pH control method for neutralization process

Abstract

To provide a control method for controlling a limestone addition timing to a preliminary neutralization step at the time of addition start of a sulfuric acid to an autoclave, and more quickly reaching a target pH.SOLUTION: A pH control method of a neutralization step, which is performed in a leachate that is an acidic process liquid through leaching of a sulfuric acid at the time of application start of a high pressure acid leaching method, starts addition of a pH modifier to the leachate in the neutralization step for 80 to 100 minutes after addition start of a sulfuric acid of the leach of the sulfuric acid, adds the pH modifier in the leachate after the sulfuric acid has been added in autoclave, wherein an addition amount of the pH modifier added between a time point after a lapse of 80 to 100 minutes from addition start of the sulfuric acid and a time point until the leachate reaches the neutralization step is defined as an equivalent of the pH modifier equal to neutralization of a total amount of a sulfuric acid added for 30 minutes after addition start of a sulfuric acid to an ore slurry in the autoclave.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a high-pressure acid leaching technology for nickel oxide ores. More specifically, it relates to a pH control method in the neutralization process during the startup of the high-pressure acid leaching process, and specifically, to a method for supplying limestone as a pH adjuster.

Background Art

[0002] In the field of nickel hydrometallurgy using nickel oxide ore as a raw material, the recovery of valuable metals from low-nickel grade ores by the high-pressure acid leaching (HPAL) method of acid leaching under high temperature and high pressure has been put into practical use. And for the recovery of valuable metals such as nickel and cobalt leached from nickel oxide ore by the HPAL method, a method of adding a sulfiding agent such as hydrogen sulfide gas to a sulfuric acid bath containing valuable metals under pressure and recovering it as a sulfide (nickel-cobalt mixed sulfide) is generally carried out.

[0003] In this high-pressure acid leaching method, various metal elements (mainly nickel, magnesium, iron, aluminum) components contained in nickel oxide ore are acid-leached by appropriately adding sulfuric acid to the ore slurry supplied to an autoclave under high temperature and high pressure conditions.

[0004] After the leaching step with sulfuric acid at high pressure (also referred to as the high-pressure sulfuric acid leaching step), a typical process flow of the high-pressure acid leaching method (HPAL method) including the process flow of nickel until nickel sulfide is obtained is shown in FIG. 1 (see Patent Document 1).

[0005] In the wet smelting process shown in Figure 1, there is an ore pretreatment step S0 in which an ore slurry containing nickel oxide ore having a predetermined particle size is prepared; a leaching step S1 (referred to as the high-pressure sulfuric acid leaching step) in which sulfuric acid is added to the ore slurry and subjected to a leaching treatment under high temperature and high pressure to generate a leaching slurry; a preliminary neutralization step S2 in which a pH adjusting agent is added to the leaching slurry to adjust the pH to a predetermined range; a solid-liquid separation step S3 in which the pH-adjusted leaching slurry is washed in multiple stages to separate the leaching liquid containing nickel and cobalt along with impurity elements from the leaching residue; a neutralization step S4 in which a neutralizing agent is added to the leaching liquid to generate a neutralized precipitate containing impurity elements, which is then separated and removed to obtain a neutralized final solution containing nickel and cobalt along with zinc. The process includes a dezincification step S5 in which a sulfidating agent is added to the neutralized final liquid to generate zinc sulfide, which is then separated and removed to obtain a nickel recovery mother liquor containing nickel and cobalt; a nickel recovery step S6 in which hydrogen sulfide and sodium hydroxide are added to the nickel recovery mother liquor as sulfidating agents to generate a NiCo mixed sulfide containing nickel and cobalt, and then the NiCo mixed sulfide is recovered by solid-liquid separation; a sulfidating agent removal step S7 in which a predetermined amount of oxidizing slurry is added to the nickel recovery final liquid discharged to the liquid phase side by solid-liquid separation in nickel recovery step S6 to decompose the sulfidating agent; and a final neutralization step S8 in which the poor liquid discharged by the decomposition treatment is detoxified together with the leaching residue discharged from the solid-liquid separation step S3.

