Microemulsion collecting agent applied to coal slime flotation and preparation method of microemulsion collecting agent

Abstract

The invention discloses a microemulsion collecting agent applied to coal slime flotation and a preparation method of the microemulsion collecting agent. The microemulsion collecting agent is prepared from the following raw materials in parts by weight: 25-45 parts of hydrocarbon oil, 10-30 parts of methyl isobutyl carbinol, 20-40 parts of a surfactant and 5-15 parts of water. Wherein the surface active agent is polyethylene glycol modified lactone type sophorolipid and / or epoxidized lactam sophorolipid. Epoxidized lactam sophorolipid and polyethylene glycol modified lactone type sophorolipid are prepared into the composite surfactant with an appropriate hydrophilic-lipophilic equilibrium value for the first time, the composite surfactant is used for preparing the microemulsion collecting agent for coal slime flotation, and the prepared collecting agent is small in oil drop particle size and can be uniformly dispersed in ore pulp, so that the floatability of coal slime is enhanced; the method has important significance on improving the coal slime flotation effect and has the effects of being low in reagent dosage, saving energy, reducing consumption and improving the clean coal recovery rate.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal flotation, and particularly relates to a microemulsion collector for coal slime flotation and a preparation method thereof. Background Art

[0002] At present, flotation is widely used in the separation of fine coal slime. With the continuous deepening of coal mining mechanization, the content of coal slime in raw coal gradually increases, and the total feed amount of the flotation system also increases. The separation of coal slime shows the characteristics of "fine, miscellaneous, and difficult". Among them, the collector plays a key role in the flotation operation.

[0003] Generally, the collectors used in coal slime flotation are mainly non-polar hydrocarbon oils, such as kerosene, diesel oil, etc., which have the problem of poor solubility. Through the action of mechanical stirring, the reagent can be evenly dispersed in the pulp. During the flotation process, the apparent volume of the formed oil droplets is large and the number is small. The probability of effective collision between the collector and the ore particles is low, and there are disadvantages such as poor adhesion. Some oil droplets will also aggregate into larger oil droplets, reducing the collecting effect of the collector.

[0004] However, for difficult-to-float and difficult-to-separate fine coal slime, its surface often contains a large number of strongly hydrophilic polar groups, and its floatability is very poor. For example, the hydrocarbon oil content of conventional collectors is usually high. Hydrocarbon oil is insoluble in water and has poor dispersibility in water. It usually exists in the form of oil droplets with larger particle sizes, while the particle size of coal in coal slime is usually small. Therefore, conventional non-polar hydrocarbon oils are not easily adhered to the surface of coal in coal slime and spread to form an oil film, resulting in a very high dosage of the reagent and poor flotation effect of coal slime. To solve the problems of the collector, current research mainly focuses on developing green, highly efficient and environmentally friendly collectors or mixing different reagents to improve the hydrophobicity of the surface of coal slime with poor floatability.

[0005] In addition, emulsifying the collector can further improve the flotation efficiency of the reagent and reduce the reagent usage cost. The microemulsion technology can make the particle size of the collector smaller and more uniformly dispersed in the pulp, with the advantages of thermodynamic stability, small dispersion particle size, and low interfacial tension. Therefore, the microemulsion technology has high application prospects in the field of coal slime flotation.

[0006] CN106000657A discloses a coal slime flotation collector, which is a transparent liquid composed of hydrocarbon oil, sec-octanol and a surfactant with a promoting effect on flotation. However, due to the relatively high production cost of Span80 and Tween80 and their difficulty in complete degradation in the environment, which may have a long-term impact on the ecosystem, their application in coal slime flotation collectors is limited.

[0007] CN103056033A discloses a fine emulsion collector for slime, comprising: 6%-15% of hydrocarbon oil, 2%-6% of main emulsifier, 2%-4% of co-surfactant, and the balance being water, wherein the hydrocarbon oil is light diesel oil, selected from No. 10 light diesel oil, No. 0 light diesel oil, No. -10 light diesel oil, No. -20 light diesel oil or No. -35 light diesel oil; the main emulsifier is an alkylphenol polyoxyethylene ether surfactant; the co-surfactant is at least selected from one of C4-C8 alcohols. However, to improve the emulsification effect, the dosage of the surfactant is large. Summary of the Invention

[0008] The object of the present invention is to provide a fine emulsion collector for slime flotation and its preparation method, so as to solve the problem that the collector has a poor collecting effect on coal in slime, improve its collectability, and achieve the purpose of saving oil and reducing consumption.

