Abstract:
The present invention relates to a method for processing acetic acid solvent in an oxidising unit of a PTA industrial apparatus; said method uses a pressure filter machine to filter crude terephthalic acid slurry, and then, using a multi-stage counter-current method, uses washing water to wash the acetic acid solvent, and, by means of setting up a bias-current and drainage, prevents residual liquid in the filtrate pipe from entering the next area following the rotation of the pressure filter machine, improving washing efficiency and reducing the amount of washing water; the present invention integrates the processes of filtering and washing crude terephthalic acid into a single pressure filter machine, such that the process is shorter, the occupied floor space is reduced, and energy consumption is lower. A method feeding nitrogen gas into the mother liquor tank and washing liquid tank is used to regulate the pressure balance of the system.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the technical field of energy conservation, in particular to a method for processing acetic acid solvent in crude terephthalic acid (CTA) in an oxidising unit of a PTA industrial apparatus. 
       DISCUSSION OF THE RELATED ART 
       [0002]    The crude terephthalic acid slurry generated by the reactions in an oxidising unit of a PTA industrial apparatus contains a lot of acetic acid. According to the process requirements, the acetic acid has to be removed from the crude terephthalic acid before a refining process of PTA. Presently, the technology of “filtration by centrifuge or vacuum filter and drying by steam rotary dryer” is widely used in the industrial production. The process is mainly composed of a filtering area and a drying area. 
         [0003]    As shown in  FIG. 1 , the process is performed as follows: conveying crude terephthalic acid slurry generated by the reactions and containing a lot of acetic acid to a centrifuge or a vacuum filter by a slurry pump at first, which in turn separates part of acetic acid from the slurry, and then conveying the resultant crude terephthalic acid filter cake to a steam rotary dryer to heat and dry it with steam indirectly, removing acetic acid from crude terephthalic acid by drying, using processing water to pulp the dried crude terephthalic acid and then conveying it to a refining unit. 
         [0004]    The technology may have the following drawbacks. 
         [0005]    1) The process flow is long and requires many accessory equipments 
         [0006]    In such a technology, the filtering process and drying process are performed in different equipments, mainly including a vacuum filter, a steam rotary dryer, a washing tower, etc., as well as multiple accessory equipments and pipe fittings such as fans, pumps and valves. The process flow is long and thus involves complicated operations, which increase the equipment failure rate and potential safety hazards. 
         [0007]    2) The equipments requires high investment cost and occupies a large floor space. 
         [0008]    In such a technology, many equipments and requires high investment for the equipments are involved. Because of the high viscosity of crude terephthalic acid, a filter cartridge of a vacuum filter should be cleaned or replaced frequently. Therefore, the vacuum filters in the industrial apparatus generally operate with one of them being active and another one being standby, which further increases the investment of the pressure filters. Further, a relative large volume of the steam rotary dryer and a large number of auxiliary pressure filter machines increase the floor space. 
         [0009]    3) High energy consumption 
         [0010]    In such a technology, many active equipments that consume plenty of electrical energy are involved, including a centrifuge or vacuum filter machine, a steam rotary dryer, a pump, a fan, or the like. The steam rotary dryer evaporates acetic acid from crude terephthalic acid with latent heat of steam, which requires a large amount of steam and thus lots of heat energy. Further, a large amount of washing water is required for washing acetic acid from the dried tail gas. In addition, a large amount of processing water is required for pulping the dried crude terephthalic acid due to low moisture content thereof. Therefore, the technology has shortcomings of high energy consumption and high water consumption. 
         [0011]    A kind of integrated equipment, namely pressure filter machine, has been used in the prior art to replace multiple sets of equipments including a vacuum centrifuge, a dryer, a washing tower, a powder tank or the like in the conventional process unit. For example, the Chinese Invention Patent Application Publication No. CN102476994A discloses a method for removing acetic acid from crude terephthalic acid by coarsely filtering crude terephthalic acid suspension by a pressure filter machine, and washing crude terephthalic acid with fresh water in several times sequentially to displace acetic acid in crude terephthalic acid. 
         [0012]    However, as shown in  FIG. 2 , the washing liquid will be conveyed from a washing area to the next washing area via a filtrate pipe during the actual operation of the pressure filter, which will increase the concentration of the washing liquid in the next washing area and thus greatly reduce washing efficiency of the washing process. The consumption of washing water has to be increased so as to meet the requirements for product. Moreover, the pressure filter machine consists of multiple chambers. The content of acetic acid in a filter cake varies with the proceeding of the washing process, resulting in different pressure drops in each of the chambers. As a result, the seals between the chambers will be damaged in long-term operation and the washing liquid will be mixed among the chambers. 
