Process and device for recycling raw materials of lime kiln of slurry plant

Abstract

The invention provides a slurry plant lime kiln raw material recovery process and device, and relates to the technical field of slurry making. The process comprises the following steps: (1) selecting waste white mud, feeding the waste white mud into a recovery tank through a grating net, adding hot water of 70-85 DEG C for dilution and mixing at the same time, and adding the hot water from the periphery in a spraying manner; in the mixing process, auxiliary stirring is carried out at the stirring speed of 20-60 revolutions per minute, and meanwhile, the residence time of the white mud in the recovery tank is controlled to be less than 15 minutes, so that recovered white mud emulsion is obtained; (2) mixing the recovered white mud emulsion with white mud generated in causticization production according to a proportion of less than 30%, and washing; and (3) filtering the washed white mud, wherein the filtered white mud is the raw material of the lime kiln. By adopting the process disclosed by the invention, the waste white mud can be efficiently recovered and reused in a system, causticization and lime kiln operation are not influenced, and the solid waste treatment cost and the cost of purchased raw materials (lime or limestone) are saved.

Description

Technical Field

[0001] The invention relates to the technical field of pulping, and in particular to a pulp mill lime kiln raw material recovery process and device. Background Art

[0002] In modern large-scale kraft pulping process, causticizing / lime kiln process is an essential component of its liquid circulation. When the lime kiln is under maintenance / abnormal, the pulp line is often maintained by supplementing with purchased lime. Due to insufficient output or shutdown of the lime kiln, the calcium carbonate (white mud) produced in the white liquor production cannot be treated and can only be discharged. The amount of white mud discharged each year accounts for about 2%-5%. The calcium carbonate lost after discharge needs to be purchased and supplemented, and the discharged white mud needs to be treated as industrial solid waste, which increases the production cost. For a pulp mill with an annual output of 1 million tons, the annual cost increase is about 4-5 million yuan.

[0003] As environmental protection requirements become increasingly stringent, abnormally discharged white mud contains a certain amount of alkali, and improper disposal will cause environmental pollution. Its main component is calcium carbonate, which is the raw material for calcining lime, and its discharge is also a waste of resources. At present, the mainstream treatment methods at home and abroad are landfill, discarding, and making building materials raw materials, which either pollute the environment or cost a lot of money. Recycling to the system will affect the normal operation of the entire causticizing, lime kiln and even the entire pulp plant, and it has not been recycled to the system for use as a raw material for the lime kiln. Due to unreasonable equipment and process design, some factories have problems such as a large number of impurities, large concentration fluctuations, and large temperature fluctuations when recycling white mud, which reduces the filtering capacity of the back-end white mud filter, reduces the dryness of the white mud, and causes unstable operation of the lime kiln, forcing the inability to recycle the discharged white mud. Summary of the invention

[0004] In view of this, the present invention proposes a pulp mill lime kiln raw material recovery process and device to solve the above problems.

[0005] The technical solution of the present invention is implemented as follows: A pulp mill lime kiln raw material recovery process comprises the following steps:

[0006] (1) selecting waste white mud, feeding the waste white mud into a recovery tank through a grid net, adding 70-85° C. hot water to dilute and mix the materials, and adding hot water from all sides in a spraying manner; assisting stirring during the mixing process, the stirring speed is 20-60 revolutions per minute, and the retention time of the white mud in the recovery tank is controlled to be less than 15 minutes, so as to obtain a recovered white mud emulsion;

[0007] (2) mixing the recovered white mud emulsion with the white mud produced in the causticizing production at a ratio of less than 30% and washing;

[0008] (3) The washed white mud is filtered, and the filtered white mud is the raw material of the lime kiln.

[0009] Furthermore, the thickness of the grille mesh is 15-25 mm, and the hole specifications of the grille mesh are 35-45 mm*35-45 mm.

[0010] Further, in step (1), the concentration of the recovered white mud emulsion is controlled at 25%-35%wt, or the density is controlled at 1.25-1.35t / m 3 .

[0011] Furthermore, in step (2), the residual alkali content after washing is less than 1.0% based on Na2O.

[0012] Furthermore, in step (3), the dryness of the filtered white mud is controlled at 70%-80%, and the residual alkali content calculated as Na2O is less than 0.3%.

[0013] Furthermore, a high-efficiency white mud recovery system is used, including a feed port, a recovery tank, a grille, a flushing device, and an agitator; the feed port is located above the recovery tank, the grille is located between the recovery tank and the feed port, and the flushing device is located above the grille; the stirring blades of the agitator are located in the recovery tank; the waste white mud is fed from the feed port through the grille into the recovery tank, and hot water enters the recovery tank through the grille in a spraying manner from all sides.

