阅读说明：本技术 一种利用白泥对循环硫化床锅炉脱硫的系统及方法 (System and method for desulfurizing circulating fluidized bed boiler by using white mud ) 是由 刘博� 王丰吉 张杨 江建平 于 2021-07-14 设计创作，主要内容包括：本发明公开了一种利用白泥对循环硫化床锅炉脱硫的系统及方法,系统包括白泥制浆池、过滤网、工艺水泵、工艺水喷头、密度计、搅拌装置、白泥挤压泵、流量计、白泥浆喷头、白泥浆输送管道和白泥清洗水管道,方法包括如下步骤：1)取白泥,加水搅拌,得到白泥浆；2)将步骤1)所得白泥浆通过白泥挤压泵送至循环流化床锅炉的顶部；3)白泥浆通过循环流化床锅炉顶部的白泥浆喷头进入循环流化床锅炉的内部。白泥浆进入炉膛后,CaCO-3在高温下分解成CO-2和CaO,CaO与炉膛中的SO-2发生反应生成CaSO-4。本发明脱硫工艺简单,采用白泥为脱硫剂降低了脱硫成本,脱硫效果好,最终达到了对白泥和二氧化硫取得双向治理的效果。(The invention discloses a system and a method for desulfurizing a circulating fluidized bed boiler by white mud, wherein the system comprises a white mud pulping pool, a filter screen, a process water pump, a process water spray head, a densimeter, a stirring device, a white mud extrusion pump, a flowmeter, a white mud spray head, a white mud conveying pipeline and a white mud cleaning water pipeline, and the method comprises the following steps: 1) taking the white mud, adding water and stirring to obtain white mud slurry; 2) pumping the white mud slurry obtained in the step 1) to the top of a circulating fluidized bed boiler through a white mud extrusion pump; 3) the lime-mud slurry enters the circulating fluidized bed boiler through a lime-mud slurry spray nozzle at the top of the circulating fluidized bed boiler. After the white mud enters the hearth, CaCO 3 Decomposition to CO at high temperature 2 And CaO, CaO and SO in the furnace 2 React to generate CaSO 4 . The invention has simple desulfurization process, reduces the desulfurization cost by adopting the white mud as the desulfurizer, has good desulfurization effect and finally achieves the effect of bidirectional treatment on the white mud and the sulfur dioxide.)

1. A system for desulfurizing a circulating fluidized bed boiler by using white mud is characterized by comprising a white mud pulping pool (1), a filter screen (2), a process water pump (3), a process water spray head (5), a densimeter (7), a stirring device (10), a white mud extrusion pump (11), a flow meter (13), a white mud spray head (16), a white mud conveying pipeline (17) and a white mud cleaning water pipeline (18), wherein the filter screen (2) is arranged at a feed inlet of the white mud pulping pool (1), the process water spray head (5) is arranged above the feed inlet of the white mud pulping pool (1), the process water spray head (5) is connected with the process water pump (3), an electric valve I (4) is arranged at a water inlet of the process water spray head (5), one end of the white mud cleaning water pipeline (18) is connected with the process water pump (3), the other end of the white mud cleaning water pipeline (18) is connected to the white mud conveying pipeline (17), install electric valve two (6) on white mud washing water pipeline (18), the one end of white mud conveying pipeline (17) is connected to in white mud slurrying pond (1), the other end and the white mud shower nozzle (16) of white mud conveying pipeline (17) are connected, the inside top at circulating fluidized bed boiler (19) is arranged in white mud shower nozzle (16), be connected with white mud extrusion pump (11), electric control valve (12), flowmeter (13) and electric valve three (14) on white mud conveying pipeline (17), electric valve four (15) are installed in the import of white mud shower nozzle (16), be provided with agitating unit (10) in white mud slurrying pond (1), install densimeter (7) in white mud slurrying pond (1).

2. The system for desulfurizing a circulating fluidized bed boiler by using white mud according to claim 1, wherein a sampling pipe (8) is arranged in the white mud slurrying pool (1), and a sampling valve (9) is arranged on the sampling pipe (8).

3. A method of operating a system for desulphurizing a circulating fluidized bed boiler with lime mud according to claim 1 or 2, comprising the steps of:

(1) taking the white mud, adding water and stirring to obtain white mud slurry;

(2) feeding the white mud slurry obtained in the step (1) to the top of a circulating fluidized bed boiler (19) through a white mud extrusion pump (11);

(3) the lime mud slurry enters the interior of the circulating fluidized bed boiler (19) through a lime mud slurry spray nozzle (16) at the top of the circulating fluidized bed boiler (19).