       SUMMARY 
       [0013]    The technical problem to be solved by the present invention is to provide a method for processing acetic acid (CTA) solvent in an oxidising unit of a PTA industrial apparatus to improve removal efficiency of acetic acid. 
         [0014]    To solve the technical problem, the method for processing acetic acid solvent in an oxidising unit of a PTA industrial apparatus of the present invention comprises the following steps: 
         [0015]    S 100 , conveying pressurized acetic acid CTA slurry from an oxidising unit of a upstream PTA industrial apparatus into a filtering area of a pressure filter to separate solid particles and acetic acid in the CTA slurry so as to form a CTA filter cake, separated mother liquor, and residual liquid in a filtrate pipe, discharging the separated mother liquor to a mother liquor tank, and biasing the residual liquid in the filtrate pipe to the mother liquor tank; 
         [0016]    S 200 , washing the CTA filter cake stage by stage through a plurality of washing areas following the rotation of the pressure filter, wherein each washing area other than a washing area for the last-stage of washing is connected to a washing liquid tank respectively, each washing liquid tank providing washing liquid to its corresponding washing area to wash the CTA filter cake; wherein the washing area for the last-stage of washing uses fresh processing water as the washing liquid, while the remaining stages of washing supplies washing liquid from a later stage of washing back to a washing liquid tank of a former stage of washing in a multi-stage counter-current washing mode; and wherein a step of biasing residual liquid in the filtrate pipe of each stage of washing to the washing liquid tank of its corresponding stage is included during the stage-by-stage washing; and 
         [0017]    S 300 , conveying the CTA filter cake which has been processed by multiple washings to a unloading area following the rotation of the pressure filter, and then conveying the CTA filter cake of the unloading area to a pulping tank with the back blowing of nitrogen gas and the gravity so as to mix the CTA filter cake with external processing water and to pulp it, wherein a step of conducting residual liquid in the filtrate pipe to a filtrate tank and further conducting it to a washing liquid tank connected to a washing area for the penultimate stage of washing is included during the process of conveying the CTA filter cake to the unloading area. 
         [0018]    For the method for processing acetic acid solvent in an oxidising unit of a PTA industrial apparatus, the first washing area is also connected to a corresponding recycling washing liquid tank, and the step S 200  further comprises a step of recycling the washing liquid from the first stage of washing by the recycling washing liquid tank. 
         [0019]    For the method for processing acetic acid solvent in an oxidising unit of a PTA industrial apparatus, the mother liquor tank and the washing liquid tanks are provided with a pressure regulating device respectively, wherein gas in a given tank is vented to a certain extent by the pressure regulating device on the tank when the pressure in the tank is higher than a preset pressure drop; while nitrogen gas is fed into a given tank by the pressure regulating device on the tank when the pressure in the tank is lower than the preset pressure drop. 
         [0020]    The method for processing acetic acid solvent in an oxidising unit of a PTA industrial apparatus further comprises a step of feeding nitrogen gas to the mother liquor tank and the respective washing liquid tanks to maintain pressure balance of the whole system. 
         [0021]    For the method for processing acetic acid solvent in an oxidising unit of a PTA industrial apparatus, in the step S 100 , the position of a separation block in a control head of the pressure filter machine is adjusted so as to discharge the mother liquor into the mother liquor tank and to bias residual liquid in the filtrate pipe into the mother liquor tank. 
         [0022]    For the method for processing acetic acid solvent in an oxidising unit of a PTA industrial apparatus, in the step S 200 , the position of a separation block in a control head of the pressure filter machine is adjusted so as to bias residual liquid in the filtrate pipe of respective stage of washing to a washing liquid tank of a corresponding stage. 
         [0023]    For the method for processing acetic acid solvent in an oxidising unit of a PTA industrial apparatus, in the step S 300 , the position of a separation block in a control head of the pressure filter machine is adjusted so as to conduct residual liquid in the filtrate pipe to the filtrate tank. 
         [0024]    For the method for processing acetic acid solvent in an oxidising unit of a PTA industrial apparatus, in the step S 300 , the washing liquid in the filtrate tank is conveyed to a washing liquid tank connected to a washing area of the penultimate stage of washing by a pump. 