[0014] Furthermore, the efficient white mud recovery system also includes a liquid level meter, a density meter, a transmission pump, a control valve, and a delivery pipeline;

[0015] The flushing device is located at the feeding port, and includes an upper hot water flushing device and a lower spray flushing device. The upper hot water flushing device is provided with a plurality of flushing nozzles, and the nozzles of the flushing nozzles are flat; the lower spray flushing device is provided with a plurality of flushing nozzles, and the plurality of flushing nozzles are connected to form a rectangle through water pipes;

[0016] The stirring blades of the stirrer are located in the recovery tank;

[0017] The liquid level meter and the density meter are located in the recovery tank;

[0018] The bottom of the recovery tank is connected to the transmission pump through a delivery pipeline.

[0019] A raw material recovery device for a lime kiln in a pulp mill is provided with a high-efficiency white mud recovery system, the high-efficiency white mud recovery system comprises a feed port, a recovery tank, a grille, a flushing device, an agitator, a level meter, a density meter, a transmission pump, and a conveying pipeline; the feed port is located above the recovery tank, the grille is located between the recovery tank and the feed port, the flushing device is located at the feed port, the flushing device comprises an upper hot water flushing device and a lower spray flushing device, the upper hot water flushing device is provided with a plurality of flushing nozzles, the nozzles of the flushing nozzles are flat; the lower spray flushing device is provided with a plurality of flushing nozzles, the plurality of flushing nozzles are connected by water pipes to form a rectangular arrangement; the stirring blades of the agitator are located in the recovery tank; the level meter and the density meter are located in the recovery tank; the bottom of the recovery tank is connected to the transmission pump through a conveying pipeline.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) The present invention is carefully designed and uses online instruments to effectively control process parameters, recover the discharged white mud and use it as a raw material for the lime kiln, thereby solving the problem that the discharged white mud cannot be effectively reused. The system recovers the white mud and reuses it in the system, which can stabilize the back-end process parameters and does not affect the causticizing and lime kiln operations. It saves the factory high solid waste treatment costs and the cost of purchasing raw materials (lime or limestone), and realizes efficient recovery of waste white mud.

[0022] (2) The present invention adds an efficient white mud recovery system, which mainly includes a feed port, a grid, a special flushing device, a recovery tank, an agitator, a liquid level meter, a density meter, a transmission pump, a control valve, a delivery pipeline, etc. After the lime kiln is in normal production, the discharged white mud is recovered to the causticizing system, and after washing and filtering, it is re-calcined into lime for causticizing as a raw material of the lime kiln, thereby reducing production costs.

[0023] (3) Since the recovery of white mud is intermittent operation, the density, concentration, temperature, etc. fluctuate, which will affect the operating efficiency of the back-end white mud filter, thereby reducing the white mud dryness and affecting the operation of the lime kiln, causing additional negative effects. For this reason, the recovery system is designed to use special dilution nozzles and grids for white mud dilution and flushing, and install density meters, density control valves, reflux pipes and other equipment, which can effectively control the concentration of recovered white mud, and in the recovery process, the white mud is recovered to the front-end process of the white mud filter entering the kiln (white mud washing machine or the back-end of the white liquor filter), thereby effectively preventing the negative impact of recovered white mud on white mud dryness and lime kiln. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The process flow chart of the pulp mill lime kiln raw material recovery process of the present invention (the dotted frame part is the newly added process).

[0025] Figure 2Schematic diagram (side view) of the spray design of the raw material recovery process of the lime kiln of the pulp mill of the present invention.

[0026] Figure 3 Schematic diagram of the spray design of the efficient white mud recovery system of the present invention (top view).

[0027] In the figure, 1 is a feeding port, 2 is a recovery tank, 3 is a grille, 401 is an upper hot water flushing device, 402 is a lower spray flushing device, 5 is a stirrer, 6 is a level meter, 7 is a density meter, 8 is a transmission pump, 9 is a control valve, 10 is a feeding tank of a white mud washing machine, 11 is a white mud washing machine, 12 is a white mud storage tank, 13 is a white mud filter, and 14 is a lime kiln. DETAILED DESCRIPTION

[0028] In order to better understand the technical content of the present invention, specific embodiments are provided below to further illustrate the present invention.

[0029] Unless otherwise specified, the experimental methods used in the embodiments of the present invention are all conventional methods.