4. The method for operating a system for desulfurizing a circulating fluidized bed boiler by using white slime as set forth in claim 3, wherein in the step (1), the white slime is poured into a feed inlet of the white slime pulping tank (1), the electric valve II (6) is closed, the electric valve I (4) is opened, the process water pump (3) is started, the process water is used for flushing the white slime into the white slime pulping tank (1) through the process water nozzle (5), and the stirring device (10) is started to prepare the white slime and prevent the white slime from settling.

5. Method of operating a system for desulphurizing a circulating fluidized bed boiler with lime mud according to claim 4 wherein the mass fraction of lime mud in the monitored lime mud slurry is controlled by controlling the amount of lime mud and process water supplied by means of a densitometer (7).

6. The method for operating the system for desulfurizing a circulating fluidized bed boiler by using white mud according to claim 3, wherein in the step (2), the white mud slurry in the white mud slurrying tank (1) is uniformly extruded into the inner part of the boiler chamber of the circulating fluidized bed boiler (19) at the top part of the circulating fluidized bed boiler (19) by using the white mud extrusion pump (11) through the white mud conveying pipeline (17) and the white mud sprayer (16); CaCO in white mud₃The part is decomposed under the action of high temperature to generate CaO, and the generated CaO and MgO contained in the white slime form the pyrolysis white slime with sulfur fixation activity.

7. The method for operating a system for desulfurizing a circulating fluidized bed boiler by using white mud according to claim 6, wherein the requirement of the boiler load change and the sulfur content in different coal qualities is met by observing the flow rate displayed by the flow meter (13) and adjusting the electric adjusting valve (12) to control the supply amount of the white mud.

8. The method for operating a system for desulfurizing a circulating fluidized bed boiler by using white mud according to claim 3, wherein in the step (3), the white mud has sufficient time to decompose and react with sulfides in the flue gas after entering the hearth of the circulating fluidized bed boiler (19); the white mud spray nozzle (16) enables the white mud to uniformly disperse and enter the hearth of the circulating fluidized bed boiler (19), and is beneficial to the decomposition reaction of the white mud in the hearth of the circulating fluidized bed boiler (19).

9. A method of operating a system for desulphurizing a circulating fluidized bed boiler with lime mud according to claim 3 wherein the lime mud slurry remains in the lime mud slurry feed line (17) when the circulating fluidized bed boiler (19) or the lime mud slurry feed system is shut down, and the lime mud slurry feed line (17) and the lime mud slurry injectors (16) are flushed;

the rinsing process was as follows: closing the first electric valve (4) and the fourth electric valve (15), opening the second electric valve (6), the third electric valve (14) and the electric adjusting valve (12), starting the process water pump (3), and flushing the white mud conveying pipeline (17); and (3) closing the first electric valve (4) and the third electric valve (14), opening the second electric valve (6) and the fourth electric valve (15), and starting the process water pump (3) to flush the white mud spray nozzle (16).

Technical Field

The invention relates to a system and a method for dry desulphurization of a circulating fluidized bed boiler, in particular to a system and a method for desulphurization of a circulating fluidized bed boiler by using white mud.

Background

The pulp and paper mill produces a large amount of causticized white mud (also called papermaking white mud) in the process of black liquor alkali recovery, and the main component of the causticized white mud is CaCO₃For paper mills, the white mud belongs to a solid waste, and is generally treated in a landfill mode or a river discharge mode. The white mud is used as solid waste generated by a paper mill, has approximately the same components as limestone, has the actual use value of desulfurization, and can effectively reduce the operation cost of environmental protection facilities and achieve the aim of comprehensively utilizing waste resources if being applied to a thermal power generation desulfurization process system on a large scale.

