JP2005296866A - Method for making cake containing iron from waste liquid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for making a cake having a low water content by treating a waste liquid containing a plurality of metals to make slurry containing iron whose filterability and drainability are improved, and dewatering the same. <P>SOLUTION: The method is characterized by supplying an acidic waste water containing a plurality of metal ions in addition to a divalent iron ion and a trivalent iron ion to a first reactor, oxidizing the divalent iron ion to the trivalent iron ion by biological oxidation means or chemical oxidation means, also adjusting the pH of the waste liquid at ≥3 and ≤5 by adding a neutralizer, preparing a particle composed mainly of iron hydroxide (III), supplying a part of slurry of the waste liquid containing the particle to a second reactor, adjusting the pH of the slurry in the second reactor at ≥5 and ≤13 by adding an alkali agent to return to the first reactor after each particle is flocculated, carrying out solid-liquid separation of the slurry containing the flocculated particle to concentrate, and thereafter dewatering the concentrated slurry to make a cake. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for recovering iron in a waste liquid by treating a waste liquid containing a plurality of metals such as pickling waste liquid and plating waste liquid of a steel sheet or a metal mine drainage containing acid and heavy metals.

  Conventionally, when surface treatment or the like is performed on the surface of a steel plate, pickling or alkali treatment is performed as a pretreatment to remove scale generated on the surface of the steel plate.

  In this pickling treatment, an acidic liquid containing sulfuric acid, nitric acid, nitric hydrofluoric acid, etc. is used. In the case of a steel sheet, an acidic waste liquid in which a metal such as iron or nickel in the steel sheet is dissolved is generated. In some cases, waste liquid is generated in which metals such as nickel and zinc contained in the plating solution and iron in the base material are dissolved.

  This waste liquid is neutralized by adding an acid or alkali solution, and then added with a flocculant to precipitate a metal hydroxide using a sedimentation basin such as a thickener to form a slurry, which is dehydrated. And then processed into sludge cake. Therefore, the sludge cake contains metal hydroxides such as nickel and zinc in addition to iron, which is a component problem for recycling as an iron source. In addition, since these metal hydroxides have small particles, the moisture content of the sludge cake is high, and in order to recycle as an iron source, the drying cost of the sludge cake becomes high. Furthermore, since the moisture content of the sludge cake is high, it is transported in a state containing a large amount of water, which causes problems such as an increase in transportation costs.

  On the other hand, in the following Patent Document 1, the present inventor performs the pH adjustment by dividing the neutralization treatment of the waste liquid into a plurality of stages, and deposits the metal hydroxide for each metal type, thereby forming the metal hydroxide. We have proposed a waste liquid treatment method that easily separates and collects metal by metal type. In addition, by further stirring the waste liquid after neutralization treatment, the precipitated metal hydroxide particles are brought into contact with each other to increase the particle size, thereby increasing the efficiency of dehydration and making sludge with a low moisture content. Proposes a processing method for use.

  Also, since metal mines mainly mine sulfide minerals, minerals such as pyrite, chalcopyrite, and sphalerite remain after mining, and these react with oxygen in groundwater and air to produce acidic and heavy metal-containing mines. Wastewater is generated. This mine drainage is neutralized by a neutralizing agent such as slaked lime, and then subjected to solid-liquid separation in a sedimentation basin. The supernatant water is discharged and the sediment is dehydrated and discarded. Although this starch contains a large amount of iron, it contains impurities such as arsenic, copper, and zinc and has a high water content of 60 to 80%, so it cannot be used as an iron source in the steel industry.

  However, in the method described in Patent Document 1 below, when the suspended matter in pickling / plating waste liquid or metal mine wastewater largely fluctuates due to the inflow of rainwater or the composition fluctuation or pH fluctuation of the waste liquid, It has been found that when the generated sludge is charged, the number of particles in the reaction vessel is remarkably increased and there are too many core particles to increase the particle size of the metal hydroxide.

JP 2003-7201 A

  In the present invention, an acidic waste liquid containing iron ions and other metal ions is supplied to the first reaction tank continuously in the first reaction tank and neutralized, In a method for treating a waste liquid containing a plurality of metals, the pH of the waste liquid is adjusted to 3 to 5 to produce particles mainly composed of iron (III) hydroxide, the particles are separated from the liquid, and the liquid part is discharged The purpose is to produce an iron-containing slurry with improved filterability and dewaterability and to dehydrate it to produce a dehydrated cake with a low water content.

  In order to solve the above-mentioned object, the present inventor continuously supplies an acidic waste liquid containing iron ions and other metal ions in the first reaction tank in the first reaction tank. And a neutralization treatment to adjust the pH of the waste liquid to 3 or more and 5 or less to generate iron (III) hydroxide-based particles, separate the particles from the liquid, and discharge the liquid part. As a result of earnest examination in the waste liquid treatment method containing iron, a slurry containing iron (III) -based particles precipitated in the first reaction tank is fed to the second reaction tank, and iron hydroxide (III) The pH is temporarily adjusted to a pH region where the main particles are aggregated, the particles are aggregated more and larger in diameter, and then the larger aggregated particles are returned to the first reaction tank. The iron hydroxide (III) -based compound in the first reaction tank is deposited on the aggregated particles, which are integrated and refined. It has been found that the filterability and dewaterability can be greatly improved by densification.

  In addition, by adding a negatively charged fine powder or mud-like substance to the first reaction tank, the agglomeration action is increased and the diameter is increased by agglomeration by electrical attraction, and then metal ions are contained. While the waste liquid is continuously supplied into the container, neutralization is performed, and the iron (III) -based compound in the first reaction tank is precipitated on the agglomerated particles that have become larger in diameter and integrated and dense. It was also discovered that filterability and dewaterability can be greatly improved.

  That is, the present invention has been made based on the above findings, and the gist thereof is as follows.

(1) An acidic waste liquid containing a plurality of metal ions in addition to divalent iron ions and trivalent iron ions is supplied into the first reaction tank, and divalent iron is obtained by biological oxidation means or chemical oxidation means. While oxidizing the ions to trivalent iron ions, adding a neutralizing agent to adjust the pH of the waste liquid to 3 or more and 5 or less to produce iron (III) -based particles, A part of the slurry is supplied to the second reaction tank, an alkaline agent is added to adjust the pH of the slurry in the second reaction tank to 5 or more and 13 or less, and the particles are aggregated. The slurry containing the agglomerated particles is solid-liquid separated and concentrated, and then the concentrated slurry is dehydrated into a cake, and then a method for producing an iron-containing dehydrated cake from waste liquid .