[0006] Here, the process liquid after leaching with sulfuric acid under high pressure is strongly acidic (pH 0.5-1.0). After going through a preliminary neutralization step and a solid-liquid separation step to obtain a leachate from which leaching residue has been separated by washing (countercurrent multi-stage washing method), it goes through a neutralization step (including solid-liquid separation) to separate impurities, and after being adjusted to a predetermined pH, nickel sulfide is obtained by reacting hydrogen sulfide gas with nickel sulfate in a dezincification step and a sulfidation reaction step.

[0007] The process liquid used after this dezincification step needs to have a specific pH to promote the sulfidation reaction, and the pH is precisely controlled (for example, pH 2.8 to 3.2) mainly in the preliminary neutralization step. In this preliminary neutralization process, limestone is used to neutralize the mixture, raising the pH and primarily neutralizing the free acid (unreacted sulfuric acid with metal components). However, sulfate ions react with the calcium in the limestone and precipitate as gypsum (see reaction equation 1).

[0008]

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[0009] In controlling the pH of the preliminary neutralization process, the ideal method would be to install a pH meter permanently in the reaction vessel and control the amount of limestone added based on the pH. However, the preliminary neutralization process is a harsh environment with high temperature, strong acidity, and slurry-like properties, making it difficult to install a permanent pH meter due to the high frequency of misreadings and malfunctions. Therefore, a management system is in place to monitor the pH by collecting the process liquid after the preliminary neutralization process and measuring the pH separately, and then changing the amount of limestone added according to the measured value.

[0010] This special pH control system works well under normal operating conditions where the pH of the process liquid after high-pressure sulfuric acid leaching is stable. In such conditions, the amount of limestone added can be kept generally stable, and adjusting the amount of limestone added according to the pH measurement after the preliminary neutralization process is sufficient. On the other hand, in cases of large operational fluctuations, such as when sulfuric acid is first added to the autoclave, the pH of the leachate changes significantly from 7-8 (before sulfuric acid addition) to 0.5-1.0 (after sulfuric acid addition). Therefore, it is necessary to adjust the amount of limestone added in the preliminary neutralization process to follow this change.

[0011] When sulfuric acid is added to the autoclave, controlling the pH to the target value (2.8-3.2) during the preliminary neutralization process is difficult. If the pH deviates significantly from the target value, the high flow rate of the process fluid and the difficulty in grasping the pH distribution make pH adjustment extremely time-consuming. As a result, the supply of fluid to the downstream process is delayed, leading to a decrease in production volume. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2020-180317 [Overview of the project] [Problems that the invention aims to solve]

[0013] This invention provides a control method for reaching the target pH more quickly by controlling the timing of limestone addition to the preliminary neutralization step at the start of sulfuric acid addition to the autoclave. [Means for solving the problem]

[0014] In view of these circumstances, the first embodiment of the present invention is a pH control method for a neutralization (operation) step performed on a leachate, which is an acidic process liquid that has undergone sulfuric acid leaching, at the start of application of a high-pressure acid leaching method, characterized in that the pH control in the neutralization step is performed by adding a pH adjusting agent to the leachate in the neutralization step between 80 and 100 minutes after the start of sulfuric acid addition for sulfuric acid leaching, the addition of the pH adjusting agent is performed on the leachate after the sulfuric acid addition has been carried out in an autoclave, and the amount of pH adjusting agent added between 80 and 100 minutes after the start of sulfuric acid addition and the time when the leachate reaches the neutralization step is an equivalent amount of pH adjusting agent that is equal to the total amount of sulfuric acid added in the first 30 minutes after the start of sulfuric acid addition to the ore slurry in the autoclave.

[0015] A second embodiment of the present invention is a method for controlling the pH of a neutralization step performed on an acidic process solution, which is an acidic process solution obtained through sulfuric acid leaching, at the start of application of a high-pressure acid leaching method, characterized in that, in the first embodiment, the pH after the neutralization step is measured at least once every 30 minutes, and the amount of the pH adjusting agent added in the neutralization step is adjusted based on the obtained pH measurement.

[0016] The third embodiment of the present invention is a pH control method for a neutralization step performed on a leachate, which is an acidic process solution that has undergone sulfuric acid leaching at the start of application of the high-pressure acid leaching method, characterized in that the pH adjuster in the first and second embodiments is limestone.