         [0025]    Compared with the prior art, the present invention has the following advantages: The present invention employs bias-current and conduction techniques to bring the residual liquid in the filtrate pipes back to the washing liquid tanks of corresponding washing areas, which prevents the washing liquid of the previous area from entering the washing area of the next area and affecting the next stage of washing. With the bias-current and conduction techniques, the actual washing process is substantially identical with the ideal washing process, which increases the washing efficiency. The residual washing liquid can be separated by the conduction technique and reused as the washing liquid of the previous stage of washing, thus being capable of reducing the consumption of washing liquid and reducing the cost. 
         [0026]    Furthermore, the present invention feeds nitrogen gas to regulate the pressures in the pressure filter, the mother liquor tank and the washing liquid tanks, thereby regulating the pressure balance of the system. The pressure adjusting method is convenient and accurate, and avoids damage of the seals between the chambers, thus avoiding the mixing of the washing liquid among the chambers. 
         [0027]    The present invention will be described in detail in combination with accompanied drawings and preferred embodiments, but the drawings and preferred embodiments do not limit the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a flow block diagram of a centrifugal or vacuum filtration system for crude terephthalic acid in the prior art. 
           [0029]      FIG. 2  is a schematic diagram of a multi-stage counter-current washing process for crude terephthalic acid without bias-current in the prior art. 
           [0030]      FIG. 3  is a process flow diagram of the present invention. 
           [0031]      FIG. 4  is a process flow block diagram of the present invention. 
           [0032]      FIG. 5  is a schematic diagram of multi-stage counter-current washing process for crude terephthalic acid in the present invention. 
           [0033]      FIG. 6  is a schematic diagram of multi-stage counter-current washing of crude terephthalic acid with bias-current and conduction in the present invention. 
           [0034]      FIG. 7  is a simplified schematic diagram of a pressure filter machine as shown in  FIG. 3 . 
           [0035]      FIG. 8  is an outline drawing of a control head as shown in  FIG. 7 . 
       
    
    
       [0036]    Marks in the Drawings: 
         [0037]      1 —Pressure filter machine 
         [0038]      2 —Mother liquor tank 
         [0039]      3 —Recycling washing liquid tank 
         [0040]      4 —Primary washing liquid tank 
         [0041]      5 —Primary washing liquid circulating pump 
         [0042]      6 —Secondary washing liquid tank 
         [0043]      7 —Secondary washing liquid circulating pump 
         [0044]      8 —Tertiary washing liquid tank 
         [0045]      9 —Tertiary washing liquid circulating pump 
         [0046]      10 —Fourth washing liquid tank 
         [0047]      11 —Fourth washing liquid circulating pump 
         [0048]      12 —Pulping tank 
         [0049]      13 —Filtrate tank 
         [0050]      14 —Conducting fluid circulating pump 
         [0051]      18 —Frame 
         [0052]      19 —Feeding area 
         [0053]      20 —Mother liquor chamber 
         [0054]      21 —Separation block A 
         [0055]      22 —Separation block a 
         [0056]      23 —Control head 
         [0057]      24 —Primary washing unit chamber 
         [0058]      25 —Separation block B 
         [0059]      26 —Primary filtrate chamber 
         [0060]      27 —Secondary washing unit chamber 
         [0061]      28 —Separation block b 
         [0062]      29 —Secondary filtrate chamber 
         [0063]      30 —Separation block C 
         [0064]      31 —Separation block c 
         [0065]      32 —Tertiary filtrate chamber 
         [0066]      33 —Tertiary washing unit chamber 
         [0067]      34 —Filtrate pipe 
         [0068]      35 —Separation block D 
         [0069]      36 —Separation block d 
         [0070]      37 —Fourth washing unit chamber 
         [0071]      38 —Fourth filtrate chamber 
         [0072]      39 —Separation block e 
         [0073]      40 —Separation block E 
         [0074]      41 —Fifth filtrate chamber 
         [0075]      42 —Fifth washing unit chamber 
         [0076]      43 —Separation block f 
         [0077]      44 —Separation block F 
         [0078]      45 —Unloading area I 
         [0079]      46 —Unloading area II 
         [0080]      47 —Separation block g 
         [0081]      48 —Separation block G 
         [0082]      49 —Regulation plate 
         [0083]      50 —Conduction port 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0084]    The present invention is described in detail in combination with accompanied drawings and preferred embodiments to further understand the object, solution and effects of the present invention, but the drawings and preferred embodiments do not limit the protection extent of appended claims of the present invention. 