(2) An acidic waste liquid containing a plurality of metal ions in addition to divalent iron ions and trivalent iron ions is supplied into the first reaction tank, and divalent iron is obtained by biological oxidation means or chemical oxidation means. Ions are oxidized to trivalent iron ions, and a neutralizer is added to adjust the pH of the waste liquid to 3 or more and 5 or less to produce iron hydroxide (III) -based particles, which are further negatively charged. A fine powder or mud substance is charged, the particles and the negatively charged fine powder or mud substance are aggregated, and the slurry containing the aggregated particles is solid-liquid separated and concentrated, and then the concentrated slurry. A method for producing an iron-containing dehydrated cake from waste liquid, characterized in that the cake is dehydrated into a cake.

(3) The method according to (1) above, wherein a negatively charged fine powder or mud substance is further charged into the first reaction vessel.

(4) As the biological oxidation means, iron-oxidizing bacteria are added into the first reaction tank, air or oxygen is blown into the waste liquid, and the waste liquid is stirred (1) to (3) ) Any one of the methods.

(5) The plurality of metal ions are one or more of nickel ions, zinc ions, and copper ions, and the slurry is subjected to solid-liquid separation by the method according to any one of (1) to (4). After the concentration, the discharged processing liquid is supplied to the third reaction tank, and the processing liquid in the third reaction tank is stirred to adjust the pH of the processing liquid to 6 or more and 10 or less. Then, one or more metal hydroxide particles of nickel, zinc and copper are produced, and the slurry containing the metal hydroxide particles is solid-liquid separated and concentrated, and then dehydrated and cake A method for producing an iron-containing dehydrated cake from waste liquid.

(6) The negatively charged substance is activated sludge generated when organic wastewater is treated by a biological treatment method, as described in any one of (2) to (5) above Method.

(7) The negatively charged substance is charged in an amount of 0.2% by mass or more to 2% by mass with respect to the mass generated per unit time of iron (III) -based particles precipitated in the first reaction tank. % Or less (dry conversion), The method in any one of said (2)-(6) characterized by the above-mentioned.

(8) In any one of (1) to (7), a part of the slurry in the first reaction tank and / or the third reaction tank is solid-liquid separated by a separation membrane. The method described.

(9) The method according to (8), wherein the pore size of the separation membrane is 1 μm or more and 100 μm or less.

(10) The separation membrane is supplemented with the particles and the metal hydroxide particles on the separation membrane in the first reaction tank and / or on the separation membrane in the third reaction tank, respectively. The method according to (8) or (9) above, wherein the thickness of the cake layer formed thereon is controlled to 1 mm to 50 mm.

(11) The suspension concentration in the acidic waste liquid is 1/3 or less (mass concentration) of the soluble iron ion concentration in the acidic waste liquid. The method described.

  By making the particles finer and finer, the filterability and dewaterability of the slurry containing them can be improved, and this dehydrated cake mainly composed of iron hydroxide (III) can be used as an iron source in the steel industry. Can be used effectively.

The present invention will be described in detail below.
In the method for producing an iron-containing dehydrated cake from the waste liquid according to the present invention, a waste liquid containing iron ions and other metal ions is used as a treatment target.

  First, the pH of a solution containing 400 mg / L, 400 mg / L, 30 mg / L, 30 mg / L and 30 mg / L of divalent iron ion, trivalent iron ion, nickel ion, zinc ion and copper ion, respectively, and FIG. 1 shows the relationship of the residual ratio of elements in the wastewater.

  From this figure, trivalent iron ions are precipitated at pH 3-5, and zinc ions, nickel ions, divalent iron ions and copper ions are precipitated at pH 6-9.

  In order to separate zinc ions, nickel ions and copper ions from iron ions, first, divalent iron ions are oxidized to trivalent iron ions by chemical oxidation or biological oxidation, and then the pH is adjusted to 3 to 3. By adjusting to 5, iron (III) hydroxide precipitates. The solid part and the liquid part are separated from the slurry by solid-liquid separation operation, and the solid part is recovered. Then, zinc hydroxide, nickel hydroxide, and copper hydroxide precipitate by making pH of a liquid part into 6-10. Only the liquid part is separated from the slurry by solid-liquid separation, and the solid part is recovered. In this way, iron ions can be separated from nickel ions, zinc ions, and copper ions.

  In other words, in the process of precipitating the metal compound, the element separation can be performed by controlling the pH to several stages and extracting the metal compound precipitated at each stage.

  On the other hand, FIG. 2 shows the charged state of precipitated iron hydroxide (III) with respect to pH. It can be seen that in the acidic region, it is slightly weakly charged, in the alkaline region, it is slightly weakly charged, and in the vicinity of neutrality it is not charged.

  Whether or not the particles of iron hydroxide (III) agglomerate is determined by the sum of the electric attractive force or electric repulsive force depending on the surface charging condition and the attractive force due to van der Waals force. Actually, particles made of iron (III) hydroxide aggregate at pH 5.0 or higher and pH 13.0 or lower, preferably aggregate at pH 6.0 or higher and pH 12.0 or lower. In other words, the pH range for precipitating iron and separating it from other elements does not match the pH range for agglomeration. To achieve particle size increase by element separation and agglomeration, the pH should be adjusted in two steps. It is important to control.

  In addition, by utilizing the fact that particles of iron hydroxide (III) are weakly charged positively in the acidic region, by adding a negatively charged substance, the particles are aggregated by electrical attraction and element separation And increase in particle diameter by aggregation can be achieved.

  Therefore, the acidic waste liquid containing iron ions and other metal ions is continuously supplied into the first reaction tank in the first reaction tank, and neutralization treatment is performed. In the method for treating a waste liquid containing a plurality of metals, the pH is adjusted to 3 or more and 5 or less to produce iron (III) -based particles, the particles are separated from the liquid, and the liquid part is discharged. A part of the slurry containing iron (III) -based particles generated in the reaction tank is continuously supplied to the second reaction tank, and the pH of the slurry in the second reaction tank is 5 or more and 13 or less, Preferably, the pH is adjusted to 6 or more and 12 or less, and the particles mainly composed of iron (III) hydroxide are aggregated, and then returned to the first reaction tank, whereby iron hydroxide (III ) By precipitating the main substance, the aggregated particles are integrated and the particle size is increased. It can be made into fine particles and the filterability and dewaterability are improved.