[0017] The fourth embodiment of the present invention is a pH control method for a neutralization step performed on a leachate, which is an acidic process solution that has undergone sulfuric acid leaching at the start of application of the high-pressure acid leaching method, characterized in that the neutralization step in the first to third embodiments is a preliminary neutralization step performed on the leachate obtained by sulfuric acid leaching by adding sulfuric acid to the ore slurry in the autoclave while increasing the pressure in the autoclave storing the ore slurry of the raw material.

Advantages of the Invention

[0018] By controlling the timing of adding limestone to the preliminary neutralization step at the start of adding sulfuric acid to the autoclave, the following effects are obtained. · The pH after the preliminary neutralization step can reach the target value in a short time. · Due to this time reduction, the supply of the process solution to subsequent steps such as the zinc removal step can also be started earlier, and the time required for startup of the entire process can be shortened, thus improving productivity.

Brief Description of the Drawings

[0019] [Figure 1] It is a process flow diagram of the high-pressure acid leaching method (HPAL method) for nickel oxide ore. [Figure 2] It is a diagram showing the transition of "the ratio of the ore slurry flow rate to the autoclave and the limestone slurry flow rate to the neutralization step" according to the elapsed time after starting the addition of sulfuric acid to the autoclave. [Figure 3] It is a diagram showing the transition of "pH after the neutralization step" according to the elapsed time after starting the addition of sulfuric acid to the autoclave.

Modes for Carrying Out the Invention

[0020] Specifically, we developed a method to adjust the amount of limestone added by starting the addition of limestone to the pre-neutralization process 80 to 100 minutes after the start of sulfuric acid addition to the autoclave, adjusting the amount of limestone added in proportion to the amount of sulfuric acid added to the autoclave, and measuring the pH after the pre-neutralization process at least once every 30 minutes. The following will provide a detailed explanation comparing RUN1, 3, and 4, which correspond to the examples, with RUN2, which corresponds to the comparative example.

[0021] {Regarding the timing of starting limestone addition} Figure 2 shows the elapsed time after the start of sulfuric acid addition to the autoclave on the horizontal axis, and the ore slurry flow rate load to the autoclave {m} on the vertical axis. 3 / H} and the flow rate of limestone slurry to the pre-neutralization process {m 3 This shows the ratio of / H}. Runs 1, 3, and 4 show the addition of limestone starting 80-100 minutes after the start of sulfuric acid addition, while Run 2 shows the case where limestone addition started simultaneously with the start of sulfuric acid addition. In other words, in Run 2, limestone addition started before the acidic process liquid from the autoclave began to reach the pre-neutralization process, resulting in a state where excess limestone exists in the pre-neutralization reaction vessel.

[0022] Figure 3 shows the pH changes after the preliminary neutralization step in these cases. Runs 1, 3, and 4 start at an initial pH of around 2, and reach the target pH of 2.8-3.2 200-300 minutes after the start of autoclave acid addition. On the other hand, RUN2 started at an initial pH of 7 due to the influence of excess limestone. Although an attempt was made to reach the target pH by reducing the amount of limestone added between 100 and 200 minutes, this coincided with the time it took for the acidic process solution to reach its target pH after sulfuric acid leaching (130 to 200 minutes), making adjustment extremely difficult. As a result, the pH dropped to 1.2 to 1.3 between 400 and 500 minutes. Consequently, it took 800 minutes to return to the target pH, requiring significantly more adjustment time compared to RUNs 1, 3, and 4 (200 to 300 minutes).

[0023] If the addition of limestone is delayed beyond 80-100 minutes, pH adjustment will have to begin only after the acidic process solution has reached the pre-neutralization reactor. If this adjustment is delayed, the pH of the pre-neutralization process will swing significantly lower than the target value, and given the residence time in the reactors such as the pre-neutralization process, it will take several hours to recover the pH. For this reason, delaying the addition of limestone beyond 80-100 minutes is not appropriate. Furthermore, the leaching process S1 requires a considerable amount of time to allow the sulfuric acid to penetrate every corner of the nickel oxide ore and allow the reaction to proceed. Although this time can be shortened somewhat by setting the reaction conditions in the autoclave where the sulfuric acid is added to high temperature and high pressure, such as 220°C to 280°C and a pressure of 3,000 to 4,500 kPaG, at least 80 minutes are still required from the start of sulfuric acid addition to the autoclave until the reaction is complete and the preliminary neutralization process begins.