The circulating fluidized bed boiler adopts the clean coal combustion technology with the highest industrialization degree. The wide fuel adaptability is one of the main advantages of the circulating fluidized bed boiler. In the circulating fluidized bed boiler, the fuel only accounts for 1-3% of the bed material by weight, and the rest is non-combustible solid particles such as a desulfurizer, ash and the like. Thus, the fresh coal particles added to the bed are surrounded by hot ash particles which behave as a "large thermal reservoir". Because of the intense mixing within the bed, these hot ash particles act as virtually an infinite "ideal arch" that heats the coal to ignition temperature and begins to burn. In the heating process, the absorbed heat only accounts for a few thousandths of the total heat capacity of the bed layer, so that the temperature of the bed layer is slightly influenced, and the combustion of the coal particles releases heat, so that the bed layer can keep a certain temperature level. Because the fuel in the circulating bed furnace is ignited and the combustion condition is good, the biomass waste materials such as high-ash, high-sulfur, high-moisture, low-heat value and low-volatile matter bituminous coal, anthracite, lignite, peat, coal gangue, oil shale, wood, rice hulls and the like can be combusted until the poor fuel such as slagging of a grate-fired boiler, gas making slag and the like is generated, the coal types are variable, and various fuel mixtures can be adapted.

According to the characteristic of wide fuel adaptability of the circulating fluidized bed boiler, if the papermaking white mud can be added to be used as a desulfurizing agent to be fed into the circulating fluidized bed boiler, the treatment of pollution by waste can be realized, so that the pollutants are comprehensively controlled, and the circular economy among enterprises is realized.

Disclosure of Invention

The invention aims to overcome the defects in the prior art and provide a system and a method for desulfurizing a circulating fluidized bed boiler by using white mud. The recycled circulating fluidized bed boiler has the advantages of wide fuel adaptability and small influence of a desulfurizer on the bed temperature, and CaCO is used as the material after white mud enters the hearth₃Will decompose to CO at high temperature₂And CaO, CaO and SO in the furnace₂React to generate CaSO₄. The method has the advantages of simple process, low cost, good desulfurization effect, and simultaneous treatment of white mud and CO₂Bidirectional treatment and the like.

The technical scheme adopted by the invention for solving the problems is as follows: the utility model provides an utilize white mud to desulfurization system of circulating fluidized bed boiler, characterized by, including white mud slurrying pond, filter screen, process water pump, technology water shower nozzle, densimeter, agitating unit, white mud extrusion pump, flowmeter, white mud shower nozzle, white mud pipeline and white mud washing water pipeline, the filter screen is arranged at the feed inlet of white mud slurrying pond, the technology water shower nozzle sets up in the feed inlet top of white mud slurrying pond, the technology water shower nozzle is connected with process water pump, and installs electric valve one at process water shower nozzle's water inlet, white mud washing water pipeline's one end and technology water pump are connected, white mud washing water pipeline's the other end is connected to white mud pipeline, white mud washing water pipeline is last to install electric valve two, white mud pipeline's one end is connected to in the white mud pond, white mud pipeline's the other end and white mud shower nozzle are connected, the white mud slurry sprayer is arranged on the top end of the inside of the circulating fluidized bed boiler, a white mud extrusion pump, an electric adjusting valve, a flowmeter and an electric valve are connected to the white mud slurry conveying pipeline, an electric valve four is installed at the inlet of the white mud slurry sprayer, a stirring device is arranged in the white mud slurry preparing pool, and a densimeter is installed in the white mud slurry preparing pool.

Further, be provided with the sampling tube in the white mud slurrying pond, install the sample valve on the sampling tube.

The method for desulfurizing the circulating fluidized bed boiler by using the white mud comprises the following steps:

(1) taking the white mud, adding water and stirring to obtain white mud slurry;

(2) pumping the white mud slurry obtained in the step (1) to the top of a circulating fluidized bed boiler through a white mud extrusion pump;

(3) the lime-mud slurry enters the circulating fluidized bed boiler through a lime-mud slurry spray nozzle at the top of the circulating fluidized bed boiler.

In the step (1), white mud from a factory is poured into a feed inlet of a white mud pulping pool, an electric valve II is closed, an electric valve I is opened, and a process water pump is started, so that process water is required to flush the white mud into the white mud pulping pool through a process water nozzle due to the large cohesiveness of the white mud; and starting the stirring device to prepare white mud, and preventing the white mud from precipitating.

In the step (1), the supply amount of the white mud and the process water is controlled by a densimeter, and the mass fraction of the white mud in the white mud slurry is controlled and monitored.

In the step (1), the mass fraction of the white mud in the prepared white mud slurry is 30-75%, preferably 40-75%, and more preferably 50-75%.

