CURRENT STATE OF OXIDATIVE PRECIPITATION OF COBALT AND STUDY OF FACTORS AFFECTING THE OXIDATIVE PRECIPITATION PROCESS

ABSTRACT

This study experimentally substantiates the effectiveness of the oxidative precipitation method for cobalt recovery from sulfate solutions obtained after cadmium removal by cementation. It was established that the use of sodium hypochlorite in combination with pH and temperature control ensures a high degree of cobalt extraction in the form of low-solubility Co(OH)₃. The influence of technological parameters (pH, temperature, reagent concentrations) on the efficiency of the process was analyzed, and side reactions reducing selectivity were identified. The optimal conditions for cobalt precipitation were determined to be: pH 4.4–4.7, temperature 75 °C, process duration 60 minutes, and a mass ratio of Co²⁺/NaOCl = 1:20, under which the cobalt recovery rate reached 86.1%, with cobalt content in the precipitate up to 55%. It was found that nickel is partially co-precipitated with cobalt, while zinc remains predominantly in solution. Potential approaches for improving process selectivity were proposed. The obtained results confirm the feasibility of applying this method for the processing of technogenic waste and the effective separation of cobalt and nickel ions in hydrometallurgical processes.

АННОТАЦИЯ

В работе экспериментально обоснована эффективность метода окислительного осаждения кобальта из сульфатных растворов, полученных после удаления кадмия цементацией. Установлено, что применение гипохлорита натрия в сочетании с контролем pH и температуры обеспечивает высокую степень извлечения кобальта в виде малорастворимого Co(OH)₃. Проведён анализ влияния технологических параметров (pH, температура, концентрации реагентов) на эффективность процесса, а также выявлены побочные реакции, снижающие его селективность. Определено, что оптимальными условиями для осаждения кобальта являются: pH 4,4–4,7, температура 75 °C, продолжительность 60 минут и массовое соотношение Co²⁺/NaOCl = 1:20, при которых степень извлечения кобальта достигает 86,1%, а содержание кобальта в осадке составляет до 55%. Установлено, что никель частично осаждается вместе с кобальтом, тогда как цинк преимущественно остаётся в растворе. Предложены возможные пути повышения селективности процесса. Полученные результаты подтверждают целесообразность применения данного метода для переработки техногенных отходов и эффективного разделения ионов кобальта и никеля в гидрометаллургических процессах.

Keywords: Cobalt solution, sodium hypochlorite, oxidative precipitation, Co²⁺/NaOCl, Co²⁺, Co³⁺, Co(OH)₃, calcium hypochlorite, chlorine, ozone.

Ключевые слова: Раствор кобальта, гипохлорит натрия, окислительное осаждение, Co²⁺/NaOCl, Co²⁺, Co³⁺, Co(OH)₃, гипохлорит кальция, хлор, озон.

Introduction. Approximately 70% of all cobalt produced worldwide is extracted using hydrometallurgical methods. In the first stage, the concentrate undergoes reductive roasting, followed by either ammonia or acid pressure leaching. After cobalt is transferred into solution, the solution is subsequently purified from harmful impurities and subjected to electrolysis.

With each passing year, the increasing demand for metallic cobalt leads to a reduction in primary resource availability. Under these conditions, one of the priority tasks is to maximize the efficiency of cobalt extraction from secondary resources.

At present, there are a number of secondary sources from which cobalt can be extracted, including:

• Tailing materials generated during the flotation of cobalt ores;
• Intermediate and technogenic wastes arising during the production of copper, nickel, and zinc;
• Spent catalysts and used batteries.

The cobalt content in these intermediate and technogenic wastes often significantly exceeds its concentration in primary cobalt ores.

Literature Review. Cobalt can exist in two oxidation states—Co²⁺ and Co³⁺—which differ in their physicochemical properties in solution. Upon oxidation of divalent cobalt (Co²⁺) to trivalent cobalt (Co³⁺), as shown in Figure 1, the concentration of cobalt in solution decreases significantly. The hydrolysis reaction and precipitation of cobalt in the form of Co(OH)₃ occur under conditions of high acidity. According to the data presented in Figure 1, the pH values at which hydrolysis and precipitation of Co³⁺ and Zn²⁺ ions are possible at 25 °C are –1.08 and 5.95, respectively. Thus, for cobalt to be present in the solution as Co(OH)₃, while zinc remains in its ionic form Zn²⁺, the pH range must lie between –1.08 and 5.95.