[0009] To achieve one aspect of the above-mentioned invention object, the present invention adopts the following technical solutions:

[0010] A fine emulsion collector for slime flotation, the fine emulsion collector is made of raw materials including the following parts by weight: 25-45 parts of hydrocarbon oil, 10-30 parts of methyl isobutyl carbinol, 20-40 parts of surfactant, and 5-15 parts of water; wherein, the surfactant is polyethylene glycol-modified lactone-type sophorolipid and / or epoxidized lactam sophorolipid.

[0011] In the present invention, the fine emulsion collector can be prepared by mixing the above raw materials and stirring and emulsifying; wherein, the hydrocarbon oil is preferably kerosene and / or diesel oil.

[0012] In a preferred embodiment, among the raw materials used for the fine emulsion collector, by weight, the dosage of hydrocarbon oil is 30-40 parts such as 32, 35 or 38 parts, the dosage of methyl isobutyl carbinol is 15-25 parts such as 17, 20 or 22 parts, the dosage of surfactant is 25-35 parts such as 28, 30 or 32 parts, and the dosage of water is 5-15 parts such as 8, 10 or 12 parts. It is understood in the art that deionized water can preferably be used.

[0013] In a preferred embodiment, the surfactant is a mixture of polyethylene glycol-modified lactone-type sophorolipid and epoxidized lactam sophorolipid. By using the different hydrophilic-lipophilic balance values of the two, a composite surfactant that cooperates with each other to obtain a hydrophilic-lipophilic balance value suitable for slime is realized, thereby improving the flotation effect; preferably, in the surfactant, the dosage ratio of polyethylene glycol-modified lactone-type sophorolipid to epoxidized lactam sophorolipid is 1:(3-10), preferably 1:(4-9) such as 1:5, 1:6 or 1:8.

[0014] In a preferred embodiment, the microemulsion collector is prepared from raw materials including the following parts by weight: 30-40 parts of hydrocarbon oil, 15-25 parts of methyl isobutyl carbinol, 2-10 parts (such as 4, 6 or 8 parts) of polyglycol-modified lactone-type sophorolipid, 20-30 parts (such as 22, 24, 26 or 28 parts) of epoxidized lactam sophorolipid, and 5-15 parts of water.

[0015] In the present invention, methyl isobutyl carbinol is used as a co-surfactant. In a preferred embodiment, the dosage ratio of the surfactant to methyl isobutyl carbinol as the co-surfactant is (1-2):1.

[0016] In a preferred embodiment, the polyglycol-modified lactone-type sophorolipid is polyglycol 200-400. For example, polyglycol 200, polyglycol 300 or polyglycol 400-modified lactone-type sophorolipid is more conducive to preparing a composite surfactant with a suitable hydrophilic-lipophilic balance value, thereby further effectively reducing the oil-water interfacial tension, increasing the dispersion of oil droplets, and improving the flotation effect compared with polyglycol 600 or polyglycol 1000-modified lactone-type sophorolipid.

[0017] For another aspect of achieving the above-mentioned invention object, the present invention also provides a preparation method of the above microemulsion collector, and the preparation method includes:

[0018] (1) Mix the surfactant and methyl isobutyl carbinol according to the ratio to obtain a composite surfactant;

[0019] (2) After stirring and mixing the composite surfactant and the hydrocarbon oil according to the ratio, gradually add the water in the ratio amount while stirring and emulsifying to obtain a homogenized microemulsion collector.