         [0085]    Refer to  FIG. 3  to  FIG. 6 , a method for processing acetic acid (CTA) solvent in an oxidising unit of a PTA industrial apparatus in the embodiments of the present invention comprises the following steps: 
         [0086]    a. Conveying crude terephthalic acid slurry generated in reactions and containing about 35% of TA solids and about 60% of acetic acid to a filtering area of a pressure filter machine  1  at a pressure of 0.3 MPG to 0.6 MPG, gradually separating the TA solid particles from the acetic acid in the slurry following the rotation of the pressure filter machine  1  to generate a CTA filter cake, and conveying the separated mother liquor to a mother liquor tank  2  via a control head of the pressure filter machine  1 ; 
         [0087]    b. Conveying the filter cake generated by filtration to a first washing area following the rotation of the pressure filter machine  1 , and at the same time, applying a pressure of 0.3 MPaG to 0.6 MPaG to the washing liquid in a primary washing liquid tank  4  and conveying the washing liquid to the first washing area by a primary washing liquid circulating pump  5  so as to continuously wash the filter cake (i.e. the first stage of washing), conveying the filter cake processed by the primary washing to a second washing area following the rotation of the pressure filter machine  1 , and after the washing process, distributing the primary washing liquid via the control head and conveying it into a recycling washing liquid tank  3 ; 
         [0088]    c. applying a pressure of 0.3 MPaG to 0.6 MPaG to the washing liquid in a secondary washing liquid tank  6  by a secondary washing liquid circulating pump  7 , and conveying the washing liquid to the second washing area so as to continuously wash the filter cake processed by the primary washing (i.e. the second stage of washing), conveying the filter cake processed by the secondary washing to a third washing area following the rotation of the pressure filter machine  1 , and after the washing process, distributing the secondary washing liquid via the control head and conveying it into a primary washing liquid tank  4 ; 
         [0089]    d. applying a pressure of 0.3 MPaG to 0.6 MPaG to the washing liquid in a tertiary washing liquid tank  8  by a tertiary washing liquid circulating pump  9 , and conveying the washing liquid to the third washing area so as to continuously wash the filter cake processed by the secondary washing (i.e. the third stage of washing), conveying the filter cake processed by the tertiary washing to a fourth washing area following the rotation of the pressure filter machine  1 , and after the washing process, distributing the tertiary washing liquid via the control head and conveying it into the secondary washing liquid tank  6 ; 
         [0090]    e. applying a pressure of 0.3 MPaG to 0.6 MPaG to the washing liquid in a fourth washing liquid tank  10  by a fourth washing liquid circulating pump  11 , and conveying the washing liquid to the fourth washing area so as to continuously wash the filter cake processed by the tertiary washing (i.e. the fourth stage of washing), conveying the filter cake processed by the fourth washing to a fifth washing area following the rotation of the pressure filter machine  1 , and after the washing process, distributing the fourth washing liquid via the control head and conveying it into the tertiary washing liquid tank  8 ; and 
         [0091]    f. conveying external process water of 90˜100° C. to a fifth washing area of the pressure filter machine  1 , to further wash the filter cake processed by the fourth washing (i.e. the fifth stage of washing) to remove the acetic acid from the crude terephthalic acid; after the washing process, distributing the fifth washing liquid via the control head and conveying it into the fourth washing liquid tank  10 , conveying the filter cake processed by the fifth washing to a unloading area following the rotation of the pressure filter machine  1 , conveying the filter cake to a pulping tank  12  with the nitrogen gas back blowing (0.1 to 0.3 MPaG) and gravity, so as to be mixed and pulped with the external process water. 
         [0092]    In the steps, if a bias-current technique is not used, the residual liquid in the filtrate pipe would be carried into the next washing area following the rotation of the pressure filter machine  1  during the filtering and washing processes of crude terephthalic acid. The residual liquid would flow through the filtering area, the first washing area, the second washing area, the third washing area, the fourth washing area, the fifth washing area and the unloading area in sequence. To avoid such a case, the present invention provides a bias-current between different areas of the pressure filter machine  1 . The bias current flows through the fifth washing area, the fourth washing area, the third washing area, the second washing area, the first washing area and the filtering area in sequence. With the bias-current, the residual liquid in the filtrate pipes can be brought back to the corresponding areas (see  FIG. 6 ). Therefore, the method of the present invention also comprises a step of biasing the residual liquid in the filtrate pipe to the mother liquor tank; the above steps b, c, d and e comprise a step of biasing the residual liquid in the filtrate pipes of all stages of washing to the washing liquid tanks at corresponding stages; and the above step f also comprises a step of conducting the residual liquid in the filtrate pipe to a filtrate tank  13 , and further conducting it to the washing liquid tank connected to the washing area for the penultimate stage of washing (i.e. the fourth washing liquid tank  10 ) during the process of conveying the CTA filter cake to the unloading area. Preferably, the washing liquid in the filtrate tank is conveyed to the fourth washing liquid tank  10  by a pump  14  (the pump is also referred as a conducting fluid circulating pump based on its function). 