In the step (2), the white mud in the white mud pulping pool is uniformly squeezed into the hearth of the circulating fluidized bed boiler at the top of the circulating fluidized bed boiler through a white mud conveying pipeline and a white mud sprayer by a white mud squeezing pump; CaCO in white mud₃And decomposing part of the waste lime sludge under the action of high temperature to generate CaO, wherein the generated CaO and MgO contained in the lime sludge jointly form the pyrolysis white sludge with sulfur fixation activity.

In the step (2), the flow displayed by the flowmeter is observed, and the electric control valve is adjusted to control the supply amount of the white mud so as to meet the requirements of the load change of the boiler and the sulfur content in different coal qualities.

In the step (3), the white mud has sufficient time to decompose after entering the hearth of the circulating fluidized bed boiler and reacts with sulfides in the flue gas; the white mud spray nozzle can enable the white mud to uniformly disperse and enter the circulating fluidized bed boiler furnace, and is beneficial to the decomposition reaction of the white mud in the circulating fluidized bed boiler furnace.

When the circulating fluidized bed boiler or the lime mud slurry supply system is stopped, lime mud slurry can remain in the lime mud slurry conveying pipeline, and the lime mud slurry has high cohesiveness, so that the lime mud slurry is easy to harden and block the pipeline if not processed in time, and the later operation of the system is influenced, so that the lime mud slurry conveying pipeline and the lime mud slurry spray head need to be washed.

The rinsing process was as follows: closing the first electric valve and the fourth electric valve, opening the second electric valve, the third electric valve and the electric adjusting valve, and starting the process water pump to flush the white mud conveying pipeline; and closing the first electric valve and the third electric valve, opening the second electric valve and the fourth electric valve, and starting the process water pump to flush the white mud sprayer.

Compared with the prior art, the invention has the following advantages and effects:

the invention dilutes and dissolves the white mud into white mud slurry, feeds the white mud slurry from the top of the furnace by adopting an extrusion pump, and feeds the white mud slurry into the high-temperature center in the pulverized coal circulating fluidized bed boiler₃Will decompose to CO at high temperature₂And CaO, CaO and SO in the furnace₂React to generate CaSO₄To reduce SO in the combustion flue gas₂The aim of discharging is fulfilled, meanwhile, the waste recycling of the waste white mud is realized, and the perfect combination of economic benefit and environmental protection and emission reduction social benefit is achieved.

Drawings

FIG. 1 is a schematic diagram of the system architecture of the present invention.

FIG. 2 is a schematic diagram of the configuration of the lime mud slurry injection head of the system of the present invention.

In the figure: the device comprises a white mud pulping pool 1, a filter screen 2, a process water pump 3, a first electric valve 4, a process water spray head 5, a second electric valve 6, a densimeter 7, a sampling pipe 8, a sampling valve 9, a stirring device 10, a white mud extrusion pump 11, an electric adjusting valve 12, a flowmeter 13, a third electric valve 14, a fourth electric valve 15, a white mud spray head 16, a white mud conveying pipeline 17, a white mud cleaning water pipeline 18 and a circulating fluidized bed boiler 19.

Detailed Description

The present invention will be described in further detail below by way of examples with reference to the accompanying drawings, which are illustrative of the present invention and are not to be construed as limiting the present invention.

Example 1.

Referring to fig. 1 to 2, in the embodiment, a system for desulfurizing a circulating fluidized bed boiler by using white slime comprises a white slime pulping tank 1, a filter screen 2, a process water pump 3, a process water nozzle 5, a densimeter 7, a stirring device 10, a white slime extrusion pump 11, a flow meter 13, a white slime spray nozzle 16, a white slime conveying pipeline 17 and a white slime cleaning water pipeline 18, wherein the filter screen 2 is arranged at a feed inlet of the white slime pulping tank 1, the process water nozzle 5 is arranged above the feed inlet of the white slime pulping tank 1, the process water nozzle 5 is connected with the process water pump 3, an electric valve one 4 is arranged at a water inlet of the process water nozzle 5, one end of the white slime cleaning water pipeline 18 is connected with the process water pump 3, the other end of the white slime cleaning water pipeline 18 is connected to the white slime conveying pipeline 17, an electric valve two 6 is arranged on the white slime pipeline 18, one end of the white slime conveying pipeline 17 is connected into the white slime pulping tank 1, the other end and the lime mud shower nozzle 16 of lime mud pipeline 17 are connected, lime mud shower nozzle 16 arranges the inside top at circulating fluidized bed boiler 19, be connected with lime mud extrusion pump 11 on the lime mud pipeline 17, electronic regulating valve 12, flowmeter 13 and electronic valve three 14, electronic valve four 15 is installed to the import of lime mud shower nozzle 16, be provided with agitating unit 10 in the lime mud slurrying pond 1, install densimeter 7 in the lime mud slurrying pond 1, be provided with sampling tube 8 in the lime mud slurrying pond 1, install sample valve 9 on the sampling tube 8.