[0020] In step (1) of the present invention, the surfactant used is epoxidized lactam sophorolipid and / or polyglycol-modified lactone-type sophorolipid; among them, the epoxidized lactam sophorolipid is a known compound and is obtained by ring-opening oxidation of lactone-type sophorolipid. For example, refer to the preparation method disclosed in "Preparation and Surface Properties of Sophorolipid Derivatives [J]" (Li Zuyi et al., Fine Chemicals, 1991, (01): 1-5). Specifically, the epoxidized lactam sophorolipid can be prepared by the following method:

[0021] Weigh 10 parts by weight of tungsten metal powder and add it to a mixed solution of 80-150 parts by weight of hydrogen peroxide solution and water with a volume ratio of (1:2)-(2:1), wherein the concentration of the hydrogen peroxide solution is 20-30 wt.%, and stir and heat at 40-60 °C for 30-60 minutes, then add 8-12 parts by weight of orthophosphoric acid and stir to dissolve to obtain a mixed solution;

[0022] 80 - 120 parts by weight of lactonic sophorolipid are pre-dissolved in a dissolution solution prepared from ethyl acetate and hydrogen peroxide solution with a volume ratio of (1 - 2):1 to obtain a lactonic sophorolipid solution, and the lactonic sophorolipid solution and 4 - 6 parts by weight of methyltrialkylammonium chloride are added to the mixed solution, and the reaction is carried out at 40 - 60 °C for 60 - 90 minutes;

[0023] After the reactants are cooled, extraction is carried out using ethyl acetate solvent, the organic layer is collected, anhydrous calcium sulfate is added for drying and filtration, and then the solvent is removed by vacuum distillation to obtain the epoxidized lactam sophorolipid.

[0024] The polyethylene glycol-modified lactonic sophorolipid is a known compound. For example, refer to the preparation method disclosed in 《A class of surfactants via PEG modification of the oleate moiety of lactonic sophorolipids: synthesis, characterisation and application[J]》(Joseph K. Ogunjobi et al., Green Chem., 2021, 23, 9906). Specifically, the polyethylene glycol-modified lactonic sophorolipid can be prepared by the following method:

[0025] Polyethylene glycol and an iron-loaded montmorillonite catalyst are mixed in a reaction vessel, and the temperature is controlled at 60 - 90 °C; wherein, the mass ratio of polyethylene glycol to the catalyst is (10 - 30):1;

[0026] Then, epoxidized lactam sophorolipid is added to the reaction vessel to obtain a mixture, and ethyl acetate is gradually added, and then the temperature is raised to 90 - 110 °C, and the reaction is carried out for 0.5 - 1 hour to obtain a reaction product; wherein, the molar amount of epoxidized lactam sophorolipid is the same as that of polyethylene glycol, and the volume amount of ethyl acetate is 2 - 10 times the volume of the mixture;

[0027] After the reaction product is cooled, the reaction product is diluted with ethyl acetate solvent and filtered to recover the catalyst. At the same time, the filtrate is extracted with distilled water, sodium chloride solution is added to the collected organic layer for extraction, the organic layer is collected again, anhydrous calcium sulfate is added for drying and filtration, and then the solvent is removed by vacuum distillation to obtain polyethylene glycol-modified sophorolipid; wherein, the polyethylene glycol used is preferably polyethylene glycol 200, polyethylene glycol 300 or polyethylene glycol 400.

[0028] The iron-supported montmorillonite catalyst used in the above synthesis is known in the art, for example, see the preparation method disclosed in "Study on the Degradation of Roxarsone by Activating Sulfite with Iron-based Modified Montmorillonite [D]" (Peng Hao, Huazhong Agricultural University, 2022.), specifically, the iron-supported montmorillonite catalyst can be prepared by the following method:

[0029] 2 parts by weight of ferric chloride are added to 100-300 parts by weight of distilled water to obtain a ferric chloride solution, 6-10 parts by weight of montmorillonite are added to the solution, and the solution is heated at 50-80° C. for 15-24 hours. After cooling, the solid-liquid separation, washing, and drying are performed to obtain the iron-supported montmorillonite catalyst.

[0030] In step (2) of the present invention, the composite surfactant and hydrocarbon oil such as kerosene are stirred and mixed according to a proportion, for example, they are fully stirred and mixed on a magnetic stirrer; then a proportion of water is gradually added, and stirred and emulsified at the same time, at which time the solution is transparent, and a homogenized microemulsion collector is obtained.

