Method for preparing high-quality calcium sulfate and magnesium hydroxide by efficiently desulfurizing coal-fired flue gas by using dolomite

Abstract

The invention relates to the technical field of inorganic chemical industry, in particular to a method for preparing high-quality calcium sulfate and magnesium hydroxide through efficient desulfurization of coal-fired flue gas by dolomite. According to the method, dolomite is calcined, calcium and magnesium are separated through one-time leaching of the ammonium nitrate solvent, then SO2 in the flue gas is removed through magnesium, the desulfurization rate is high, the reaction speed is high, high-quality calcium sulfate and magnesium hydroxide are obtained with the desulfurization process as a medium, and perfect combination of flue gas environment-friendly desulfurization and high-added-value product manufacturing is achieved. Meanwhile, the generated ammonia gas is recycled in the system, so that the cost of raw materials is saved, the gas generated by the process is fully utilized, and the waste of resources and the environmental pollution are avoided. Meanwhile, the raw materials are basically solved on the spot, so that the technical scheme can be completed without providing conditions (large chemical fertilizer plants or large coking plants) for nearby specific matched enterprises, the applicability is higher, and the cost is lower.

Description

Technical Field

[0001] The invention relates to the technical field of inorganic chemical industry, in particular to a method for preparing high-quality calcium sulfate and magnesium hydroxide by efficiently desulfurizing coal-fired flue gas using dolomite. Background Art

[0002] With the continuous development of social industry, the degree of industrialization is getting higher and higher, the scale of production is expanding, and the flue gas discharged into the atmosphere is also increasing. Among the various gaseous pollutants that affect the air quality, SO2 flue gas has the largest amount and the greatest impact. It causes acid rain, damages the ecological environment, and affects human health. Therefore, before these industrial flue gases are discharged into the atmosphere, they need to be treated, among which the more important one is desulfurization treatment.

[0003] About flue gas desulfurization: At present, most domestic enterprises use calcium carbonate-lime method to remove SO2 from flue gas. Cheap calcium carbonate powder or lime milk is used to absorb SO2 in flue gas, and the generated CaSO3 is oxidized to generate CaSO4. However, due to its low quality and large output, the market can only digest a small part of it and it is stored in large quantities, which not only increases the cost of desulfurization, but also causes secondary pollution. At present, some domestic enterprises also use magnesium oxide for desulfurization, which is generally suitable for urban boiler desulfurization (no gypsum storage site). Since the light-burned magnesium oxide used is concentrated in the northeast, the production and transportation costs are high, and the value of the by-product magnesium sulfate is relatively low, resulting in high desulfurization costs for enterprises. Therefore, considering the use of cheap and easily available dolomite as a desulfurizer is the best way to solve the problem of flue gas desulfurization.

[0004] Although there are other schemes using dolomite at present, they usually use dolomite as the source of magnesium oxide. For example, patent application 201710768665.0 (a combined method for flue gas desulfurization and gas deammoniation pollutant resource recovery and waste treatment) uses magnesium oxide obtained after calcining dolomite as a raw material. This technical scheme requires a large amount of input ammonia, mainly for coke oven gas or ammonia-containing gas produced by various gasification furnaces in coal chemical industry or petrochemical industry. When the downstream users of ammonia are relatively close, the separated magnesium sulfate solution can still be directly transported; if the users are particularly far away, it needs to be dehydrated before transportation. At this time, the energy consumption and processes required for drying and packaging are relatively large costs. Since the power plants that currently produce coal-fired delays are often far away from the coking plants, this method is restricted by certain objective conditions.

[0005] Therefore, the current key research direction is to find a method that can use dolomite as a medium to obtain high-quality calcium sulfate and magnesium hydroxide through the desulfurization process, achieve a perfect combination of environmentally friendly flue gas desulfurization and high value-added product manufacturing, and be applicable to coal-fired flue gas desulfurization in most factories. Summary of the invention

[0006] In order to solve the above technical problems existing in the prior art, the present invention provides a method for efficiently desulfurizing coal-fired flue gas using dolomite to produce high-quality calcium sulfate and magnesium hydroxide, comprising the following steps:

[0007] A method for efficiently desulfurizing coal-fired flue gas using dolomite to produce high-quality calcium sulfate and magnesium hydroxide comprises the following steps:

[0008] (1) calcining dolomite powder at 700-1200° C. to obtain calcined ore powder;

[0009] (2) adding calcined ore powder and ammonium nitrate solution into a reaction tank for stirring and reacting, generating ammonia gas and effectively recovering it, and filtering and separating to obtain leachate A and leach residue a, wherein the leachate A is a mixed solution of calcium nitrate and ammonium nitrate, and the leach residue a is a mixture of magnesium hydroxide containing a small amount of calcium hydroxide;

[0010] (3) The leached residue a is slurried with water to a suitable concentration and enters a circulation tank and is pumped into a desulfurization tower for countercurrent mixing with the dedusted coal-fired flue gas to remove SO2 from the flue gas. At this time, MgSO3 and CaSO3 are generated. During this process, an appropriate amount of air is introduced for oxidation to obtain MgSO4 solution and CaSO4, which are separated by plate and frame filter press to obtain filtrate B and filtered residue b; filtrate B is a magnesium sulfate solution, and the filtered residue b is washed to obtain a coarse calcium sulfate filter cake;

[0011] (4) adding the filtrate A to the clear purified filtrate B in a reaction tank and stirring for reaction; after the reaction is completed, the filtrate C and filter cake C are separated by plate and frame filter pressing; the filtrate C is a magnesium nitrate solution containing a small amount of ammonium nitrate, and the filter cake C is washed and dried to obtain high-quality calcium sulfate.

[0012] (5) Adding ammonia water to the filtrate C in a reaction tank and stirring the mixture to generate ammonia gas for effective recovery. After the reaction is completed, the filtrate D and filter cake d are separated by plate and frame filter press. The filtrate D is an ammonium nitrate solution containing a small amount of magnesium nitrate. The filter cake d is washed and dried to obtain high-quality magnesium hydroxide.

[0013] Furthermore, the step (1) is to carry out calcination at 700-1200° C. for 1-2 hours. In this case, the calcination has a better effect on decomposing dolomite.

[0014] Furthermore, the dolomite has a specific chemical formula of CaCO3·MgCO3. In this case, the calcium and magnesium oxide obtained has high activity and good reaction effect.

[0015] Furthermore, in step (2), the concentration of the ammonium nitrate solution is 10-40%, the reaction temperature is 35-70°C, and the reaction time is 30-120 minutes. At this time, the calcium and magnesium in the slurry are better separated.

[0016] Furthermore, the step (3) is to slurry the leached residue a with water to 10-40%, and control the liquid-gas ratio in the desulfurization tower to 3-10 L / m 3 , the reaction temperature is 80-160℃, and the reaction time is 30-90min. At this time, the magnesium hydroxide in the slurry reacts fully with the SO2 in the flue gas, and the obtained MgSO3 is oxidized by air to obtain MgSO4 solution B.

[0017] Furthermore, in step (4), filtrate B is added to filtrate A in a reaction tank for stirring and reacting, and the reaction temperature is controlled at 30-120° C. and the reaction time is controlled at 30-90 min. Under this condition, high-quality calcium sulfate is obtained with good economic value.

[0018] Furthermore, the ammonia water in step (5) is prepared from the ammonia gas effectively recovered in step (2). The recycling of ammonia gas in the present application can theoretically achieve a non-consumption ammonia cycle without the need for adding ammonia gas. Although there will be some losses in actual production, it is still not necessary to add a large amount of ammonia gas as a whole. The technical solution of the present application successfully does not require a large amount of external ammonia gas input, and gets rid of the dependence on coking plants and the like.

[0019] Furthermore, in step (5), the filtrate C is added with ammonia water in the reaction tank and stirred at a reaction temperature of 60-120° C. and a reaction time of 30-90 min. Under these conditions, high-quality magnesium hydroxide is obtained with high economic value. The filtrate D in step (5) is reused in step (2).