[0024] In other words, it is desirable to start adding limestone to the preliminary neutralization process between 80 and 100 minutes to minimize the amount of excess limestone in the neutralization reactor, while gradually increasing the amount of limestone added as the acidic process liquid arrives from the autoclave.

[0025] {Regarding the amount of limestone added} The amount of limestone added between 80-100 minutes after the start of limestone addition and 130-200 minutes after the preliminary neutralization step of the acidic process solution should preferably be the equivalent amount of limestone needed to neutralize the total amount of sulfuric acid added to the autoclave during the first 30 minutes after the start of sulfuric acid addition.

[0026] By implementing this control, pH changes can be sufficiently detected at a 30-minute pH measurement interval after the preliminary neutralization process, and a state can be created where even rapid pH changes can be noticed. If a limestone amount below this standard is added, the pH may drop rapidly within 30 minutes, which is not appropriate. If a limestone amount exceeding this standard is added, as shown in RUN2 of Figure 2, it will create a situation where the initial pH rises, which is also not appropriate.

[0027] Comparing the "pH adjustment time" under conditions RUN1, 3, and 4 according to the present invention with that of the conventional condition RUN2, it can be seen that a significant reduction in the required time has been achieved, as shown below. (1) RUN2 • Time required for pH adjustment when adding sulfuric acid to the autoclave and simultaneously supplying limestone to the preliminary neutralization process = 800 minutes. (2) RUN1, 3, 4 • When limestone is supplied 80-100 minutes after sulfuric acid is added to the autoclave, the time required for pH adjustment is 200-300 minutes. The time reduction effect in the comparison between (1) and (2) above is 800 - 200 minutes = 600 minutes, making it possible to reduce the time by approximately 10 hours. This time reduction also allows for a 600-minute reduction in the start of supplying the process liquid to the dezincification process, and thus shortens the start-up of subsequent processes, leading to improved productivity. [Explanation of symbols]

[0028] S01 Sorting process S02 Concentration process S0 Ore pretreatment process S1 Leaching process S2 Pre-neutralization process S3 Solid-liquid separation process S4 Neutralization process S5 Dezincification process S6 Nickel Recovery Process S7 Sulfide Removal Process S8 Final neutralization step

Claims

1. A method for controlling the pH of a preliminary neutralization step performed on an acidic process solution, which is the leachate obtained after sulfuric acid leaching, at the start of application of the high-pressure acid leaching method, A method for controlling the pH in a preliminary neutralization step performed on an acidic process liquid, which is a leachate obtained through sulfuric acid leaching, at the start of application of a high-pressure acid leaching method, characterized in that the pH control in the preliminary neutralization step is performed by adding a pH adjusting agent to the leachate in the preliminary neutralization step between 80 and 100 minutes after the start of sulfuric acid addition in the sulfuric acid leaching, the addition of the pH adjusting agent is performed on the leachate after the sulfuric acid addition has been carried out in an autoclave, and the amount of pH adjusting agent added between 80 and 100 minutes after the start of sulfuric acid addition and the time when the leachate reaches the preliminary neutralization step is an equivalent amount of pH adjusting agent that is equal to the total amount of sulfuric acid added in the first 30 minutes after the start of sulfuric acid addition to the ore slurry in the autoclave.

2. A method for controlling the pH of a pre-neutralization step performed on an eluate, which is an acidic process solution obtained through sulfuric acid leaching, at the start of application of the high-pressure acid leaching method according to claim 1, characterized in that the pH after the pre-neutralization step is measured at least once every 30 minutes, and the amount of pH adjusting agent added in the pre-neutralization step is adjusted based on the obtained pH measurement.

3. A method for controlling the pH of a preliminary neutralization step performed on an acidic process solution obtained through sulfuric acid leaching at the start of application of the high-pressure acid leaching method according to claim 1 or 2, characterized in that the pH adjusting agent is limestone.

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