         [0093]    Because the pressure filter machine consists of multiple chambers, i.e., the filtering area and the first to fifth washing unit chambers, the content of the acetic acid in the filter cake varies during the washing processes, resulting in different pressure drops in each of the chambers. In such a case, the seals between the chambers will be damaged in long-term operation and the washing liquid will mixed among the chambers. To avoid it, the present invention feeds nitrogen gas to regulate the pressures in the pressure filter, the mother liquor tank and the respective washing liquid tanks, so as to regulate the pressure drops in the respective chambers and to make it substantially identical. Therefore, the method of the present invention also comprises a step of feeding nitrogen gas of 0.1 to 0.2 MPaG to the mother liquor tank and the respective washing liquid tanks so as to adjust the pressures in the mother liquor tank and the washing liquid tanks with nitrogen gas to maintain a pressure balance of the whole system. The mother liquor tank and the washing liquid tanks are provided with a pressure regulating device respectively, so that gas in a given tank may be vented to a certain extent by the corresponding pressure regulating device on the tank when the pressure in the tank is higher than a preset pressure drop; and a certain amount of nitrogen gas may be fed into a given tank by the pressure regulating device on the tank when the pressure in the tank is lower than the preset pressure drop, thereby maintaining the pressure balance of the whole system. 
         [0094]    The present invention implements the bias-current and conduction by adjusting a position of a separation block in the control head of the pressure filter. That is, the residual liquid in the filtrate pipe is biased to the mother liquor tank by adjusting the position of the separation block in the control head of the pressure filter; the residual liquid in the filtrate pipes at respective stages of washing is biased to the washing liquid tank at corresponding stages by adjusting the position of the separation block in the control head of the pressure filter; and the residual liquid in the filtrate pipe is conducted to the filtrate tank by adjusting the position of the separation block in the control head of the pressure filter. 
         [0095]    Refer to  FIG. 7  and  FIG. 8 , the pressure filter machine  1  used in the present invention comprises a frame  18  and a control head  23 . The frame  18  is divided into a feeding area  19 , a primary washing unit chamber  24 , a secondary washing unit chamber  27 , a tertiary washing unit chamber  33 , a fourth washing unit chamber  37 , a fifth washing unit chamber  42  and an unloading area II  46  by a separation block A 21 , a separation block B 25 , a separation block C 30 , a separation block D 35 , a separation block E 40 , a separation block F 44  and a separation block G 48 . The control head  23  is divided into a mother liquor chamber  20 , a primary filtrate chamber  26 , a secondary filtrate chamber  29 , a tertiary filtrate chamber  32 , a fourth filtrate chamber  38 , a fifth filtrate chamber  41  and an unloading area I  45  by a separation block a 22 , a separation block b 28 , a separation block c 31 , a separation block d 36 , a separation block e 39 , a separation block f 43  and a separation block g 47 . The separation block A 21 , the separation block B 25 , the separation block C 30 , the separation block D 35 , the separation block E 40 , the separation block F 44  and the separation block G 48  correspond to the separation block a 22 , the separation block b 28 , the separation block c 31 , the separation block d 36 , the separation block e 39 , the separation block f 43  and the separation block g 47  one by one. An adjusting plate  49  is arranged on the control head  23 . A conduction port  50  is arranged at the end of the fifth filtrate chamber  41 . The primary washing unit chamber  24 , the secondary washing unit chamber  27 , the tertiary washing unit chamber  33 , the fourth washing unit chamber  37  and the fifth washing unit chamber  42  are respectively connected to the primary filtrate chamber  26 , the secondary filtrate chamber  29 , the tertiary filtrate chamber  32 , the fourth filtrate chamber  38  and the fifth filtrate chamber  41  one by one, through a filtrate pipe  34 . 
         [0096]    To bias the residual liquid in the filtrate pipe to the mother liquor tank, the separation block a 22  of the control head  23  can be adjusted to allow the separation block a 22  to lead the separation block A 21  of the corresponding frame  18  by an angle θ 1 , so that the residual mother liquor may enter the corresponding mother liquor chamber  20  within the period in which a drum rotates by the angle θ 1 . The mother liquor chamber  20  is communicated with the mother liquor tank  2  through a pipe. 