[0020] The chemical reaction principle of each step in this application:

[0021] In step (1), dolomite (CaCO3·MgCO3) begins to decompose at a high temperature above 750°C. The specific equation is as follows:

[0022] CaCO3·MgCO3=CaO+MgO+2CO2

[0023] In step (2), the equation for ammonium nitrate leaching is as follows:

[0024] CaO+2NH4NO3=Ca(NO3)2+2NH3+H2O

[0025] MgO+H2O=Mg(OH)2

[0026] In step (3), the specific equation is as follows:

[0027] SO2+Mg(OH)2=MgSO3+H2O

[0028] MgSO3+1 / 2O2=MgSO4

[0029] Ca(OH)2+SO2+1 / 2O2+H2O=CaSO4·2H2O

[0030] In step (4), the specific equation is as follows:

[0031] MgSO4+Ca(NO3)2+2H2O=CaSO4·2H2O+Mg(NO3)2

[0032] In step (5), the specific equation is as follows:

[0033] Mg(NO3)2+2NH4OH=Mg(OH)2+2NH4NO3

[0034] The present invention uses dolomite calcination and ammonium nitrate solvent to leach calcium and magnesium once, and then uses magnesium to remove SO2 in the flue gas. The desulfurization rate is high and the reaction speed is fast. High-quality calcium sulfate and magnesium hydroxide are obtained through the desulfurization process as a medium, achieving a perfect combination of environmentally friendly flue gas desulfurization and high value-added product manufacturing. It not only solves the problem of poor economic benefits of low-quality gypsum in existing calcium carbonate desulfurization, but also solves the problem of low value and high production cost of magnesium sulfate in existing magnesium oxide desulfurization raw material area restrictions. Compared with the prior art, the technical effects created by the present invention are embodied in:

[0035] 1. The present invention uses dolomite as a desulfurizing agent for the removal of SO2 from flue gas of coal-fired kilns and power plant boilers. Compared with the existing calcium carbonate-lime method, it has the characteristics of fast reaction speed, high desulfurization rate, low investment cost and low operating cost. It cleverly combines the unique calcium and magnesium characteristics of dolomite with flue gas desulfurization to produce high-value-added calcium sulfate and magnesium hydroxide, so that low-cost dolomite and coal-fired flue gas are combined to produce high-quality calcium sulfate and high-purity magnesium hydroxide with high value and large market demand, and realize the transformation of flue gas desulfurization and environmental protection treatment from negative economic output to high returns. In particular, the use of magnesium oxide desulfurization in the existing calcium carbonate desulfurization system of power generation enterprises can increase the desulfurization capacity by more than 30%, and it has a lot of room for reducing raw material costs and increasing profits for power plants in high-sulfur coal raw material production areas to improve the sulfur content index of raw coal, especially for the application of high-sulfur coal. It has opened up an effective way.

[0036] 2. The technical solution of the present invention can be improved on the original desulfurization system of the enterprise. It only needs to change the desulfurization raw materials and basically no other modifications are required. Only the raw material calcination and the following two high-end product production process equipment need to be added. The original enterprise's desulfurization system can be fully utilized, saving money and having wide applicability.

[0037] 3. The present invention achieves flue gas desulfurization under more optimized conditions through a reasonable division of labor between the functions of calcium and magnesium in dolomite, while avoiding the risk of secondary environmental pollution and the burden of high disposal costs caused by the production of low-quality desulfurization gypsum.

[0038] 4. The present invention uses dolomite instead of limestone or light-burned magnesium oxide to treat flue gas desulfurization, which can basically save raw material costs by using local materials, turning flue gas desulfurization from a burden on enterprises into an economic growth point, effectively promoting the enthusiasm of enterprises for environmental protection investment and improving the social benefits of the ecological environment.

[0039] 5. In the process of the present invention, the ammonia produced in steps (2) and (5) is recycled in the system, which not only saves the cost of raw materials, but also makes full use of the gas produced by the process, avoiding waste of resources and environmental pollution. At the same time, the recycling of ammonia in the present application makes the technical solution of the present application not need a large amount of external ammonia input, and can be carried out without relying entirely on a large coking plant or fertilizer plant nearby, which is more applicable and has lower cost. In the process of the present invention, since the liquid-to-gas ratio of the desulfurization tower is greatly reduced, the water consumption is reduced, and the full recycling of wash water has significant environmental significance.

[0040] 6. The technical solution of the present application does not require a large amount of external ammonia input to produce magnesium hydroxide products. There is no need to dehydrate and dry the magnesium sulfate product and transfer it to a place where ammonia is produced and then re-prepared into magnesium hydroxide products. There is no dilemma of only being able to produce magnesium sulfate products with low economic value when there is no ammonia input.