Moreover, the reduction of cobalt concentration in solution is achieved by its oxidation and subsequent precipitation, since the solubility of Co(OH)₃ is extremely low (Kₛ = 2 × 10⁻⁴⁴). Therefore, the oxidative precipitation method, based on the conversion of Co²⁺ to Co³⁺ followed by the separation of cobalt and zinc, is highly effective.

Oxidizing agents such as H₂O₂ [1], NaClO, NaClO₃, KMnO₄, Na₂S₂O₈, and others can be used. Studies by Hong and Guo [2, 3] examined the relationship between the redox potential of various oxidizers and the pH of the medium, the results of which are presented in reactions 1–6 and illustrated in Figure 2.

The results showed that with increasing pH, the oxidation potential of most oxidizers decreases (with the exception of the S₂O₈²⁻ ion). Numerous scientific works have investigated the application of Na₂S₂O₈ as a precipitating agent for cobalt (reaction 1.7) [4, 5, 6].

To regulate the pH of the medium for effective cobalt recovery from solution, sodium hydroxide is often used. In this case, cobalt precipitates (with a content exceeding 50%), while zinc remains in solution. The resulting solution, containing zinc and less than 1 mg/L of cobalt, is directed to the zinc electrorefining process.

Additionally, Guler and Seyrankaya [7] studied the oxidative precipitation of cobalt using ammonium persulfate. Their research examined the effects of pH, temperature, and oxidizer concentration on process efficiency.

Experimental studies have shown that, depending on the concentration of the precipitating agent, the degree of cobalt recovery from solution into the precipitate ranges from 70% to 90%. In this case, cobalt is predominantly precipitated in the form of CoOOH. In an alkaline medium, Co(OH)₃ is oxidized by air to form the brown compound CoO(OH); to enhance this process, sodium hypochlorite or hydrogen peroxide is used [8].

Co(OH)₃ + 3H⁺ + e = Co²⁺ ε = 1.870 – 0.18 pH                                                              (1)

H₂O₂ + 2H⁺ +2e = 2H₂O ε = 1.78 – 0.0591 pH                                                              (2)

 + 8H⁺ + 5e = Mn²⁺ + 4H₂O ε = 1.742 – 0.095 pH                                                    (3)

 + 6H⁺ + 6e = Cl^- + 3H₂O ε = 1.45 – 0.0591 pH                                                    (4)

ClO ^- + 2H⁺ + 2e = Cl^- + H₂O ε = 1.715 – 0.0591 pH                                                  (5)

S₂ + 2e = 2 ε = 2.080                                                               (6)

S₂  + 2Co²⁺ + 6H₂O = 2 + 2Co (OH)₃↓ + 6H⁺                                             (7)

3O₃ + 2Co²⁺ + 3H₂O = 3O₂ + 2Co (OH)₃↓                                                          (8)

Ozone is also used as a pure oxidizing agent, and the reaction is accompanied by the formation of oxygen and oxides [9]. A number of scientific studies have been conducted on the use of ozone (reaction 8) [10, 11, 12, 13]. Using these methods, up to 98% of cobalt can be oxidatively precipitated from solution, with the optimal pH for cobalt recovery determined to be 4.0 [14]. However, the high cost of ozone limits its widespread use. In addition to ozone, the use of KMnO₄ [15, 16] and NaClO [17, 18] for the separation of cobalt and zinc in solution has also been extensively studied.

Table 1.

Standard electrochemical potentials (E⁰) of Several important oxygen-containing oxidizers depending on the pH of the medium

Object and Methodology. Following the separation of cadmium from a sulfate solution using zinc powder, a cobalt-enriched solution was obtained. The composition of this cobalt-containing solution is shown in Table 1.

The volume of experimental work was 2 liters, conducted in a heat-resistant glass reactor equipped with a stirrer. The optimal process temperature was maintained by placing the reactor on an electric heater. The pH of the solution during the experiments was measured using a pH meter (pH-150MI) with an electrode set (ESK-10603/7 K80.7, TDL-1000-06, ShU-05). The concentrations of metal ions in the solution, as well as the metal content in the cobalt-containing precipitate, were determined using an atomic absorption spectrometer. A sieve set (Rotap) was used to measure particle size. The chemical composition of the raw materials and products was analyzed by X-ray fluorescence spectroscopy.