  Further, a fine powder or a muddy substance whose surface is negatively charged is put into the first reaction tank, and agglomerated with iron (III) -based particles, and iron (III) hydroxide is aggregated on the surface of the aggregated particles. By precipitating the main substance, the aggregated particles can be integrated into fine particles having a large particle size, and the filterability and dewaterability can be improved. When the negatively charged fine powder or mud substance is charged, it is possible to omit the second reaction tank and the treatment there.

  As the negatively charged fine powder or mud substance, pulverized coal, activated carbon, biomass and the like are suitable. Biomass includes activated food sludge generated when biologically treating food residues and organic wastewater discharged in the food processing process. The charged amount of the negatively charged fine powder or mud substance is 2% by mass or less (dry matter) with respect to the mass generated per unit time of the iron (III) hydroxide-based particles precipitated in the first reaction tank. Conversion), preferably 1% by mass or less. When the amount exceeds 2% by mass, the amount of negatively charged particles increases, and the entire surface of the negatively charged particles cannot be covered with iron hydroxide (III) -based particles, and is composed of iron (III) hydroxide. There is a space between the particles, which makes it difficult to form dense particles. On the other hand, if it is less than 0.2% by mass, the number of negatively charged particles is too small, and iron (III) -based particles that remain unaggregated remain, which is not good. The mass of iron hydroxide (III) -based particles generated per unit time depends on the amount of iron hydroxide (III) contained in the acidic waste liquid and divalent iron ions in the acidic waste liquid in the first reaction tank. The sum of the amount of iron hydroxide (III) that has been oxidized to trivalent iron ions and then precipitated, and the amount of iron hydroxide (III) in which the trivalent iron ions in the acidic waste liquid have precipitated in the first reactor is Now, iron hydroxide (III) generated per unit time can be calculated from the total iron concentration in the acidic waste liquid and the input amount of the acidic waste liquid.

  Further, the diameter of the negatively charged particles is preferably 200 μm or less. If it is larger than 200 μm, the entire surface of the negatively charged particles cannot be covered with particles mainly composed of iron (III) hydroxide, and there is a space between particles made of iron (III) hydroxide, making it difficult to form dense particles. Become.

  The suspension concentration (SS) in the acidic waste liquid is preferably 1/3 or less (mass concentration) of the soluble iron ion concentration in the acidic waste liquid. If the suspended substance concentration is greater than one-third (mass concentration) of the soluble iron ion concentration, the amount of iron ions that precipitate on the aggregate surface and integrate and densify the aggregate is increased in the first reaction vessel. The number of fine particles supplied increases, making it difficult to integrate. In addition, the number of primary nuclei is excessive and the particle size is difficult to increase. The soluble iron ion concentration refers to iron ions in a dissolved state and refers to the sum of divalent iron ion concentration and trivalent iron ion concentration.

  As a method of reducing the suspension concentration in the acidic waste liquid to 1/3 or less (mass concentration) of the iron ion concentration, by adding acid to the acidic waste liquid and lowering the pH, the metal component is dissolved in acid and suspended. The substance concentration can be reduced. By adjusting the pH to 2 or less, many metal components are dissolved. In addition, when there are three or more types of acidic waste liquid and the pH of one or more of the waste water is 2 or less, the suspended water concentration is lowered by mixing waste water with a pH of 2 or less into waste water having a high suspended solid concentration, Without mixing with the waste water, it is possible to reduce the concentration of suspended solids in the acidic waste water by directly feeding it into the first reaction tank.

  The iron (III) hydroxide-based particles that precipitate in the first reaction tank contain a minute amount of nickel, zinc, and copper that cannot be separated from the waste liquid, and are contained in the waste water. Contains trace amounts of arsenic and chromium.

Next, an embodiment embodying the present invention will be described.
In FIG. 3, an example of the processing apparatus which concerns on the manufacturing method of the iron containing dehydrated cake from the waste liquid of this invention is shown.
As shown in FIG. 3, the waste water treatment apparatus 10 applied to the iron-containing slurry reforming method according to the embodiment of the present invention includes, for example, a thin steel plate made of an acid solution such as sulfuric acid, nitric acid, or nitric hydrofluoric acid. Pickling wastewater that is an example of waste liquid after pickling treatment, pickling plating wastewater consisting of plating waste liquid after surface treatment, or acid and heavy metal-containing mine drainage generated after mining a metal mine, etc. The drainage tank 12 for temporarily storing the wastewater 11, the first reaction tank 16 for receiving the drainage 11 from the drainage tank 12 via the pump 13, and the neutralization treatment, and the slurry in the first reaction tank 16 are the second To the reaction tank 17, a pump 18 for feeding the slurry, an alkaline liquid tank 14 containing a neutralizing agent such as caustic soda, calcium carbonate, slaked lime or magnesium hydroxide, and an alkaline liquid are added to the second reaction tank 17. Pump 15, agitator 19 for agitating the inside of the second reaction tank 17, a pipe for introducing the slurry overflowed from the second reaction tank 17, a hopper 20 for temporarily storing a negatively charged substance, A cutting device 21 that cuts out a small amount from the hopper 20 and a pump 27 that sucks and pumps waste liquid after being processed in the first reaction tank 16 provided on the outlet side of the first reaction tank 16. ing.

  A pH meter 22 that measures the pH in the reaction tank 16 and a filter body 23 that is an example of a separation membrane that filters excess water in the first reaction tank 16 are attached to the first reaction tank 16. . The filter body 23 uses a filter cloth made of a material such as polypropylene, polyester, polyvinylidene chloride, polyacryl, polyvinyl alcohol, etc., has a pore diameter of 1 to 100 μm, and sucks treated waste water into the filter body 23. The pump 27 is in communication. When the pore diameter is smaller than 1 μm, fine particles mainly composed of iron hydroxide are clogged and the filtration rate is remarkably reduced. On the other hand, when the pore diameter is larger than 100 μm, the number of particles that pass through the pores of the separation membrane increases, and the recovery efficiency of particles mainly composed of iron (III) hydroxide decreases. Further, a compressed gas header 25 is attached to which gas is blown from a large number of blowing holes 25a connected to a supply pipe 24 communicated with a compressed gas source (not shown), and the waste water in the first reaction tank 16 is stirred and circulated. . The compressed gas is preferably air or oxygen.

