JP6728906B2 - Heat exchanger descaling method - Google Patents

Description

The present invention relates to a method for removing scale attached to a heat exchanger. More specifically, the present invention relates to a method for removing scale adhering to a heat exchanger used for cooling a deferred iron final solution obtained from a deironing step of removing iron from an aqueous nickel chloride solution containing iron.

In nickel smelting, a dry smelting method in which nickel ore or nickel concentrate is dissolved in a dry furnace such as a blast furnace or an electric furnace, nickel in ore or nickel concentrate is leached into an aqueous solution to remove impurities. After that, there is a hydrometallurgical method for recovering nickel, and the most suitable method is industrialized according to the purpose and application.

Among them, in the hydrometallurgical method, for example, as described in Patent Document 1, nickel matte and mixed sulfides are leached by utilizing the oxidizing action of chlorine gas, and the leached nickel ions and cobalt ions are removed. The chlorine leaching process, which is commercialized as electrolytic nickel and electrolytic cobalt by electrowinning, has been put to practical use.

In the chlorine leaching step of the chlorine leaching process, the mixed sulfide and cementation residue described below are repulped into a chloride aqueous solution, and then chlorine gas is blown into the slurry to leach nickel and cobalt into the chloride aqueous solution. ..

Next, in the cementation step, a crushed Ni ₃ S ₂ and nickel matte containing metallic nickel as a main component was added to a chlorine leaching solution containing a divalent copper chloro complex ion as an oxidizing agent obtained in the chlorine leaching step. By bringing them into contact with each other to carry out a substitution reaction of copper and nickel, nickel in the nickel mat is leached by substitution into the liquid, and copper ions become a form of Cu ₂ S or Cu ⁰ (metal copper), which is a solid (cementation residue). Part).

The cementation final solution, the cement leaching residue consisting of the substitution leaching residue of the nickel matt and the solid precipitated in the form of Cu ₂ S or Cu ⁰ (metallic copper) are subjected to solid-liquid separation, and then the cementation. The final solution is sent to the next purification step, and the solid cementation residue is sent to the chlorine leaching step.

Here, since the cementation final solution contains impurities such as copper, iron, lead, and zinc in addition to nickel and cobalt that are the recovery target metals, cobalt is separated and recovered to remove impurities. In order to obtain a high-purity nickel chloride aqueous solution suitable for electrowinning, a purification process is configured. The cementation final solution is processed through a deironing step, a cobalt separating step, a deleading step, and a dezincing step to remove impurities, and is supplied to the electrowinning step.

In the iron removal step, for example, an oxidative neutralization method using chlorine gas as an oxidizing agent and carbonate as a neutralizing agent is used. The oxidative neutralization method utilizes the property that some heavy metals such as iron easily become hydroxides even in a low pH range when they become ions in a higher oxidation state. It is a method that is widely used for wastewater treatment including heavy metals.

In the cobalt separation step, for example, a solvent extraction method is used. Solvent extraction methods using amine-based extractants are commonly practiced in chloride baths, although cobalt forms chloro complex ions when the chloride ion concentration in the aqueous solution is sufficiently high. , Nickel takes advantage of the fact that it does not form chloro complex ions. The cobalt separated from the nickel chloride aqueous solution containing cobalt in the solvent extraction step becomes a cobalt chloride aqueous solution by the back extraction operation, and after impurities are removed in the cobalt chloride purification step, the product is electro-produced in the electrolytic extraction step. Becomes cobalt.

For the lead removal step and the zinc removal step, known methods are appropriately selected as necessary.

The pH of the nickel chloride aqueous solution that has been subjected to the above-mentioned purification process is adjusted, and then sent to the electrowinning process to become electric nickel in the electrowinning process.

In this chlorine leaching process, the reaction is operated at high temperature in order to increase the reaction efficiency in the cementation step. Therefore, the cementation final solution treated in the subsequent deironing step has a high temperature, and the deironization final solution after deironing also has a high temperature.