[0041] 7. The high-quality calcium sulfate obtained by the present invention has a purity of more than 98% and a whiteness of more than 97%, and the market price is about 1,200 yuan / ton; the high-purity magnesium hydroxide has a purity of more than 98% and a whiteness of more than 95%, and the market price is about 4,500 yuan / ton. The low-value dolomite is processed by the flue gas desulfurization high-efficiency combination to obtain high value-added products and double its value, which has good economic benefits. The high-quality calcium sulfate obtained by this application is in short supply in the market, which solves the problem that the current desulfurization product is gypsum, which can only be digested by the market due to excessive production, so it not only increases the desulfurization cost, but also causes secondary pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a schematic diagram of the process of the present invention. DETAILED DESCRIPTION

[0043] The technical solution of the present invention is further defined below in conjunction with specific implementation methods, but the scope of protection required is not limited to the description.

[0044] Example 1

[0045] (1) calcining dolomite at 950° C. for 1.5 h to obtain calcined ore powder;

[0046] (2) adding calcined ore powder and 25% ammonium nitrate solution into a reaction tank and stirring for reaction at a reaction temperature of 45° C. for a reaction time of 60 min to generate ammonia gas, which is filtered and separated to obtain leachate A and leach residue a. The leachate A is a mixed solution of calcium nitrate, magnesium nitrate and ammonium nitrate, and the leach residue a is a mixture of magnesium hydroxide containing a small amount of calcium hydroxide;

[0047] (3) Leaching slag a (mainly magnesium hydroxide and a small amount of calcium hydroxide) is slurried with water to a concentration of about 20% to absorb SO2 in the flue gas and control the liquid-gas ratio in the desulfurization tower to 5L / m 3 The reaction temperature is 120°C and the reaction time is 60 min. After the reaction is completed, air is introduced for oxidation to obtain CaSO4MgSO4, which is filtered and separated to obtain crude calcium sulfate and magnesium sulfate solution B. (.

[0048] (4) The magnesium sulfate solution B and the leaching solution A are stirred for reaction, the reaction temperature is controlled to be 60° C. and the reaction time is 60 min, and the mixture is filtered, washed and separated. The filter cake is dried to obtain fine CaSO4·2H2O. The filtrate is a magnesium nitrate solution D containing a small amount of ammonium nitrate.

[0049] (5) The filtrate D is treated with aqueous ammonia to precipitate magnesium hydroxide. The temperature is controlled at 60°C and the reaction time is 60 min. The filtrate is filtered, washed and separated. The filter cake is dried to obtain Mg(OH)2. The ammonium nitrate solution in the filtrate is returned for recycling.

[0050] Example 2

[0051] (1) calcining dolomite at 900° C. for 2 h to obtain calcined ore powder;

[0052] (2) adding calcined ore powder and 10% ammonium nitrate solution into a reaction tank and stirring for reaction at a reaction temperature of 70° C. for a reaction time of 120 min to generate ammonia gas, which is filtered and separated to obtain leachate A and leach residue a. The leachate A is a mixed solution of calcium nitrate, magnesium nitrate and ammonium nitrate, and the leach residue a is a mixture of magnesium hydroxide containing a small amount of calcium hydroxide;

[0053] (3) Leaching slag a (mainly magnesium hydroxide and a small amount of calcium hydroxide) is slurried with water to a concentration of about 10% to absorb SO2 in the flue gas and control the liquid-gas ratio in the desulfurization tower to 3L / m 3 , the reaction temperature is 100°C, the reaction time is 120 min, after the reaction is completed, air is introduced for oxidation to obtain CaSO4MgSO4, and crude calcium sulfate and magnesium sulfate solution B are obtained after filtration and separation. (.

[0054] (4) The magnesium sulfate solution B and the leaching solution A are stirred for reaction, the temperature is controlled at 50° C., the reaction time is 90 min, and the mixture is filtered, washed and separated. The filter cake is dried to obtain fine CaSO4·2H2O. The filtrate is a magnesium nitrate solution D containing a small amount of ammonium nitrate.

[0055] (5) The filtrate D is treated with aqueous ammonia to precipitate magnesium hydroxide. The temperature is controlled at 60°C and the reaction time is 60 min. The filtrate is filtered, washed and separated. The filter cake is dried to obtain Mg(OH)2. The ammonium nitrate solution in the filtrate is returned for recycling.