  Further, in the first reaction tank 16, there is a scraper support structure 28 to which a scraper 29 is fixed in order to make the cake thickness of the filter body constant. A connector 31 is provided for connecting 30 and the scraper support structure 28. The length of the scraper, the number of scrapers, and the number of peeling of the scrapers are preferably designed so that the following formula is established.

Filtration area (m ² ) ÷ (Scraper length (m / piece) × Scraper number) ÷ Scraper peeling frequency (times / min) <0.3

  However, the filtration area is constant.

  If it is larger than this value, the cake thickness becomes large and a high filtration rate cannot be obtained.

  That is, the scraper 29 always scrapes the cake layer surface deposited on the surface of the filter body 23 to make the cake thickness constant, and the circulated flow causes the scraped cake to be discharged from the space between the filter bodies 23. A filtration rate can be obtained. Further, as the time elapses, the cake layer formed on the surface of the filter body 23 becomes dense and the filtration performance deteriorates. Therefore, the washing water is supplied into the filter body 23 by the backwash pump 32 at regular intervals. It press-fits and peels the cake deposited on the surface of the filter body 23. At the bottom of the first reaction tank 16, there is provided a slurry discharge pipe 26 for discharging a slurry containing a hydroxide such as precipitated iron hydroxide (III).

  The third reaction tank 40 includes an alkaline liquid tank 41 containing a neutralizing agent such as caustic soda, calcium carbonate, slaked lime, or magnesium hydroxide, and a pump 42 for charging the alkaline liquid into the third reaction tank 40. Furthermore, a pH meter 46 for measuring the pH of the waste liquid and a filter body 47 as an example of a separation membrane for filtering excess water in the third reaction tank 40 are attached. The filter body 47 uses a filter cloth made of materials such as polypropylene, polyester, polyvinylidene chloride, polyacryl, polyvinyl alcohol, etc., has a pore diameter of 1 to 100 μm, and sucks waste liquid after treatment into the filter body 47. The pump 48 is in communication. When the pore diameter is smaller than 1 μm, fine particles of metal hydroxide are clogged and the filtration rate is remarkably reduced. On the other hand, when the pore diameter is larger than 100 μm, the number of particles that pass through the pores of the separation membrane increases, and the metal hydroxide recovery efficiency decreases. Further, a compressed gas header 49 is attached for blowing gas from a number of blowing holes 49a connected to a supply pipe 24 communicated with a compressed gas source (not shown) and stirring the waste liquid in the third reaction tank 40. The compressed gas is preferably air or oxygen.

  Further, in the third reaction tank 40, there is a scraper support structure 51 to which a scraper 52 is fixed in order to make the cake thickness of the filter body constant, a driving device 53 for operating the scraper up and down, and a driving device. A connector 54 for connecting 53 and the scraper support structure 51 is provided. It is preferable to design the scraper length and the number of times the scraper is peeled off so that the following formula is established.

Filtration area (m ² ) ÷ (Scraper length (m / piece) × Scraper number) ÷ Scraper peeling frequency (times / min) <0.3

  However, the filtration area is constant.

  When it is larger than this value, the cake thickness becomes large and a high filtration rate cannot be obtained.

  In other words, the scraper 52 always scrapes the cake layer surface deposited on the surface of the filter body 47 to make the cake thickness constant, and the circulated flow always discharges the scraped cake from the space between the filter bodies 47. A high filtration rate can be obtained. Moreover, since the cake layer formed on the surface of the filter body 47 becomes dense as time elapses, washing water is press-fitted into the filter body 47 with a backwash pump 55 at regular intervals. The cake deposited on the surface is peeled off. At the bottom of the third reaction tank 40, a slurry discharge pipe 50 for discharging the deposited metal hydroxide slurry is provided.

  Next, based on FIG. 3, one Embodiment of the manufacturing method of the iron containing dewatering cake which is this invention is described.

The pump 13 connected to the drainage tank 12 storing the pickling waste liquid or the plating waste liquid after pickling treatment of the steel sheet or the mine drainage 11 is operated, and the drainage 11 is subjected to the first reaction. It is preferable to supply continuously into the tank 16 at 6 to 3,000 m ³ / h. In the case of 6 m ³ / h or less, the target water amount is small, and the merit for the investment amount for carrying out the present invention is small and not economical. On the other hand, in the case of 3,000 m ³ / h or more, the solid-liquid separator becomes too large, and it is difficult to maintain a stable operation of the solid-liquid separator. The general waste water 11 has a pH of 2 or less and dissolves metals such as Fe, Zn and Ni in a total amount of 0.01 to 1% by mass, and most of these metals exist as metal ions. Small amounts are present as suspended particles.

The waste water 11 is continuously supplied into the first reaction tank 16 and stirred in the first reaction tank. While part of the liquid in the first reaction tank 16 is sent to the second reaction tank 17 by the pump 18, the pump 15 connected to the alkaline liquid tank 14 is operated therein, and NaOH, Ca (OH ) ₂ or an alkaline solution composed of Mg (OH) ₂ solution is added, stirred with a stirrer 19, and then charged from the second reaction tank 17 to the first reaction tank 16. The pH in the first reaction tank 16 is adjusted to a range of 3 to 5. Here, when the pH is 3 or less, the precipitation of trivalent iron ions in the acidic plating waste liquid becomes worse. On the other hand, when the pH is 5 or more, metal hydroxides such as nickel and zinc are likely to precipitate, and it becomes difficult to separate the iron metal hydroxide. That is, by the first neutralization treatment, as shown in FIG. 1, particles mainly composed of iron (III) hydroxide are precipitated, and trivalent iron ions can be separated from the waste water 11.

Air is blown into the first reaction tank 16 from the blowing holes 25 a of the compressed gas header 25, and the inside of the first reaction tank 16 is agitated. Although the particle size of the particles mainly composed of iron hydroxide (III) just precipitated in the first reaction tank 16 is small, it is mainly composed of iron hydroxide (III) by stirring and continuous supply of waste water. The iron ions contained in the waste water come into contact with the particles, so that iron (III) hydroxide can be further precipitated on the surface of the particles, or particles mainly composed of iron hydroxide (III) can be bonded to each other. The diameter can be increased to some extent. The amount of air or oxygen supplied from the blowout hole 25a of the compressed gas header 25 is 0.003 to 0.2 Nm ³ / m ³ / min per unit volume of the reaction tank, although it depends on the size of the reaction tank. preferable. If it is 0.003 Nm ³ / m ³ / min or less, the precipitated iron hydroxide (III) particles settle and deposit on the bottom of the reaction tank, and if it is 0.2 Nm ³ / m ³ / min or more, power is applied to the blast. Not too economical. Moreover, in order to control the thickness of the cake layer deposited on the filter body surface, the cake thickness is controlled to 1 to 50 mm by moving the scraper 29 up and down once every 30 seconds. The thinner the cake thickness is, the larger the filtration rate is. However, for example, when the cake thickness is 0 mm, the quality of the treated water is deteriorated, and the cake thickness of 1 mm or more is necessary for controlling the cake thickness. Further, when the cake thickness is 50 mm or more, the filtration rate is lowered, and at the same time, there is a problem that the apparatus becomes large in order to secure the cake thickness. Therefore, the cake thickness is desirably 1 to 50 mm.