[0031] In the present invention, unless otherwise specified, the dosage ratios involved are dosage ratios by weight.

[0032] Compared with the prior art, the present invention has the following advantages:

[0033] The present invention prepares epoxidized lactam sophorolipids and polyethylene glycol-modified lactone sophorolipids into a hydrophilic-lipophilic balanced composite surfactant for preparing a microemulsion collector for coal slime flotation. The prepared collector has a small oil droplet size and can be evenly dispersed in the ore pulp, thereby enhancing the floatability of the coal slime, which is of great significance for improving the flotation effect of the coal slime, and has the effects of low reagent dosage, energy saving and consumption reduction, and improved clean coal recovery rate.

[0034] In addition, the raw materials used in the surfactant epoxidized lactam sophorolipid and polyethylene glycol-modified lactone sophorolipid prepared by the present invention are widely available, such as waste cooking oils, etc., and the synthesis process is simple, which has good industrial application prospects.

[0035] In addition, the adsorption strength and adsorption stability of the microemulsion collector of the present invention on coal particles are higher than those of traditional hydrocarbon oils, thereby improving the adhesion strength between coal particles and bubbles, making it more difficult for coal particles to fall off in the process of rising with bubbles. It also shows that the epoxidized lactam sophorolipids and polyethylene glycol-modified lactone sophorolipids act as promoters, allowing oil droplets to spread better on the surface of coal particles, which is beneficial to improving the yield of clean coal. DETAILED DESCRIPTION

[0036] To enable those skilled in the art to better understand the solution of the present invention, the technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0037] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values, such as values within ±10% of the endpoint values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. Without conflict, the embodiments in this application and the features in the embodiments can be combined with each other.

[0038] It should be noted that the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0039] In the following examples / comparative examples, unless otherwise specified, the reagents used are of analytical purity.

[0040] The description of the relevant raw materials is as follows:

[0041] Kerosene: sourced from Shanghai Aladdin Biochemical Technology Co., Ltd., hereinafter also referred to as Reagent A.

[0042] Methyl isobutyl carbinol: sourced from Shanghai Macklin Biochemical Co., Ltd., hereinafter also referred to as Reagent B.

[0043] Water: deionized water.

[0044] Epoxidized lactone trehalose lipid: Weigh 0.75 g of tungsten metal powder and add it to a mixed solution of hydrogen peroxide solution (3.5 ml, concentration 25 wt.%) and water (3.5 mL) with a volume ratio of 1:1, and stir and heat at 50 °C for 30 minutes; then dilute orthophosphoric acid with water and add it to the above solution to stir to obtain a mixed solution; pre-dissolve lactone-type trehalose lipid (CAS No.: 148409-20-5) in a dissolution solution prepared from ethyl acetate and hydrogen peroxide solution with a volume ratio of 3:2 to obtain a lactone-type trehalose lipid solution, and add the lactone-type trehalose lipid solution and methyltrialkylammonium chloride to the mixed solution, and react at 40-60 °C for 90 minutes; cool the reaction mixture, extract with 300 mL of ethyl acetate, collect the organic layer, dry with anhydrous calcium sulfate, filter, and remove the solvent under vacuum to obtain epoxidized trehalose lipid (hereinafter also referred to as reagent C). Among them, the mass ratio of lactone-type trehalose lipid to tungsten metal powder, orthophosphoric acid, and methyltrialkylammonium chloride is 100:10:9:5.

[0045] Iron-loaded montmorillonite catalyst: Add 2 g of ferric chloride to 200 mL of distilled water, and continue to add 8 g of montmorillonite to the solution, and heat at 70 °C for 20 h; after cooling and settling for a period of time, centrifuge and wash to obtain a solid product; then place the solid product in an oven at 85 °C and dry for 5 h to obtain an iron-loaded montmorillonite catalyst.