The method for desulfurizing the circulating fluidized bed boiler by using the white mud comprises the following steps:

(1) taking the white mud, adding water and stirring to obtain white mud slurry;

(2) feeding the white mud slurry obtained in the step (1) to the top of a circulating fluidized bed boiler 19 through a white mud extrusion pump 11;

(3) the lime mud slurry is introduced into the interior of the circulating fluidized bed boiler 19 through the lime mud slurry injection nozzle 16 at the top of the circulating fluidized bed boiler 19.

In the step (1), white mud from a factory is poured into a feed inlet of the white mud pulping pool 1, the electric valve II 6 is closed due to strong caking property of the white mud, the electric valve I4 is opened, and the process water pump 3 is started, so that process water needs to be flushed into the white mud pulping pool 1 through the process water nozzle 5 due to large caking property of the white mud; the stirring device 10 is started to make white mud slurry and prevent the white mud from settling. The mass fraction of the white mud in the white mud slurry is controlled and monitored by controlling the supply of the white mud and the process water by a densimeter 7. The mass fraction of the white mud in the prepared white mud slurry is 50-75%.

In the step (2), the white mud in the white mud pulping pool 1 is uniformly squeezed into the hearth of the circulating fluidized bed boiler 19 at the top of the circulating fluidized bed boiler 19 through the white mud conveying pipeline 17 and the white mud sprayer 16 by using the white mud squeezing pump 11; CaCO in white mud₃And decomposing part of the waste lime sludge under the action of high temperature to generate CaO, wherein the generated CaO and MgO contained in the lime sludge jointly form the pyrolysis white sludge with sulfur fixation activity. The flow displayed by the flowmeter 13 is observed, and the electric regulating valve 12 is regulated to control the supply amount of the white mud slurry so as to meet the requirements of load change of a boiler and the sulfur content in different coal qualities.

In the step (3), the white mud has sufficient time to decompose after entering a hearth of a circulating fluidized bed boiler 19 and reacts with sulfides in the flue gas; the white mud spray nozzle 16 can make the white mud uniformly dispersed and enter the hearth of the circulating fluidized bed boiler 19, which is beneficial to the decomposition reaction of the white mud in the hearth of the circulating fluidized bed boiler 19.

When the circulating fluidized bed boiler 19 or the lime mud slurry supply system is stopped, the lime mud slurry remains in the lime mud slurry conveying pipe 17, and the lime mud slurry has high cohesiveness, so that if the lime mud slurry is not treated in time, the lime mud slurry is easy to harden and block the pipe, and the subsequent operation of the system is influenced, so that the lime mud slurry conveying pipe 17 and the lime mud slurry spray nozzles 16 need to be washed.

The rinsing process was as follows: closing the first electric valve 4 and the fourth electric valve 15, opening the second electric valve 6, the third electric valve 14 and the electric adjusting valve 12, starting the process water pump 3, and flushing the white mud conveying pipeline 17; and (3) closing the first electric valve 4 and the third electric valve 14, opening the second electric valve 6 and the fourth electric valve 15, starting the process water pump 3, and flushing the white mud spray nozzle 16.

Experimental example 2.

A desulfurization device of a power plant adopts a high-efficiency limestone-gypsum wet flue gas desulfurization process and a 350MW supercritical circulating fluidized bed thermal power generating unit with one furnace and one tower. When the designed coal is burned, the concentration of sulfur dioxide at the inlet of the absorption tower is 3000 mg/m³(Standard, dry, 6% O)₂) Design, outlet SO of the desulfurization unit₂The concentration is not more than 35 mg/m³(Standard, dry, 6% O)₂). The elemental analysis composition of the coal sample used during the unit operation was Car 49.6%, Har 3.38%, Nar 1.01%, sar0.69%, and Oar 8.11%. The load of the unit is 320MW, and the concentration of sulfur dioxide at the inlet of the absorption tower is 2860 mg/m³(Standard, dry, 6% O)₂）。

Pouring the white mud into a feed inlet of the white mud pulping pool 1, closing the electric valve II 6, opening the electric valve I4, starting the process water pump 3 and flushing the white mud into the white mud pulping pool 1 by process water through the process water nozzle 5 due to the strong caking property of the white mud; the stirring device 10 is started to make white mud slurry and prevent the white mud from settling. The mass fraction of the white mud in the white mud slurry is controlled and monitored by controlling the supply of the white mud and the process water by a densimeter 7. The mass fraction of the white mud in the prepared white mud slurry is about 65 percent.