  Further, in order to increase the particle diameter, a part of the slurry is drawn out from the first reaction tank 16 by the pump 18 and charged into the second reaction tank 17. The alkaline liquid 14 is charged into the second reaction tank 17, and the pH in the second reaction tank 17 varies from 5 to 13. In this pH region, particles mainly composed of iron (III) hydroxide aggregate to form coarse particles. However, the shape is unstable and cannot be maintained only by the agglomeration action, but the iron hydroxide (III) precipitated in the first reaction tank 16 is returned to the first reaction tank 16 in the agglomerated state. By precipitating on the surface of the particle group in an agglomerated state, strong bonding occurs between the particles, and dense particles are formed. The amount of slurry drawn by the pump 18 is preferably 1/500 or less of the capacity of the first reaction tank 16 per minute. The pH in the second reaction tank is 5 to 13, which is outside the active region of iron-oxidizing bacteria to be described later, and when the withdrawal amount is large, the iron oxidation in the first reaction tank when returned to the first reaction tank. Since the activity of bacteria decreases, the number of bacteria does not increase easily, and it is difficult to maintain the oxidation ability of divalent iron ions, the amount of slurry fed into the second reaction tank is 1 minute per minute. It is preferable to set it to 1/500 or less of the capacity of the tank 16.

  In order to further increase the particle size, a small amount of finely charged fine powder or mud substance is cut out from the hopper 20 by the cutting device 21 and put into the first reaction tank. The particles mainly composed of iron hydroxide (III) deposited in the first reaction tank are positively charged in the acidic region, and therefore, the negatively charged fine powder or mud substance and van derle introduced from the hopper 20 Agglomeration and particle size increase due to Waals force and electrical attraction. However, the shape is unstable and cannot be maintained only by the aggregating action, but the iron hydroxide (III) precipitated in the first reaction tank 16 is precipitated on the surface of the particles in the aggregated state. Strong bonds occur between the particles, and dense particles are formed.

  Further, in the process of increasing the particle size and densifying, it is possible to omit the process in the second reaction tank 17 and perform only the process of charging a negatively charged substance into the first reaction tank 16. is there.

  In the first reaction tank 16, iron-oxidizing bacteria are added to oxidize divalent iron ions of iron ions to trivalent iron ions, thereby facilitating generation of iron hydroxide (III). This reaction is preferably at a pH of 2 or more and 5 or less where the reaction activity of iron-oxidizing bacteria is high. The precipitated iron hydroxide (III) -based particles act as a carrier for iron-oxidizing bacteria, so that the iron-oxidizing bacteria can stay in the first reaction tank 16 for a long time, and 3 of divalent iron ions. The oxidation reaction to valent iron ions can be carried out stably and continuously. As the iron-oxidizing bacteria, filamentous bacteria that grow in the neutral region, filamentous bacteria that grow in the acidic region, and non-filamentous bacteria can be used. For example, Thiobacillus ferrosioxydans, which is a non-filamentous bacterium that grows in a commonly used acidic region and is a chemical autotrophic bacterium, can be used.

  Alternatively, an oxidant such as ozone gas, hydrogen peroxide, or chlorine gas may be introduced into the first reaction tank 16 to oxidize divalent iron ions to trivalent iron ions.

  A suction pump 27 is arranged on the outlet side of the first reaction tank 16, and the suction pump 27 is operated to suck the inside of the filter body 23 to a negative pressure to separate the waste liquid into solid and liquid, and metal ions such as nickel and zinc Is continuously supplied to the third reaction tank 40. The slurry in the first reaction tank 16 after discharge of the processing liquid is concentrated to 3 to 50% as a solid content, and is discharged from the slurry discharge pipe 26.

Next, the acid plating waste liquid treated in the first reaction tank 16 is continuously supplied into the third reaction tank 40 and communicated with the alkaline liquid tank 41 as the second neutralization treatment. The pump 42 is actuated to add an alkaline solution made of NaOH or Ca (OH) ₂ solution to adjust the pH of the acidic plating waste solution to a range of 6 or more and 10 or less. By this neutralization treatment, a metal hydroxide mainly composed of nickel, zinc or the like is deposited. As shown in FIG. 1, by setting the pH to 6 or more, nickel, zinc and remaining divalent iron ions are precipitated, and almost all can be deposited at a pH of 10 or more.

  Air is blown into the third reaction tank 40 from the blowing hole 49a of the compressed gas header 49, and the waste liquid in the third reaction tank 40 is stirred. The just-deposited metal hydroxide has small particles, but the metal hydroxide contained in the waste liquid comes into contact with the newly-deposited metal hydroxide by stirring and continuous supply of the acid plating waste liquid. Metal ions can be further deposited on the surface of the object, and metal hydroxide particles can be bonded to each other, and the particle size of the metal hydroxide can be increased.

The amount of air or oxygen supplied from the blowout hole 49a of the compressed gas header 49 is 0.003 to 0.2 Nm ³ / m ³ / min per unit volume of the reaction tank, although it depends on the size of the reaction tank. preferable.

If it is 0.003 Nm ³ / m ³ / min or less, the precipitated metal hydroxide particles will settle and accumulate on the bottom of the reaction tank, and if it is 0.2 Nm ³ / m ³ / min or more, it will take too much power to blow the economy. Not right. Moreover, in order to control the thickness of the cake layer deposited on the filter body surface, the cake thickness is controlled to about 1 to 50 mm by moving the scraper 29 up and down once every 30 seconds. The thinner the cake thickness is, the larger the filtration rate is. However, for example, when the cake thickness is 0 mm, the quality of the treated water is deteriorated, and the cake thickness of 1 mm or more is necessary for controlling the cake thickness. Further, when the cake thickness is 50 mm or more, the filtration rate is lowered, and at the same time, there is a problem that the apparatus becomes large in order to secure the cake thickness. Therefore, the cake thickness is desirably 1 to 50 mm.