[0030] Unless otherwise specified, the materials, reagents, etc. used in the embodiments of the present invention can be obtained from commercial sources.

[0031] Due to unreasonable equipment and process design, some factories have problems such as a lot of impurities, large concentration fluctuations, and large temperature fluctuations when recycling white mud, which reduces the filtering capacity of the back-end white mud filter, reduces the dryness of the white mud, and causes unstable operation of the lime kiln, forcing them to be unable to recycle the discharged white mud. This system has been carefully designed, reasonably selected, and installed with online equipment automatic control measures to recycle the discharged white mud as raw material for the lime kiln. It solves the problem that the discharged white mud cannot be effectively reused. The system recycles the white mud and can stabilize the back-end process parameters without affecting the causticizing and lime kiln operation. It saves the factory the cost of solid waste treatment and the cost of purchasing raw materials (lime or limestone).

[0032] (1) Design of efficient spraying structure: Special nozzles are used for spraying and dilution at the feeding port to reduce water consumption and avoid too low density (density>1.3kg / L). The nozzles are arranged horizontally with a spacing of 20-30cm. Spray pipes are arranged around and spraying is controlled by a control valve. In addition to horizontal spraying, a vertical flushing pipe is set at the top. The flushing pipe mouth is designed with a flat head and different angles, which is conducive to vertical flushing and is also used for density control. For details, see Figure 2-3 The spray flushing system effectively solves the problem of low density of white mud. The material inlet is designed with a special grid mesh, which is laser cut with a 20mm steel plate. The opening size is 40mmx40mm and the grid thickness is 6mm, ensuring the opening rate, which is conducive to removing foreign matter and feeding. The agitator of the recovery tank uses a high stirring intensity and a moderate speed (20-60rpm) to ensure that the white mud is fully dispersed and mixed in a short time.

[0033] (2) Process design: The recovered white mud does not directly enter the feed tank of the white mud filter, but enters the white mud washing machine (such as Figure 1 ), thus avoiding the interference of intermittent white mud recovery on the original system, and reducing the residual alkali of white mud to below 0.5%. In addition, the density of the recovered white mud is kept consistent with the density in the later process as much as possible, both controlled at 1.3kg / L, and hot water is used for recycling white mud, and the hot water temperature is controlled at 70-75℃ to avoid reducing the efficiency of the later white mud treatment and the dryness of the white mud.

[0034] (3) Density control design: Density and flushing water are interlocked. When the density is lower than the set value, the vertical flushing water is automatically turned off. The reflux control valve can be used to reflux and reduce the amount of clean water. When the density is higher than the set value, the flushing water control valve is automatically opened. The liquid level transmission pump is interlocked with the liquid level. When the liquid level is lower than the set value, the pump outlet control valve is automatically closed and the reflux valve is opened. When the liquid level continues to drop to another set value, the transmission pump automatically stops. Effective control of density and liquid level is the key to stable system operation. Through automatic control and DCS personnel monitoring, the density of white mud can be effectively controlled to the desired density, normally 1.3kg / L.

[0035] An efficient white mud recovery system is added on site, which mainly includes a feeding port, a feeding grid, a flushing device (special flushing nozzle), a recovery tank, a stirrer, a level meter, a density meter, a transmission pump, a control valve, a transmission pipeline and other equipment. After the lime kiln is in normal production, the high-efficiency recovery system is used to recover the discharged white mud to the causticizing system, and after filtering, it is re-calcined into lime for causticizing as a raw material of the lime kiln, so that the discharged white mud returns to the system circulation.

[0036] Because the recovery of white mud is intermittent operation, the density, concentration, temperature, etc. fluctuate, which will affect the operating efficiency of the back-end white mud filter, resulting in reduced white mud dryness, unstable lime kiln operation, and high fuel consumption. For this reason, the recovery system is designed to use special dilution nozzles and special feed grids for white mud dilution and flushing, and install density meters, density control valves, reflux pipes and other equipment to effectively control the concentration of recovered white mud. In the recovery process, the white mud is recovered to the front-end process of the white mud filter entering the kiln (white mud washing machine or white mud washing clarifier), thereby effectively preventing the negative impact of recovered white mud on white mud dryness and lime kiln.

[0037] By designing this white mud recovery system, the discharged white mud can be efficiently recovered without negatively affecting the existing process parameters. For a pulp mill with an annual output of 1 million tons, about 15,000 tons of white mud can be recovered each year, and the white mud recovery rate can reach more than 97%, reaching the world's advanced level, and saving 4-5 million yuan in production costs each year.