[0046] Polyethylene glycol-modified lactone-type trehalose lipid: Mix 0.765 mmol of polyethylene glycol and an iron-loaded montmorillonite catalyst in a round-bottom flask, and control the temperature at 80 °C. Among them, the ratio of polyethylene glycol to the catalyst is 20:1. Add 0.765 mmol of epoxidized lactone trehalose lipid to the round-bottom flask, and add ethyl acetate dropwise, and then raise the temperature to 100 °C and react for 1 hour. The amount of ethyl acetate used is 5 mL. After cooling to room temperature, dilute the reaction mixture with ethyl acetate and filter to recover the catalyst. The filtrate is extracted with 30 mL of distilled water in a separating funnel, the organic phase is collected and then extracted with a sodium chloride solution, the organic phase is collected again and dried over anhydrous calcium sulfate, and the solvent is removed under vacuum to obtain polyethylene glycol-modified trehalose lipid. In this way, lactone-type trehalose lipid modified with polyethylene glycol 200 (hereinafter also referred to as reagent D), lactone-type trehalose lipid modified with polyethylene glycol 400 (hereinafter also referred to as reagent E), lactone-type trehalose lipid modified with polyethylene glycol 600 (hereinafter also referred to as reagent F), and lactone-type trehalose lipid modified with polyethylene glycol 1000 (hereinafter also referred to as reagent G) are synthesized respectively.

[0047] Examples 1-8 for the preparation of collectors

[0048] (1) Mix the surfactant and methyl isobutyl carbinol according to the ratio to obtain a composite surfactant;

[0049] (2) After stirring and mixing the composite surfactant and kerosene according to the ratio, gradually add the measured amount of water while stirring and emulsifying to obtain the homogenized microemulsion collector 1-6;

[0050] The raw material composition of the obtained microemulsion collector 1-8 is shown in Table 1.

[0051] Table 1 Raw material composition of microemulsion collector

[0052]

[0053]

[0054] Coal slime flotation experiment

[0055] Using the microemulsion collectors 1-8 obtained in Examples 1-8 of this example and the comparative coal slime collector to conduct flotation on the coal slime sample of a coal preparation plant (the relevant parameters are shown in Table 2) (the flotation test is carried out on a 0.5L flotation machine in the laboratory, the impeller speed is 1900r / min, the ventilation volume is 0.1m 3 / min, control the pulp mixing time to be 2min, add the collector and stir for 3min, add the frother and stir for 30s, the scraping time is 3min, and the pulp concentration is controlled at 80g / L).

[0056] The flotation results are shown in Table 3.

[0057] Table 2 Coal slime properties

[0058]

[0059] Table 3 Flotation results

[0060]

[0061]

[0062] From the comparison between Examples 1, 5 and 7 and the comparison between 4, 6 and 8 above, it can be seen that the lactone-type sophorolipid modified by polyethylene glycol 200-400 is more conducive to preparing a composite surfactant with a suitable hydrophilic-lipophilic balance value compared with the lactone-type sophorolipid modified by polyethylene glycol 600-1000, thereby further effectively reducing the oil-water interfacial tension and improving the flotation effect; in addition, compared with using only the lactone-type sophorolipid modified by polyethylene glycol or the epoxidized lactam sophorolipid alone, the surfactant is a mixture of the lactone-type sophorolipid modified by polyethylene glycol and the epoxidized lactam sophorolipid, which can better utilize the different hydrophilic-lipophilic balance values of the two and improve the flotation effect.

Claims

1. A microemulsion collector applied to coal slime flotation, characterized in that, The microemulsion collector is prepared from the following raw materials in parts by weight: 25-45 parts of hydrocarbon oil, 10-30 parts of methyl isobutyl carbinol, 20-40 parts of surfactant, and 5-15 parts of water; wherein, the surfactant is polyglycol-modified lactone-type sophorolipid and / or epoxidized lactam sophorolipid.

2. The microemulsion collector according to claim 1, wherein The hydrocarbon oil is kerosene and / or diesel oil.

3. The microemulsion collector according to claim 1 or 2, characterized in that, The microemulsion collector is prepared from the following raw materials in parts by weight: 30-40 parts of hydrocarbon oil, 15-25 parts of methyl isobutyl carbinol, 25-35 parts of surfactant, and 5-15 parts of water.