  A suction pump 48 is disposed on the outlet side of the third reaction tank 40, and the suction pump 48 is operated to suck the inside of the filter body 47 to a negative pressure to continuously filter the waste liquid. The slurry in the third reaction tank after the filtration waste liquid is discharged can be 1 to 10% as a solid content, and is discharged from the slurry discharge pipe 50.

  The slurry discharged from the first and third reaction tanks is dehydrated and processed into a sludge cake. Examples of the dehydration method include a method of dehydrating using a dehydrator such as a filter press, a vacuum dehydrator, and a centrifugal dehydrator, and a method of dehydrating by sun drying. Each dehydrated sludge cake can be effectively used as a resource. Although the water content after dehydration varies greatly depending on the dehydration method, when dehydrated with a filter press, the iron (III) hydroxide-based dehydrated cake recovered from the first reaction tank has a water content of 20 to 40%, The dehydrated cake made of metal hydroxide recovered from the third reaction tank has a moisture content of 65 to 85%. The large difference in moisture occurs because the metal hydroxide recovered from the third reaction tank contains crystal water.

  Although the embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and all changes in conditions and the like that do not depart from the gist are within the scope of the present invention.

  For example, in FIG. 3, the pH meter 22 is installed in the first reaction tank 16 to control the amount of alkaline solution input, but the pH meter is also installed in the reaction tank 17 of the younger brother 2 It is also within the scope of the present invention to control the pH in the reaction vessel to fall within the range of 5-13.

  Moreover, although cake filtration was shown as an example of a solid-liquid separator, you may carry out with a precipitation system or a microfiltration system.

  Moreover, although the flat filter body is used as a filter body which performs cake filtration, you may use a rotary disk-shaped filter body.

Next, although the Example of the processing method of the acidic waste liquid containing the some metal of this invention is given, this invention is not limited to a following example.
In addition, a present Example is an Example conducted by the indoor experiment, taking a correlation with actual wastewater, and this experimental result is a desirable result on sludge recycling, and is considered to be no different from actual wastewater.

Example 1
As a pickling plating simulated waste liquid, a solution containing divalent iron ions, trivalent iron ions, nickel ions, zinc ions, and suspended solids (SS) shown in Table 1 is used. The pickling plating simulated waste liquid was treated. The capacities of the first reaction tank, the second reaction tank, and the third reaction tank are 3L, 0.05L, and 3L, respectively. Note that iron-oxidizing bacteria are previously introduced into the first reaction tank.

  While the pickling plating simulated waste liquid is continuously supplied to the first reaction tank 16 at 20 mL / min, an aqueous caustic soda solution is added into the first reaction tank 16 via the second reaction tank 17, The pH of the simulated pickling plating waste liquid in one reaction tank 16 is controlled by 4, and air is further supplied at 0.5 NL / min from the blowing hole 24a of the compressed gas header 24 installed at the bottom of the first reaction tank 16. It blows in and the inside of the 1st reaction tank 16 is stirred. In the first reaction tank 16, iron-oxidizing bacteria are put in advance. The divalent iron ions in the pickling plating simulated wastewater charged into the first reaction tank 16 are almost entirely oxidized to trivalent iron ions by the iron oxidizing bacteria and are contained in the pickling plating simulated waste liquid. Iron ions precipitated as iron hydroxide (III). The slurry containing the iron (III) hydroxide thus precipitated is continuously fed to the second reaction tank 17 at a rate of 3 mL / min by a pump 18, and an aqueous caustic soda solution is intermittently charged therein. The pH in the second reaction vessel varies within a range of 6 to 12, and the cohesiveness of particles made of iron (III) hydroxide increases. The slurry containing the agglomerated particles is returned to the first reaction tank 16 by overflow, and iron (III) hydroxide precipitated in the first reaction tank is precipitated on the surface of the particle group, and the particles are densified. Integrated to increase the particle size. The excess liquid portion in the first reaction tank 16 was solid-liquid separated by the filter body 23, and the filtrate was sucked by the pump 27. Moreover, in order to control the thickness of the cake layer deposited on the filter body surface, the cake thickness was controlled to about 10 mm by moving the scraper 29 up and down once every 30 seconds. The filtrate contains zinc ions and nickel ions at the same level as in the pickling plating simulated waste liquid, and almost no zinc ions and nickel ions are precipitated in the first reaction tank 16. On the other hand, the slurry in the first reaction tank 16 was pulled out intermittently and dehydrated with a filter press dehydrator having a pressure of 0.5 MPa to obtain a dehydrated cake.

  Next, the filtrate from the first reaction tank 16 is supplied to the third reaction tank 40 and an aqueous caustic soda solution is added to the third reaction tank 40 to adjust the pH in the third reaction tank 40. The air is blown at a rate of 0.5 NL / min from the blowout hole 49a of the compressed gas header 49 controlled at 8.5 and further installed at the bottom of the third reaction tank 40 to stir the third reaction tank 40. It was. Zinc ions and nickel ions contained in the filtrate from the first reaction tank 16 were precipitated as metal hydroxides. Furthermore, the particle diameter was increased by depositing zinc ions and nickel ions as metal hydroxides on the surface of the deposited metal hydroxides, or by depositing metal hydroxides between the particles. The liquid part surplus in the third reaction tank 40 was subjected to solid-liquid separation with a filter 47, and the filtrate was sucked with a pump 48 and discharged as treated water. Moreover, in order to control the thickness of the cake layer deposited on the filter body surface, the cake thickness was controlled to about 10 mm by moving the scraper 52 up and down once every 30 seconds. On the other hand, the slurry in the third reaction vessel 40 was withdrawn intermittently and dehydrated with a filter press dehydrator having a pressure of 0.5 MPa to obtain a dehydrated cake.

  The quality of the filtered water from the first reaction tank 16 and the quality of the filtrate from the third reaction tank 40 at this time are dehydrated in the first reaction tank 16 and the third reaction tank 40 in Table 2 below. Table 3 below shows the components of the later dehydrated cake and the average particle size of the enlarged particles in the steady state.

  As shown in Table 4 below, the dehydrated cake component from the first reaction vessel is almost iron only, nickel and zinc are lost, and moisture is as low as 30%. By adding operations such as kneading, it can be used as an iron source.

