrous methanol in a beaker, then add KOH, and gently shake until completely dissolved to obtain a clear alcohol alkali solution; Step (17), quickly pour the alcohol alkali solution into the three-necked flask, continue to react at 50-60℃ and 150-200 r·min⁻¹ for 0.5-1h.
5. The preparation method according to claim 2, characterized in that, In step (2), the surfactant is one or any combination of rhamnolipid, sophorolipid, and soybean lecithin, and the mass concentration of the active agent solution is 0.1-1%.
6. A process for the preparation of a microemulsion collector for the application of jatropha oil to the flotation of low-rank coal according to claim 2, characterized by, In step (3), the co-surfactant is one or any combination of n-butanol, n-pentanol, n-hexanol, or n-octanol, and the amount of the co-surfactant is 5-20% of the mass of the Jatropha oil biodiesel.
7. A process for the preparation of a microemulsion collector for the application of jatropha oil to the floatation of low rank coal as claimed in claim 2, wherein the process is characterized by, In step (3), the mass ratio of Jatropha oil biodiesel to surfactant solution is 1:1-1:5; the stirring speed is 450-550rpm, and the stirring time is 20-30 minutes.
8. A process for the preparation of a microemulsion collector for the application of jatropha oil to the flotation of low-rank coal according to claim 2, characterized by, In step (3), the particle size D50 of the microemulsion is ≤1μm, the Zeta potential is-30 mV to-45 mV, and the centrifugal stability index K is <5%; in step (4), a microporous filter membrane with a pore size of 0.22-0.45μm is used for filtration.
9. Use of the microemulsion collector of claim 1 or the microemulsion collector prepared by the method of any one of claims 2-8 in low-rank coal flotation.
10. Use according to claim 9, characterized in that, The flotation process comprises the following steps: S1. crushing the low-rank coal lumps with an e-type crusher and sieving to a particle size of ≤5 mm to prepare a coal slurry; S2. placing the coal slurry in a flotation machine for pre-stirring, then adding the microemulsion collector and a frother, stirring, opening the air inlet valve to introduce air and scraping foam until no obvious foam is generated, to obtain flotation clean coal and tailings; wherein the concentration of the coal slurry is 50 g / L-150 g / L, the rotation speed of the flotation machine is 1800 r / min-2000 r / min; the addition amount of the microemulsion collector is 2 kg / t-3 kg / t, and the stirring time is 2 min-4 min; the frother is sec-octyl alcohol or methyl isobutyl carbinol, and the amount used is 0.1 kg / t-1 kg / t, and the stirring time after addition is 2 min-4 min, and the air charge amount is 0.15-0.20 m3 / h.