Closing the electric valve I4, opening the electric adjusting valve 12, the electric valve III 14 and the electric valve IV 15, opening the process water pump 3, and uniformly extruding the white mud slurry in the white mud pulping tank 1 into the hearth of the circulating fluidized bed boiler 19 at the top of the circulating fluidized bed boiler 19 through the white mud slurry conveying pipeline 17 and the white mud slurry nozzle 16 under the action of the white mud extrusion pump 11; CaCO in white mud₃Decomposing part of the lime mud into CaO under the action of high temperature, wherein the generated CaO and the lime mud contain the CaO and the lime mudSome CaO and MgO jointly form the pyrolysis white mud with sulfur fixation activity. By observing the outlet SO of the 19 hearth of the circulating fluidized bed boiler₂The concentration change and the combustion condition of the circulating fluidized bed boiler 19 are adjusted, the electric regulating valve 12 is regulated to control the supply amount of the lime mud slurry, and finally the hearth outlet SO of the circulating fluidized bed boiler 19₂The concentration is stabilized at 990 mg/m³(Standard, dry, 6% O)₂) The flow of the flowmeter 13 is stabilized at 8.3m³/h。

By means of the desulfurization in the lime mud furnace, the concentration of sulfur dioxide at the inlet of the desulfurization absorption tower is 2860 mg/m from the original value³(Standard, dry, 6% O)₂) Reduced to 990 mg/m³(Standard, dry, 6% O)₂). The working pressure of the desulfurization absorption tower is reduced, and the power consumption and limestone consumption of the desulfurization absorption tower are reduced.

Experimental example 3.

A desulfurization device of a power plant adopts a high-efficiency limestone-gypsum wet flue gas desulfurization process and a 200MW supercritical circulating fluidized bed thermal power generating unit with one furnace and one tower. When the coal is designed for combustion, the concentration of sulfur dioxide at the inlet of the absorption tower is 1270 mg/m³(Standard, dry, 6% O)₂) Design, outlet SO of the desulfurization unit₂The concentration is not more than 35 mg/m³(Standard, dry, 6% O)₂). The load of the unit is 110MW, two circulating pumps of the absorption tower run, and the inlet sulfur dioxide concentration is 960 mg/m³(Standard, dry, 6% O)₂) Outlet sulfur dioxide concentration 26 mg/m³(Standard, dry, 6% O)₂). Since the absorption tower can already complete the desulfurization task at this time, and the operation of the circulating pump can no longer be stopped. Therefore, the operation of the desulfurization system in the lime mud furnace needs to be stopped.

When the circulating fluidized bed boiler 19 or the lime mud slurry supply system is stopped, the lime mud slurry remains in the lime mud slurry conveying pipe 17, and the lime mud slurry has high cohesiveness, so that if the lime mud slurry is not treated in time, the lime mud slurry is easy to harden and block the pipe, and the subsequent operation of the system is influenced, so that the lime mud slurry conveying pipe 17 and the lime mud slurry spray nozzles 16 need to be washed.

The rinsing process was as follows: closing the white mud extrusion pump 11, closing the first electric valve 4 and the fourth electric valve 15, opening the second electric valve 6, the third electric valve 14 and the electric adjusting valve 12, starting the process water pump 3, and flushing the white mud conveying pipeline 17; and (3) flushing for 5 minutes, closing the first electric valve 4 and the third electric valve 14, opening the second electric valve 6 and the fourth electric valve 15, starting the process water pump 3, and flushing the white mud spray nozzle 16. The stirring device 10 is kept running.

Those not described in detail in this specification are well within the skill of the art.

Although the present invention has been described with reference to the above embodiments, it should be understood that the scope of the present invention is not limited thereto, and that various changes and modifications can be made by those skilled in the art without departing from the spirit and scope of the present invention.