Example 2
As a pickling plating simulated waste liquid, a solution containing divalent iron ions, trivalent iron ions, nickel ions, zinc ions and suspended solids (SS) shown in Table 4 is used. The pickling plating simulated waste liquid was treated. The capacity of the first reaction tank and the third reaction tank is 3 L each. Note that iron-oxidizing bacteria are previously introduced into the first reaction tank.

  While pickling plating simulated waste liquid is continuously supplied to the first reaction tank 16 at 20 mL / min, negatively charged activated sludge with a water content of 85% is intermittently charged once a day at 5 g / day. Then, an aqueous caustic soda solution is added to the first reaction tank 16 to control the pH of the pickling plating simulated waste liquid in the first reaction tank 16 to 4 and further to the bottom of the first reaction tank 16. Air was blown at 0.5 NL / min from the blowing hole 24 a of the installed compressed gas header 24 to stir the first reaction tank 16. In the first reaction tank 16, iron-oxidizing bacteria are put in advance. The divalent iron ions in the pickling plating simulated wastewater charged into the first reaction tank 16 are almost entirely oxidized to trivalent iron ions by the iron oxidizing bacteria and are contained in the pickling plating simulated waste liquid. Iron ions precipitated as iron hydroxide (III). Since the precipitated iron hydroxide (III) is positively charged at pH 4, it aggregates with the negatively charged activated sludge to form a particle group. Iron hydroxide (III) precipitated in the first reaction vessel was precipitated on the surface of the particle group, and the particles were densified and integrated to increase the particle diameter. The excess liquid portion in the first reaction tank 16 was solid-liquid separated by the filter body 23, and the filtrate was sucked by the pump 27. Moreover, in order to control the thickness of the cake layer deposited on the filter body surface, the cake thickness was controlled to about 10 mm by moving the scraper 29 up and down once every 30 seconds. The filtrate contains zinc ions and nickel ions at the same level as in the pickling plating simulated waste liquid, and almost no zinc ions and nickel ions are precipitated in the first reaction tank 16. On the other hand, the slurry in the first reaction tank 16 was intermittently drawn and dehydrated with a filter press dehydrator having a pressure of 0.5 MPa to obtain a dehydrated cake.

  Next, the filtrate from the first reaction tank 16 is supplied to the third reaction tank 40 and an aqueous caustic soda solution is added to the third reaction tank 40 to adjust the pH in the third reaction tank 40. The air is blown at a rate of 0.5 NL / min from the blowout hole 49a of the compressed gas header 49 controlled at 8.5 and further installed at the bottom of the third reaction tank 40 to stir the third reaction tank 40. It was. Zinc ions and nickel ions contained in the filtrate from the first reaction tank 16 were precipitated as metal hydroxides. Furthermore, the particle diameter was increased by depositing zinc ions and nickel ions as metal hydroxides on the surface of the deposited metal hydroxides, or by depositing metal hydroxides between the particles. The liquid part surplus in the third reaction tank 40 was subjected to solid-liquid separation with a filter 47, and the filtrate was sucked with a pump 48 and discharged as treated water. Moreover, in order to control the thickness of the cake layer deposited on the filter body surface, the cake thickness was controlled to about 10 mm by moving the scraper 52 up and down once every 30 seconds. On the other hand, the slurry in the third reaction tank 40 was withdrawn intermittently and dehydrated with a filter press dehydrator having a pressure of 0.5 MPa to obtain a dehydrated cake.

  The quality of the filtered water from the first reaction tank 16 and the quality of the filtrate from the third reaction tank 40 at this time are shown in Table 5 below, and dehydration in the first reaction tank 16 and the third reaction tank 40 is performed. The components of the later dehydrated cake and the average particle size of the enlarged particles in the steady state are shown in Table 6 below.

  As shown in Table 6, the dehydrated cake component from the first reaction vessel is almost iron only, nickel and zinc are eliminated, and the moisture is as low as 33%, so that the moisture is kneaded with other raw materials. It became possible to use as an iron source by adding operations such as.

  In Example 2, since the amount of charged activated sludge negatively charged was small, it was intermittently charged. However, when the amount of charging is increased, the effect does not change even if continuously charged.

As a comparative example pickling plating waste solution, using a solution containing divalent iron ions, trivalent iron ions, nickel ions, zinc ions, suspended substances shown in Table 2-1, using the treatment apparatus of FIG. The pickling plating simulated waste liquid was treated. The capacities of the first reaction tank and the third reaction tank are 3L and 3L, respectively.

  While the pickling plating simulated waste liquid is continuously supplied to the first reaction tank 16 at 20 mL / min, an aqueous caustic soda solution is added to the first reaction tank 16 to pickle the plating in the first reaction tank 16. The pH of the simulated waste liquid is controlled by 4, and further, air is blown at 0.5 NL / min from the blowing hole 24a of the compressed gas header 24 installed at the bottom of the first reaction tank 16, and the inside of the first reaction tank 16 is reached. Was stirred. In the first reaction tank 16, iron-oxidizing bacteria are put in advance. The divalent iron ions in the pickling plating simulated wastewater charged into the first reaction tank 16 are almost entirely oxidized to trivalent iron ions by the iron oxidizing bacteria and are contained in the pickling plating simulated waste liquid. Iron ions precipitated as iron hydroxide (III). Further, on the surface of the precipitated iron hydroxide (III) particles, trivalent iron ions are precipitated as iron hydroxide (III), or iron hydroxide (III) is precipitated between the particles. The diameter was increased to some extent. The excess liquid portion in the first reaction tank 16 was solid-liquid separated by the filter body 23, and the filtrate was sucked by the pump 27. Moreover, in order to control the thickness of the cake layer deposited on the filter body surface, the cake thickness was controlled to about 10 mm by moving the scraper 29 up and down once every 30 seconds. The filtrate contains zinc ions and nickel ions at the same level as in the pickling plating simulated waste liquid, and almost no zinc ions and nickel ions are precipitated in the first reaction tank 16. On the other hand, a part of the slurry in the first reaction tank 16 was extracted and dehydrated with a filter press dehydrator having a pressure of 0.5 MPa to obtain a dehydrated cake.