[0038] Example 1

[0039] like Figures 1 to 3As shown, a high-efficiency white mud recovery system includes a feed port 1, a recovery tank 2, a grille 3, a flushing device (not marked), an agitator 5, a level meter 6, a density meter 7, a transmission pump 8, a plurality of control valves 9, and a conveying pipeline (not marked), wherein the plurality of control valves 9 are located on the conveying pipeline.

[0040] The feeding port 1 is located above the recovery tank 2, and the grille 3 is located between the recovery tank 2 and the feeding port 1. The feeding port 1 is provided with an upper hot water flushing device and a lower spray flushing device from top to bottom, and the hot water enters the recovery tank 2 through the grille 3.

[0041] The thickness of the grid mesh 3 is 20 mm, and the hole size of the grid mesh 3 is 40 mm*40 mm;

[0042] The flushing device 4 is located at the feeding port 1, and includes an upper hot water flushing device 401 and a lower spray flushing device 402. The upper hot water flushing device 401 is provided with a plurality of flushing nozzles, and the nozzles of the flushing nozzles are flat; the lower spray flushing device 402 is provided with a plurality of flushing nozzles, and the plurality of flushing nozzles are connected by water pipes to form a rectangular arrangement (arranged around the periphery);

[0043] The stirring blades of the stirrer 5 are located in the recovery tank 2;

[0044] The liquid level meter 6 and the density meter 7 are located in the recovery tank 2;

[0045] The bottom of the recovery tank 2 is connected to the transmission pump 8 through a delivery pipeline.

[0046] Example 2

[0047] A pulp mill lime kiln raw material recovery process, the process flow is as follows Figure 1 The high-efficiency white mud recovery system of Example 1 is used.

[0048] 1) Waste white mud should be stored in warehouses, factories or uncontaminated places.

[0049] 2) Use a forklift to feed the stockpiled waste white mud from the feeding port 1 through the grid net 3 into the recovery tank 2, add 70-85°C hot water into the recovery tank 2 for dilution and mixing, and use the upper hot water flushing device 401 and the lower spray flushing device 402 to add hot water from all sides in a spraying manner, with the proportion of the lower spray water being greater than 50%.

[0050] 3) During the mixing process, the agitator 5 is turned on to assist in stirring. The speed of the agitator 5 is 20-60 rpm. The white mud is fully mixed in the white mud recovery tank 2. The concentration after mixing is controlled to be 25%-35% (mass percentage), or the density is controlled to be 1.25-1.35 tons / cubic meter. At the same time, the residence time of the white mud in the white mud recovery tank 2 is controlled to be less than 15 minutes to avoid excessive stirring to damage the white mud particle size.

[0051] 4) The recovered white mud emulsion is mixed with the white mud produced in the causticizing production at a ratio of ≤30%. In this embodiment, the mass ratio of the recovered white mud emulsion to the white mud produced in the causticizing production is not more than 30:70, and then the recovered white mud emulsion is fed into the white mud washing machine 11 through the white mud washing machine feeding trough 10 for washing to remove residual alkali, and the residual alkali content after washing is controlled to be less than 1.0% (calculated as Na2O).

[0052] 5) The washed mixed white mud enters the white mud storage tank 12 and is then sent to the white mud filter 13 for filtration. The filtered white mud can be used as a raw material for the lime kiln 14. The dryness of the filtered white mud is controlled at 70%-80%, and the residual alkali content is less than 0.3% (in terms of Na2O).

[0053] The filtered white mud was compared with the waste white mud, and the results are shown in Table 1 below.

[0054] Table 1 Comparison of waste white mud and filtered white mud

[0055] <![CDATA[CaCO3 content]]> CaO content Dryness Residual alkali content Waste white mud >90％ ＜0.5％ >80％ ＜2.0％ Filtered white mud >90％ ＜0.5％ 70％-80％ Less than 0.3%

[0056] As can be seen from Table 1, the dryness of the waste white mud after treatment is controlled at 70%-80%, and the residual alkali content is less than 0.3% (calculated as Na2O), which can be used as a raw material for a lime kiln and meet the requirements of the raw materials for the lime kiln. The recovery rate of the waste white mud using the method of the present invention is 100%.

[0057] Comparative Example 1

[0058] The difference of Example 2 is that no flushing device is provided, and hot water is directly added. The results show that a large amount of water is required, the density of the treated white mud is too low, and the recovery rate of the white mud is slow.

[0059] Comparative Example 2

[0060] The difference of Example 2 is that no grid is provided. The results show that there are foreign matters remaining, and the density of the treated white mud is uneven, which easily causes the equipment and pipelines to be blocked and cannot be used normally.