However, in the next solvent extraction step, in consideration of the evaporation of the organic solvent, the decomposition of the organic solvent, the heat resistance of the material of the apparatus, etc., it is necessary to operate at a certain temperature or lower, and the deferred iron final solution is treated by a heat exchanger. To be cooled.

However, since the deironization final solution is a high-concentration nickel chloride aqueous solution, calcium was contained on the deironization final solution flow path side of the heat transfer surface of the heat exchanger used for cooling the deironization final solution. Scale gradually precipitates and grows. This scale causes a decrease in the amount of liquid passing through, making it difficult to continue operation.Therefore, the flow of deferred iron final liquid to the heat exchanger is stopped periodically to prevent the scale from adhering to the heat transfer surface. Had to be removed.

However, the scale removal work requires a great deal of labor and time. Normally, by arranging multiple heat exchangers in parallel, the remaining heat exchangers can be operated and the deferred iron final solution can continue to flow even during scale removal work. If the removal work time is prolonged, the production of electrolytic nickel may be reduced due to the decrease in the flow rate of the final iron removal solution.

Patent Document 2 discloses a method for preventing scaling of piping, but since conditions such as liquid composition and pH are different, it cannot be applied to the final iron removal liquid in the chlorine leaching process in nickel smelting.

JP 2012-026027 A Japanese Patent Laid-Open No. 10-202213

Therefore, the present invention has been devised in view of the above-mentioned problems of the prior art, and is used to cool the final deironization solution obtained from the deironing step of removing iron from an aqueous nickel chloride solution containing iron. It is intended to provide a scale removing method for a heat exchanger, which can remove the scale adhering to the heat exchanger without requiring a lot of trouble and time.

In order to achieve the above object, the present inventor has conducted extensive studies especially on a method for dissolving scale, and as a result, the scale was removed by passing an aqueous hydrochloric acid solution having a concentration of 5 to 10 wt% into a heat exchanger. The inventors have found out what can be done and have completed the present invention.

A method for removing scale of a heat exchanger according to a first aspect of the present invention comprises forming an iron(III) hydroxide precipitate from a nickel chloride aqueous solution containing iron, and solid-liquid separating a nickel chloride aqueous solution slurry containing the iron(III) hydroxide. A method for removing scale of a plate-type heat exchanger for cooling the deironization final solution obtained from the deironing step, comprising: A hydrochloric acid aqueous solution having a concentration of 10% by weight is circulated between the heat exchanger and liquid to pass therethrough, and the hydrochloric acid concentration of the hydrochloric acid aqueous solution to be passed through the hydrochloric acid tank is adjusted to provide the heat exchanger. It is characterized in that the scale adhering to the deironization final liquid flow path side of the heat transfer surface made of pure titanium or corrosion resistant titanium alloy for cooling the deironization final liquid is removed.

The scale removing method of the heat exchanger of the second invention is characterized in that, in the first invention, the concentration of the hydrochloric acid aqueous solution is 5 to 6% by weight.

According to the scale removing method of the heat exchanger of the present invention, the following effects can be obtained.
a) In a heat exchanger in a deironing step of removing iron from an aqueous nickel chloride solution containing iron, it is possible to remove scale attached to the heat exchanger without requiring a great deal of trouble and time . For this reason , there is no possibility that the production of electrolytic nickel will be reduced due to the decrease in the flow rate.
b) The readjustment of the hydrochloric acid concentration can be done in the hydrochloric acid tank.
c) Since the deferred iron final solution and the passage of cooling water are stopped, the hydrochloric acid aqueous solution can be circulated and used.

It is a schematic flow diagram of the iron removal process according to the present invention. It is a figure showing the weight change of the titanium test piece in an example and a comparative example.

The present invention relates to a deferred iron obtained from a deironing step of producing an iron(III) hydroxide precipitate from a nickel chloride aqueous solution containing iron and solid-liquid separating a nickel chloride aqueous solution slurry containing the iron(III) hydroxide. A method for removing scale of a heat exchanger for cooling a final liquid, wherein the deironization final liquid provided in the heat exchanger by passing an aqueous hydrochloric acid solution having a concentration of 5 to 10% by weight through the heat exchanger. It is characterized in that the scale adhering to the deironization final liquid flow path side of the heat transfer surface made of pure titanium or corrosion resistant titanium alloy for cooling is removed.