  Next, the filtrate from the first reaction tank 16 is supplied to the third reaction tank 40 and an aqueous caustic soda solution is added to the third reaction tank 40 to adjust the pH in the third reaction tank 40. The air is blown at a rate of 0.5 NL / min from the blowout hole 49a of the compressed gas header 49 controlled at 8.5 and further installed at the bottom of the third reaction tank 40 to stir the third reaction tank 40. It was. Zinc ions and nickel ions contained in the filtrate from the first reaction tank 16 were precipitated as metal hydroxides. Furthermore, the particle diameter was increased by depositing zinc ions and nickel ions as metal hydroxides on the surface of the deposited metal hydroxides, or by depositing metal hydroxides between the particles. The liquid part surplus in the third reaction tank 40 was subjected to solid-liquid separation with a filter 47, and the filtrate was sucked with a pump 48 and discharged as treated water. Moreover, in order to control the thickness of the cake layer deposited on the filter body surface, the cake thickness was controlled to about 10 mm by moving the scraper 52 up and down once every 30 seconds. On the other hand, a part of the slurry in the third reaction tank 40 was drawn and dehydrated with a filter press dehydrator having a pressure of 0.5 MPa to obtain a dehydrated cake.

  In this case, the quality of the filtered water from the first reaction tank 16 and the quality of the filtrate from the third reaction tank 40 are dehydrated in the first reaction tank 16 and the third reaction tank 40 in Table 7 below. The components of the later dehydrated cake and the average particle size in the steady state are shown in Table 8 below.

  As shown in Table 8, the dehydrated cake component in the first reaction tank is almost iron only and nickel and zinc are lost, but the average particle size is as small as 4.2 μm. When used as an iron source, the handling property is poor and uniform mixing with other raw materials when used as an iron source is difficult.

It is the figure which showed the relationship between pH of a waste liquid, and the residual ratio (mass ratio) of the metal ion in a waste liquid. It is the figure which showed the charging condition (zeta potential) with respect to pH of iron hydroxide (III). It is an example of the processing apparatus which concerns on the manufacturing method of the iron containing dewatering cake from the wastewater of this invention. The processing apparatus applied in Example 1. FIG. The processing apparatus applied in Example 2. The processing apparatus applied in the comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... The processing apparatus 11 which concerns on the manufacturing method of the iron containing dehydration cake from waste liquid Drainage 12 ... Drain tank 13 ... Pump 14 ... Alkaline liquid tank 15 ... Pump

16 ... 1st reaction tank 17 ... 2nd reaction tank 18 ... Pump 19 ... Stirrer 20 ... Hopper

21 ... Cutting device 22 ... pH meter 23 ... Filter body 24 ... Supply pipe 25 ... Compressed gas header 25a ... Blowout hole

26 ... Slurry discharge pipe 27 ... Pump 28 ... Scraper support structure 29 ... Scraper 30 ... Drive device

31 ... Connector 32 ... Backwash pump 40 ... Third reaction tank

41 ... Alkaline liquid tank 42 ... Pump 46 ... pH meter 47 ... Filter body 48 ... Pump 49 ... Compressed gas header 49a ... Blowout hole

50 ... Slurry discharge pipe 51 ... Scraper support structure 52 ... Scraper 53 ... Drive device 54 ... Connector 55 ... Backwash pump

Claims (11)

  An acidic waste liquid containing a plurality of metal ions in addition to divalent iron ions and trivalent iron ions is supplied into the first reaction tank, and divalent iron ions are converted into 3 by biological oxidation means or chemical oxidation means. While oxidizing to valent iron ions, a neutralizing agent is added to adjust the pH of the waste liquid to 3 or more and 5 or less to produce iron (III) -based particles, and a waste liquid slurry containing the particles 1 part is supplied to the second reaction tank, an alkali agent is added to adjust the pH of the slurry in the second reaction tank to 5 or more and 13 or less, and the particles are aggregated together. The method for producing an iron-containing dehydrated cake from waste liquid is characterized in that the slurry containing the aggregated particles is separated into solid and liquid and concentrated, and then the concentrated slurry is dehydrated into a cake.   An acidic waste liquid containing a plurality of metal ions in addition to divalent iron ions and trivalent iron ions is supplied into the first reaction tank, and divalent iron ions are converted into 3 by biological oxidation means or chemical oxidation means. While oxidizing to valent iron ions, adding a neutralizing agent to adjust the pH of the waste liquid to 3 or more and 5 or less to produce iron hydroxide (III) -based particles, and further negatively charged fine powder or mud The particles and the negatively charged fine powder or mud substance are aggregated, and the slurry containing the aggregated particles is solid-liquid separated and concentrated, and then the concentrated slurry is dehydrated. A method for producing an iron-containing dehydrated cake from waste liquid, characterized in that the cake is made into a cake.   The method according to claim 1, wherein a finely charged fine powder or mud substance is further charged into the first reaction vessel.   The iron oxide bacteria are added into the first reaction tank as the biological oxidation means, air or oxygen is blown into the waste liquid, and the waste liquid is stirred. The method described in 1.   The plurality of metal ions are one or more of nickel ions, zinc ions, and copper ions, and the slurry is solid-liquid separated and concentrated by the method according to any one of claims 1 to 4. Thereafter, the discharged processing liquid is supplied to the third reaction tank, and the processing liquid in the third reaction tank is stirred, and the pH of the processing liquid is adjusted to 6 or more and 10 or less. 1 or 2 or more types of metal hydroxide particles of zinc and copper are produced, the slurry containing the metal hydroxide particles is solid-liquid separated and concentrated, and then dehydrated to form a cake. A method for producing an iron-containing dehydrated cake from the waste liquid.   The method according to any one of claims 2 to 5, wherein the negatively charged substance is activated sludge generated when organic wastewater is treated by a biological treatment method.   The input amount of the negatively charged substance is 0.2% by mass or more and 2% by mass or less with respect to the mass generated per unit time of the iron (III) hydroxide-based particles precipitated in the first reaction tank ( The method according to any one of claims 2 to 6, wherein the method is dry conversion.   The method according to any one of claims 1 to 7, wherein a part of the slurry in the first reaction tank and / or the third reaction tank is subjected to solid-liquid separation with a separation membrane.   The method according to claim 8, wherein the pore size of the separation membrane is 1 μm or more and 100 μm or less.   On the separation membrane in the first reaction tank and / or on the separation membrane in the third reaction tank, the particles and the metal hydroxide particles are supplemented and formed on the separation membrane, respectively. The method according to claim 8 or 9, wherein the thickness of the cake layer to be controlled is controlled to 1 mm or more and 50 mm or less.   The method according to any one of claims 1 to 10, wherein the suspension concentration in the acidic waste liquid is 1/3 or less (mass concentration) of the soluble iron ion concentration in the acidic waste liquid.

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