4. The microemulsion collector according to any one of claims 1-3, characterized in that, The surfactant is polyglycol-modified lactone-type sophorolipid and epoxidized lactam sophorolipid; The microemulsion collector is prepared from the following raw materials in parts by weight: 30-40 parts of hydrocarbon oil, 15-25 parts of methyl isobutyl carbinol, 2-10 parts of polyglycol-modified lactone-type sophorolipid, 20-30 parts of epoxidized lactam sophorolipid, and 5-15 parts of water.

5. The microemulsion collector according to claim 4, characterized in that, Wherein, The dosage ratio of the surfactant to methyl isobutyl carbinol is (1-2):1; Among the surfactants, the dosage ratio of polyglycol-modified lactone-type sophorolipid to epoxidized lactam sophorolipid is 1:(3-10), preferably 1:(4-9).

6. The microemulsion collector according to any one of claims 1-5, characterized in that, The polyglycol-modified lactone-type sophorolipid is polyglycol 200-400-modified lactone-type sophorolipid.

7. The preparation method of the microemulsion collector according to any one of claims 1-6, characterized in that, The preparation method includes: (1) Mix the surfactant and methyl isobutyl carbinol according to the ratio to obtain a composite surfactant; (2) After stirring and mixing the composite surfactant and hydrocarbon oil according to the ratio, gradually add the measured amount of water while stirring and emulsifying to obtain a homogenized microemulsion collector.

8. The preparation method according to claim 7, characterized in that, The epoxidized lactam sophorolipid is prepared by the following method: Weigh 10 parts by weight of tungsten metal powder and add it to a mixed solution of 80-150 parts by weight of a hydrogen peroxide solution and water with a volume ratio of (1:2)-(2:1), wherein the concentration of the hydrogen peroxide solution is 20-30 wt.%, and stir and heat at 40-60 °C for 30-60 minutes, then add 8-12 parts by weight of orthophosphoric acid and stir to dissolve to obtain a mixed solution; Pre-dissolve 80-120 parts by weight of lactone-type sophorolipid in a dissolution solution prepared from ethyl acetate and hydrogen peroxide solution with a volume ratio of (1-2):1 to obtain a lactone-type sophorolipid solution, and add the lactone-type sophorolipid solution and 4-6 parts by weight of methyl trialkyl ammonium chloride to the mixed solution, and react at 40-60 °C for 60-90 minutes; After the reactant is cooled, extract it with ethyl acetate solvent, collect the organic layer, add anhydrous calcium sulfate for drying and filtering, and then distill off the solvent under reduced pressure to obtain the epoxidized lactam sophorolipid.

9. The preparation method according to claim 8, characterized in that, The polyglycol-modified sophorolipid is prepared by the following method: Mix polyglycol and an iron-loaded montmorillonite catalyst in a reaction vessel and control the temperature at 60-90 °C; wherein, the mass ratio of polyglycol to the catalyst is (10-30):1; Then, epoxidized lactam sophorolipid is added to the reaction vessel to obtain a mixture, and ethyl acetate is gradually added. Then the temperature is raised to 90 - 110 °C and the reaction is carried out for 0.5 - 1 hour to obtain a reaction product; wherein, the molar amount of the epoxidized lactam sophorolipid is the same as that of polyethylene glycol, and the volume of ethyl acetate used is 2 - 10 times the volume of the mixture. After the reaction product is cooled, it is diluted with an ethyl acetate solvent and filtered to recover the catalyst. At the same time, the filtrate is extracted with distilled water, sodium chloride solution is added to the collected organic layer for extraction, the organic layer is collected again, anhydrous calcium sulfate is added for drying and filtering, and then the solvent is removed by vacuum distillation to obtain polyethylene glycol-modified sophorolipid.

10. The preparation method according to claim 9, characterized in that, The iron-loaded montmorillonite catalyst is prepared by the following method: 2 parts by weight of ferric chloride is added to 100 - 300 parts by weight of distilled water to obtain a ferric chloride solution. Then, 6 - 10 parts by weight of montmorillonite is added to the solution, and the mixture is heated at 50 - 80 °C for 15 - 24 h. After cooling, solid-liquid separation, washing, and drying are carried out to obtain the iron-loaded montmorillonite catalyst.

Images