Therefore, here, as one embodiment of the present invention, a nickel smelt and a mixed sulfide are leached by utilizing the oxidizing action of chlorine gas, which is a hydrometallurgical method for recovering nickel, and the leached nickel ions and cobalt are used. In the chlorine leaching process for producing ions as electro-nickel and electro-cobalt by electrowinning, the application to a heat exchanger used for cooling the de-ironing final solution obtained from the de-ironing step will be described below as an example. ..

1. Chlorine leaching process In the chlorine leaching process, the nickel matte used as a raw material refers to nickel sulfide produced from dry smelting, and mixed sulfide is nickel-cobalt produced from sulfuric acid leaching from low-grade laterite ore. Refers to mixed sulfides. In the chlorine leaching step of the chlorine leaching process, the mixed sulfide and cementation residue described below are repulped into an aqueous chloride solution, and then chlorine gas is blown into the slurry to leach nickel and cobalt into the aqueous chloride solution. ..

Next, in the cementation step, a crushed Ni ₃ S ₂ and nickel matte containing metallic nickel as a main component was added to a chlorine leaching solution containing a divalent copper chloro complex ion as an oxidizing agent obtained in the chlorine leaching step. By carrying out the substitution reaction of copper and nickel by bringing them into contact with each other, nickel in the nickel mat is displacement-leached into the liquid, and the copper ion becomes a form of Cu ₂ S or Cu ⁰ (metal copper), which is a solid (cementation residue). Part).

The cementation final solution, the substitution leaching residue of the nickel matte, and the solidification residue composed of the solid precipitated in the form of Cu ₂ S or Cu ⁰ (metallic copper) are subjected to solid-liquid separation, and then the cementation. The final solution is sent to the next purification step, and the solid cementation residue is sent to the chlorine leaching step.

Here, since the cementation final solution contains impurities such as copper, iron, lead, and zinc in addition to nickel and cobalt that are the recovery target metals, cobalt is separated and recovered to remove impurities. In order to obtain a high-purity nickel chloride aqueous solution suitable for electrowinning, a purification process is configured.

The cementation final solution is processed through a deironing step, a cobalt separating step, a deleading step, and a dezincing step to remove impurities, and is supplied to the electrowinning step.

2. Outline of Iron Removal Step FIG. 1 is a schematic flow chart of the iron removal step according to the present invention. In the deferring step, chlorine gas was blown into the cementation final solution to adjust the redox potential (Ag/AgCl electrode reference) to 900 to 1000 mV, and nickel carbonate was added to adjust the pH to 1.5 to 3.0. A ferrous hydroxide (III) hydroxide precipitate is produced, and a nickel chloride aqueous solution slurry containing the iron (III) hydroxide is subjected to solid-liquid separation to obtain a deferred iron final solution and a deferred iron precipitate. is there.

The temperature of the cementation final solution treated in the deironing step is as high as about 70°C, and the deironization final solution after deironing is also 65 to 70°C.

In the cobalt separation step, which is the next step of the iron removal step, when the hot deironization final solution is fed, the evaporation amount of the organic solvent increases and the retained organic solvent amount decreases. Due to the decrease in the amount of organic solvent retained, process problems such as an increase in the amount of nickel mixed into the organic solvent from which cobalt was extracted due to the deterioration of oil-water separability, and cost problems such as an increase in the amount of organic solvent replenishment have occurred. To do. Furthermore, when the evaporation amount of the organic solvent increases, safety and security problems such as generation of odor also occur. Further, since the decomposition of the organic solvent is promoted when the hot deironization final solution is fed, there is a concern that the COD load of the wastewater may increase due to the inclusion of decomposition products in the back extraction solution. Further, since resin materials such as polyvinyl chloride and FRP are often used in the solvent extraction equipment, problems such as softening and deformation due to high temperature, and liquid leakage due to such softening occur.