[0061] Comparative Example 3

[0062] The difference of Example 2 is that the hot water temperature is 90° C. The results show that the energy consumption is too large, and a large amount of dust and water vapor are generated on site, the environmental pollution is serious, and the white mud treatment effect is poor.

[0063] Comparative Example 4

[0064] The difference of Example 2 is that the hot water temperature is 60° C. or below. The results show that the recycled white mud enters the back-end filtration system, resulting in reduced filter processing capacity and reduced white mud dryness, reduced white mud processing efficiency, and poor white mud dryness.

[0065] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A pulp mill lime kiln raw material recovery process, characterized in that: The following steps are involved: (1) selecting waste white mud, feeding the waste white mud into a recovery tank through a grid net, adding 70-85° C. hot water to dilute and mix the materials, and adding hot water from all sides in a spraying manner; assisting stirring during the mixing process, the stirring speed is 20-60 revolutions per minute, and the retention time of the white mud in the recovery tank is controlled to be less than 15 minutes, so as to obtain a recovered white mud emulsion; (2) mixing the recovered white mud emulsion with the white mud produced in the causticizing production at a ratio of less than 30% and washing; (3) The washed white mud is filtered, and the filtered white mud is the raw material of the lime kiln.

2. The pulp mill lime kiln raw material recovery process according to claim 1, characterized in that: The thickness of the grille mesh is 15-25 mm, and the hole specifications of the grille mesh are 35-45 mm*35-45 mm.

3. The pulp mill lime kiln raw material recovery process according to claim 1, characterized in that: Step (1), the concentration of the recovered white mud emulsion is controlled at 25%-35%wt, or the density is controlled at 1.25-1.35t / m 3 .

4. The pulp mill lime kiln raw material recovery process according to claim 1, characterized in that: In step (2), the residual alkali content after washing is less than 1.0% based on Na2O.

5. The pulp mill lime kiln raw material recovery process according to claim 1, characterized in that: Step (3), the dryness of the filtered white mud is controlled at 70%-80%, and the residual alkali content calculated as Na2O is less than 0.3%.

6. The pulp mill lime kiln raw material recovery process according to claim 1, characterized in that: An efficient white mud recovery system is used, including a feed port, a recovery tank, a grille, a flushing device, and an agitator; the feed port is located above the recovery tank, the grille is located between the recovery tank and the feed port, and the flushing device is located above the grille; the stirring blades of the agitator are located in the recovery tank; waste white mud is fed from the feed port through the grille into the recovery tank, and hot water enters the recovery tank through the grille in a spraying manner from all sides.

7. The pulp mill lime kiln raw material recovery process according to claim 6, characterized in that: The high-efficiency white mud recovery system also includes a liquid level meter, a density meter, a transmission pump, a control valve, and a transmission pipeline; The flushing device is located at the feeding port, and includes an upper hot water flushing device and a lower spray flushing device. The upper hot water flushing device is provided with a plurality of flushing nozzles, and the nozzles of the flushing nozzles are flat; the lower spray flushing device is provided with a plurality of flushing nozzles, and the plurality of flushing nozzles are connected to form a rectangle through water pipes; The stirring blades of the stirrer are located in the recovery tank; The liquid level meter and density meter are located in the recovery tank; The bottom of the recovery tank is connected to the transmission pump through a delivery pipeline.

8. A pulp mill lime kiln raw material recovery device, characterized in that: A high-efficiency white mud recovery system is provided, which includes a feed port, a recovery tank, a grille, a flushing device, an agitator, a liquid level meter, a density meter, a transmission pump and a conveying pipeline; the feed port is located above the recovery tank, the grille is located between the recovery tank and the feed port, the flushing device is located at the feed port, the flushing device includes an upper hot water flushing device and a lower spray flushing device, the upper hot water flushing device is provided with a plurality of flushing nozzles, and the nozzles of the flushing nozzles are flat; the lower spray flushing device is provided with a plurality of flushing nozzles, and the plurality of flushing nozzles are connected by water pipes to form a rectangular arrangement; the stirring blades of the agitator are located in the recovery tank; the liquid level meter and the density meter are located in the recovery tank; the bottom of the recovery tank is connected to the transmission pump through a conveying pipeline.

9. A pulp mill lime kiln raw material recovery device according to claim 8, characterized in that: The thickness of the grille mesh is 15-25 mm, and the hole specifications of the grille mesh are 35-45 mm*35-45 mm.

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