After cadmium was precipitated by cementation, the resulting cobalt-containing solution was first adjusted to pH = 1. The solution was then transferred to a heat-resistant glass reactor and heated with an electric heater to 75 °C. To oxidize Co²⁺ to Co³⁺, 50 mL/L of sodium hypochlorite was added to the solution [19]. After some time, 25 mL/L of NaOH was added to precipitate cobalt in the form of Co(OH)₃, and the solution was stirred with a mixer to accelerate the process. The duration of the process was 60 minutes. The research results are presented in Table 2.

Table 2.

Chemical composition of Products obtained after cementation

After the oxidative precipitation process, the solution is filtered, and the cobalt-rich precipitate is analyzed using atomic absorption spectrometry to determine the impurity content.

Results and Discussion. The hypochlorite method is based on the oxidation of divalent cobalt to trivalent cobalt. Since nickel is less stable in solution than cobalt, when sodium hypochlorite is added to sulfate or chloride solutions, cobalt hydroxide precipitates first. However, nickel hydroxide also co-precipitates with cobalt, so complete separation of cobalt and nickel requires repeated application of the oxidative precipitation method. This method is considered effective primarily for solutions containing cobalt and nickel ions.

The precipitation of cobalt as a hydroxide using the hypochlorite method is not a simple reaction described by a single equation. The formation of trivalent cobalt hydroxide depends on many factors, and other reactions may also occur during the process.

The industrial application of this method proceeds according to the following reactions [20]:

2CoSO₄ + NaClO + 4NaOH + H₂O = 2Co (OH)₃ +2Na₂SO₄ + NaCl,                                       (9)

2CoSO₄ +3NaClO + NaСl + 3H₂O = 2Co (OH)₃ +2Na₂SO₄ + 2Cl₂,                                     (10)

2CoSO₄ + 2NaClO + 2NaOH + 2H₂O = 2Co (OH)₃ +2Na₂SO₄ + Cl₂,                                   (11)

2CoSO₄ + NaClO + 2NaOH + 3H₂O = 2Co (OH)₃ +Na₂SO₄ + H₂SO₄ +NaCl.                           (12)

Based on the above reactions, simultaneous oxidation of cobalt with its precipitation as a hydroxide occurs, along with the release of chlorine and the formation of acid. Nickel, which precipitates together with cobalt, subsequently re-dissolves back into the solution:

Ni (OH)₃ + CoSO₄ = Co (OH)₃ + NiSO₄,                                                             (13)

2Ni (OH)₃ + 3H₂SO₄ + 2NaCl = 2NiSO₄ +Na₂SO₄ +Cl₂ + 6H₂O.                                     (14)

The dissolution of nickel hydroxide never occurs completely. In addition to the aforementioned reactions, the catalytic decomposition of sodium hypochlorite involving cobalt and nickel hydroxides also plays an important role. Depending on the complexity of the process, the composition of the cobalt-rich precipitate depends on several factors: temperature, alkaline environment, solution concentration, the initial cobalt-to-nickel ratio in the solution, reagent feed rate, and other conditions (see Table 3).

Table 3.

Chemical composition of the Product obtained by the oxidative precipitation method using hypochlorite

The conducted studies showed that calcium hypochlorite can also be used as an oxidative-precipitating agent. Additionally, direct chlorine dosing is possible instead of sodium hypochlorite. Theoretically, besides chlorine and sodium hypochlorite, peroxides of titanium, lead, and other metals can be used as oxidizers. In particular, complete precipitation of cobalt as a hydroxide is possible by boiling a sulfate solution with lead peroxide. However, due to the high cost of other oxidizers, corresponding experiments were not conducted.

Conclusion. Sodium hypochlorite was used for the selective separation of cobalt and nickel in solution and for cobalt precipitation at a temperature of 75 °C and a pH range of 4.4–4.7. The duration of the oxidative precipitation process was 60 minutes. The optimal Co²⁺/NaOCl ratio for converting Co²⁺ to Co³⁺ was found to be 1:20. Under these process parameters, the cobalt recovery rate into the precipitate reached 86.1%. Increasing the temperature improved the filtration of the cobalt-rich precipitate. Raising the pH enhanced the oxidative precipitation of cobalt but simultaneously caused oxidation of nickel. To prevent the oxidation of Ni²⁺, it is necessary to strictly control the amount of sodium hypochlorite added to the solution.

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