Therefore, the deironization final solution needs to be cooled to about 55° C. by the heat exchanger. The heat exchange method is not particularly limited as long as it can cool the deferred iron final solution, such as a shell and tube heat exchanger. Among them, the plate heat exchanger is preferable. Therefore, as one embodiment of the present invention, cooling is performed by using a plate cooler with water as a coolant. Further, as the refrigerant, water is most suitable because it is convenient and has a high cooling efficiency, but it is not particularly limited whether cold air is used or a special refrigerant such as ethylene glycol is used.

3. Scale removal method of heat exchanger As described above, when cooling the deironing final solution with a plate cooler using water or the like as a coolant, the deironing final solution flow path side of the heat transfer surface of this plate cooler contains calcium. The scale gradually precipitates and grows. This scale causes a decrease in the amount of final deironization solution passed, making it difficult to continue operation.Therefore, stop the passing of heat to the heat exchanger regularly to remove the adhered scale. Had to do.

Therefore, normally, by arranging multiple heat exchangers in parallel, the remaining heat exchangers can be operated and the deferred iron final liquid can continue to flow even during scale removal work. If the removal work time is prolonged, the production of electrolytic nickel may be reduced due to the decrease in the flow rate of the final iron removal solution. In FIG. 1, two plate coolers are arranged in parallel, and during the scale removing work, the deironization final solution is passed through only one of the scales which is not performing the scale removing work.

In the conventional scale removing work, since the plate cooler is disassembled and each plate is carefully cleaned, it takes a lot of time and labor and requires a high level maintenance technique. Further, in order to disassemble the plate cooler, the gasket that seals between the plates needs to be renewed each time it is disassembled, resulting in a high maintenance cost. Further, since the disassembly and maintenance work of the plate cooler involves the work of reassembling the disassembled plate cooler, there is always a risk of liquid leakage at the start of liquid passage.

Meanwhile, results of the investigation, the main component of the scale was found to be gypsum _{(CaSO 4 · 2H 2 O)} . Therefore, in the present invention, hydrochloric acid is used to dissolve the scale.

In the scale removing operation of the present invention, the deferred iron final solution and the cooling water are stopped to be passed, and a 5 to 10 wt% hydrochloric acid aqueous solution is passed to the deferred final solution flow path side. In addition, at this time, it is preferable that the hydrochloric acid aqueous solution be circulated. Necessary equipment includes a hydrochloric acid tank, a pump for feeding hydrochloric acid into the plate cooler, and outgoing and return piping and a switching valve. The hydrochloric acid concentration of the hydrochloric acid aqueous solution passed through the hydrochloric acid tank may be adjusted, and when the hydrochloric acid concentration decreases, readjustment may be performed.

Dissolution of the scale with hydrochloric acid follows the reaction shown in (Equation 1).
CaSO ₄ .2H ₂ O+2HCl→CaCl ₂ +H ₂ SO ₄ +2H ₂ O (Formula 1)

Since the plate that forms the heat transfer surface of the plate cooler is pure titanium, when high-concentration hydrochloric acid is used, wear is caused by corrosion of the plate. The plate is made extremely thin, which ensures a high heat transfer coefficient, but if holes are made in the plate, the nickel chloride aqueous solution will leak to the cooling water side, which is an environmental problem. Also occurs.

Therefore, the concentration of the hydrochloric acid aqueous solution that is passed to the deironization final solution flow path side of the plate cooler heat transfer surface is set to 5 to 10% by weight. If it is less than 5% by weight, the scale removing effect is reduced, and if it exceeds 10% by weight, the titanium material may be corroded. Furthermore, the concentration of the hydrochloric acid aqueous solution is more preferably 5 to 6% by weight.

According to the scale removing method of the heat exchanger of the present invention, since it is not necessary to disassemble and clean the heat exchanger, it is possible to remove the scale adhering to the heat exchanger without requiring much labor and time. , There is no possibility that the production of electrolytic nickel will be reduced due to the decrease in the final iron removal solution flow rate. Further, since the work of reassembling the disassembled heat exchanger does not occur, there is no risk of liquid leakage at the start of the passage of the deferred iron final liquid. Furthermore, in the case of the plate heat exchanger, there is no need to disassemble and reassemble, so there is no need to replace the gasket, and there is no cost.

Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples, but the scope of the present invention is not particularly limited by the description of the Examples and Comparative Examples.

(Example 1)
The scale adhering to the plate cooler for cooling the final de-ironing solution in the de-ironing process in actual operation is sampled, 10 g of the scale is put into 200 mL of hydrochloric acid aqueous solution adjusted to a hydrochloric acid concentration of 5% by weight, and immersed at room temperature for 20 hours. The calcium concentration in the hydrochloric acid aqueous solution at that time was measured.

(Example 2)
10 g of the same scale as in Example 1 was put into 200 mL of an aqueous hydrochloric acid solution adjusted to have a hydrochloric acid concentration of 10% by weight, and the calcium concentration in the aqueous hydrochloric acid solution when immersed for 20 hours at room temperature was measured.

(Comparative Example 1)
10 g of the same scale as in Example 1 was placed in 200 mL of an aqueous hydrochloric acid solution adjusted to a hydrochloric acid concentration of 18% by weight, and the calcium concentration in the aqueous hydrochloric acid solution when immersed for 20 hours at room temperature was measured.

In Example 1, Example 2, and Comparative Example 1, the calcium concentration was measured by an ICP emission spectroscopic analyzer. The results of Example 1, Example 2 and Comparative Example 1 are shown in Table 1.

From Example 1, Example 2, and Comparative Example 1, it was possible to dissolve about 4 g of scale with 5 wt% hydrochloric acid and 10 wt% hydrochloric acid by immersing at room temperature for 20 hours. On the contrary, with 18% by weight hydrochloric acid, the amount of scale dissolved decreased. With 18% by weight hydrochloric acid, there is a possibility that a reverse reaction due to sulfuric acid generated by dissolution of gypsum occurred.

(Example 3)
About 21.8 g of a pure titanium test piece was immersed in an aqueous hydrochloric acid solution adjusted to a hydrochloric acid concentration of 5% by weight, and allowed to stand at room temperature for 60 days to examine the weight change.

(Example 4)
About 21.8 g of a pure titanium test piece was immersed in a hydrochloric acid aqueous solution adjusted to a hydrochloric acid concentration of 10% by weight, and allowed to stand at room temperature for 60 days to examine the weight change.

(Comparative example 2)
About 21.7 g of a pure titanium test piece was immersed in an aqueous hydrochloric acid solution adjusted to have a hydrochloric acid concentration of 35% by weight, and allowed to stand at room temperature for 60 days to examine the weight change. The results of Example 3, Example 4, and Comparative Example 2 are shown in FIG.

From FIG. 2, with 5% by weight hydrochloric acid and 10% by weight hydrochloric acid, there was no large change in weight even after immersion for 60 days, but with 35% by weight hydrochloric acid, there was a decrease of about 1 g.

Thus, when removing the scale attached to the deironization final solution flow path side of the heat transfer surface made of pure titanium, it was confirmed that the titanium material was not corroded by using the hydrochloric acid aqueous solution having a concentration of 5 to 10% by weight. It was

Claims (2)

An iron (III) hydroxide precipitate is formed from a nickel chloride aqueous solution containing iron, and a deironization final solution obtained from a deironing step of solid-liquid separating a nickel chloride aqueous solution slurry containing the iron(III) hydroxide is cooled. A method for removing scale of a plate heat exchanger, comprising:
With the deferred iron final solution and the cooling solution being stopped, a hydrochloric acid aqueous solution having a concentration of 5 to 10% by weight stored in a hydrochloric acid tank is circulated between the heat exchanger and the solution ,
By adjusting the hydrochloric acid concentration of the hydrochloric acid aqueous solution that is passed through the hydrochloric acid tank ,
The heat exchanger is provided, for cooling the deironization final solution, to remove scale attached to the deironization final solution flow path side of the heat transfer surface made of pure titanium or a corrosion resistant titanium alloy,
Descaling method for a heat exchanger, characterized in that. The method according to claim 1, wherein the hydrochloric acid aqueous solution has a concentration of 5 to 6% by weight .

Images