[0056] Example 3

[0057] (1) calcining dolomite at 1100° C. for 2 h to obtain calcined ore powder;

[0058] (2) adding calcined ore powder and 40% ammonium nitrate solution into a reaction tank and stirring for reaction at a reaction temperature of 35° C. for a reaction time of 30 min to generate ammonia gas, which is filtered and separated to obtain leachate A and leach residue a. The leachate A is a mixed solution of calcium nitrate, magnesium nitrate and ammonium nitrate, and the leach residue a is a mixture of magnesium hydroxide containing a small amount of calcium hydroxide;

[0059] (3) Leaching slag a (mainly magnesium hydroxide and a small amount of calcium hydroxide) is slurried with water to a concentration of about 40% to absorb SO2 in the flue gas and control the liquid-gas ratio in the desulfurization tower to 10L / m 3 , the reaction temperature is 80°C, the reaction time is 30 min, after the reaction is completed, air is introduced for oxidation to obtain CaSO4MgSO4, which is filtered and separated to obtain crude calcium sulfate and magnesium sulfate solution B. (.

[0060] (4) The magnesium sulfate solution B and the leaching solution A are stirred for reaction, the reaction temperature is controlled at 120° C. and the reaction time is controlled at 30 min, and the mixture is filtered, washed and separated. The filter cake is dried to obtain fine CaSO 4 ·2H 2 O. The filtrate is a magnesium nitrate solution D containing a small amount of ammonium nitrate.

[0061] (5) The filtrate D is treated with aqueous ammonia to precipitate magnesium hydroxide. The temperature is controlled at 60°C and the reaction time is 60 min. The filtrate is filtered, washed and separated. The filter cake is dried to obtain Mg(OH)2. The ammonium nitrate solution in the filtrate is returned for recycling.

[0062] Example 4

[0063] (1) calcining dolomite at 950° C. for 1.5 h to obtain calcined ore powder;

[0064] (2) adding calcined ore powder and 25% ammonium nitrate solution into a reaction tank and stirring for reaction at a reaction temperature of 45° C. for a reaction time of 60 min to generate ammonia gas, which is filtered and separated to obtain leachate A and leach residue a. The leachate A is a mixed solution of calcium nitrate, magnesium nitrate and ammonium nitrate, and the leach residue a is a mixture of magnesium hydroxide containing a small amount of calcium hydroxide;

[0065] (3) Leaching slag a (mainly magnesium hydroxide and a small amount of calcium hydroxide) is slurried with water to a concentration of about 20% to absorb SO2 in the flue gas and control the liquid-gas ratio in the desulfurization tower to 5L / m 3, the reaction temperature is 90°C, the reaction time is 60 min, after the reaction is completed, air is introduced for oxidation to obtain CaSO4MgSO4, which is filtered and separated to obtain crude calcium sulfate and magnesium sulfate solution B. (.

[0066] (4) The magnesium sulfate solution B and the leaching solution A are stirred for reaction, the reaction temperature is controlled to be 60° C. and the reaction time is 60 min, and the mixture is filtered, washed and separated. The filter cake is dried to obtain fine CaSO4·2H2O. The filtrate is a magnesium nitrate solution D containing a small amount of ammonium nitrate.

[0067] (5) The filtrate D is treated with aqueous ammonia to precipitate magnesium hydroxide. The temperature is controlled at 60° C. and the reaction time is 60 min. The filtration, washing and separation are performed. The filter cake is dried to obtain Mg(OH)2. The ammonium nitrate solution in the filtrate is treated with aqueous ammonia to recover magnesium hydroxide and then returned for recycling.

[0068] The quality of the products obtained in Examples 1-4 is as follows:

[0069]

[0070]

[0071] The calcium sulfate and magnesium hydroxide products obtained in each embodiment of the present invention have a calcium sulfate content of more than 98% and a whiteness of more than 97%, and a magnesium hydroxide content of more than 98% and a whiteness of more than 95%, all of which are excellent products.

[0072] Finally, it should be pointed out that the above embodiments are only representative examples of the present invention. Obviously, the technical solution of the present invention is not limited to the above embodiments, and there are many variations. All variations that can be directly derived or associated with the content disclosed by ordinary technicians in this field should be considered as the protection scope of the present invention.