         [0097]    To bias the residual liquid in the filtrate pipe which has been processed by the first stage of washing to the primary washing liquid tank  4 , the position of the separation block b 28  between the primary filtrate chamber  26  and the secondary filtrate chamber  29  can be adjusted to allow the separation block b 28  to lead the separation block B 25  of the corresponding frame  18  by an angle θ 2 , so that the residual primary filtrate may enter the corresponding primary filtrate chamber  26  within the period in which the drum rotates by the angle θ 2 . The primary filtrate chamber  26  is communicated with the primary washing liquid tank  4  through a pipe. 
         [0098]    To bias the residual liquid in the filtrate pipe which has been processed by the second stage of washing to the secondary washing liquid tank  6 , the position of the separation block c 31  between the secondary filtrate chamber  29  and the tertiary filtrate chamber  32  can be adjusted to allow the separation block c 31  to lead the separation block C 30  of the corresponding frame  18  by an angle θ 3 , so that the residual secondary filtrate may enter the corresponding secondary filtrate chamber  29  within the period in which the drum rotates by the angle θ 3 . The secondary filtrate chamber  29  is communicated with the secondary washing liquid tank  6  through a pipe. 
         [0099]    To bias the residual liquid in the filtrate pipe which has been processed by the third stage of washing to the tertiary washing liquid tank  8 , the position of the separation block d 36  between the tertiary filtrate chamber  32  and the fourth filtrate chamber  38  can be adjusted to allow the separation block d 36  to lead the separation block D 35  of the corresponding frame  18  by an angle θ 4 , so that the residual tertiary filtrate may enter the corresponding tertiary filtrate chamber  32  within the period in which the drum rotates by the angle θ 4 . The tertiary filtrate chamber  32  is communicated with the tertiary washing liquid tank  8  through a pipe. 
         [0100]    To bias the residual liquid in the filtrate pipe which has been processed by the fourth stage of washing to the fourth washing liquid tank  10 , the position of the separation block e 39  between the fourth filtrate chamber  38  and the fifth filtrate chamber  41  can be adjusted to allow the separation block e 39  to lead the separation block E 40  of the corresponding frame  18  by an angle θ 5 , so that the residual fourth filtrate may enter the corresponding fourth filtrate chamber  38  within the period in which the drum rotates by the angle θ 5 . The fourth filtrate chamber  38  is communicated with the fourth washing liquid tank  10  through a pipe. 
         [0101]    To conduct the residual liquid in the filtrate pipe which has been processed by the fifth stage of washing to the filtrate tank  13 , the position of the separation block f 43  between the fifth filtrate chamber  41  and the unloading area  145  can be adjusted to correspond the separation block f 43  to the corresponding separation block F 40  of the frame  18 . The residual liquid is further conducted from the conduction port  50  of the filtrate pipe to the filtrate tank  13  by a pumping unit, and then conveyed to the fourth washing liquid tank  10  by a conducting fluid circulating pump  14  connected to the filtrate tank  13 . 
         [0102]    After five times of washing, the filter cake enters the unloading area  1146  in the frame  18  following the rotation of the drum, and unloading gas enters from the unloading area  145  of the control head  23  to back flush the filter cake, so as to unload the filter cake to the pulping tank  12  at which the filter cake is pulped to obtain a slurry, and to discharge the slurry. 
         [0103]    Of course, the present invention may have other multiple embodiments. Those skilled in the art can make various corresponding changes and modifications according to the present invention without departing from the spirit and essence of the present invention, but these changes and modifications should be incorporated in the protection scope of the claims appended to the present invention. 
       INDUSTRIAL APPLICABILITY 
       [0104]    The present invention employs bias-current and conduction techniques to bring the residual liquid in the filtrate pipes back to the washing liquid tanks of corresponding washing areas, which prevents the washing liquid of the previous area from entering the washing area of the next area and affecting the next stage of washing, and increases the washing efficiency. The residual washing liquid can be separated by the conduction technique and reused as the washing liquid of the previous stage of washing, thus being capable of reducing the consumption of washing liquid and the cost. Furthermore, the present invention feeds nitrogen gas to regulate the pressures in the pressure filter, the mother liquor tank and the washing liquid tanks, thereby regulating the pressure balance of the system. The pressure adjusting method is convenient and accurate, and avoids damage of the seals between the chambers, thus avoiding the mixing of the washing liquid among the chambers.