Claims

1. A method for efficiently desulfurizing coal-fired flue gas using dolomite to produce high-quality calcium sulfate and magnesium hydroxide, characterized in that: The steps include: (1) calcining dolomite powder at 700-1200° C. to obtain calcined ore powder; (2) adding calcined ore powder and ammonium nitrate solution into a reaction tank for stirring and reacting, generating ammonia gas and effectively recovering it, and filtering and separating to obtain leachate A and leach residue a, wherein the leachate A is a mixed solution of calcium nitrate and ammonium nitrate, and the leach residue a is a mixture of magnesium hydroxide containing a small amount of calcium hydroxide; (3) The leached residue a is slurried with water to a suitable concentration and enters a circulation tank and is pumped into a desulfurization tower for countercurrent mixing with the dedusted coal-fired flue gas to remove SO2 from the flue gas. At this time, MgSO3 and CaSO3 are generated. During this process, an appropriate amount of air is introduced for oxidation to obtain MgSO4 solution and CaSO4, which are separated by plate and frame filter press to obtain filtrate B and filtered residue b; filtrate B is a magnesium sulfate solution, and the filtered residue b is washed to obtain a coarse calcium sulfate filter cake; (4) adding the filtrate A to the reaction tank to react with the purified filtrate B, and performing a stirring reaction. After the reaction is completed, the filtrate C and the filter cake C are separated by plate and frame filter pressing; the filtrate C is a magnesium nitrate solution containing a small amount of ammonium nitrate, and the filter cake C is washed and dried to obtain high-quality calcium sulfate; (5) Adding ammonia water to the filtrate C in a reaction tank and stirring the mixture, a small amount of ammonia gas is effectively recovered, and after the reaction is completed, the filtrate D and filter cake d are obtained by plate and frame filter pressing. The filtrate D is an ammonium nitrate solution containing a small amount of magnesium nitrate, and the filter cake d is washed and dried to obtain high-quality magnesium hydroxide.

2. The method for producing high-quality calcium sulfate and magnesium hydroxide by using dolomite to efficiently desulfurize coal-fired flue gas according to claim 1, characterized in that: The step (1) is to carry out calcination at 700-1200° C. for 1-2 hours.

3. The method for producing high-quality calcium sulfate and magnesium hydroxide by utilizing dolomite for efficient desulfurization of coal-fired flue gas according to claim 1, characterized in that: The dolomite has a specific chemical formula of CaCO3·MgCO3.

4. The method for producing high-quality calcium sulfate and magnesium hydroxide by using dolomite to efficiently desulfurize coal-fired flue gas according to claim 1, characterized in that: In the step (2), the concentration of the ammonium nitrate solution is 10-40%, the reaction temperature is 35-70° C., and the reaction time is 30-120 min.

5. The method for producing high-quality calcium sulfate and magnesium hydroxide by using dolomite to efficiently desulfurize coal-fired flue gas according to claim 1, characterized in that: The step (3) is to slurry the leached slag a with water to 10-40%, and control the liquid-gas ratio in the desulfurization tower to 3-10 L / m 3 , the reaction temperature is 80-160℃, and the reaction time is 30-90min.

6. The method for producing high-quality calcium sulfate and magnesium hydroxide by using dolomite to efficiently desulfurize coal-fired flue gas according to claim 1, characterized in that: In the step (4), filtrate B is added to filtrate A in a reaction tank and stirred for reaction, and the reaction temperature is controlled at 30-120° C. and the reaction time is controlled at 30-90 min.

7. The method for producing high-quality calcium sulfate and magnesium hydroxide by using dolomite to efficiently desulfurize coal-fired flue gas according to claim 1, characterized in that: The ammonia water in step (5) is prepared from the ammonia gas effectively recovered in step (2).

8. The method for producing high-quality calcium sulfate and magnesium hydroxide by using dolomite to efficiently desulfurize coal-fired flue gas according to claim 1, characterized in that: In the step (5), filtrate C is added with ammonia water in a reaction tank and stirred, and the reaction temperature is controlled at 30-120° C. and the reaction time is controlled at 30-90 min.

9. The method for producing high-quality calcium sulfate and magnesium hydroxide by using dolomite to efficiently desulfurize coal-fired flue gas according to claim 1, characterized in that: The filtrate D in step (5) is returned to step (2) for reuse.

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