Removal of Fe²⁺ And Mn²⁺ by Ozone Micro-Nano Bubble Technology in Zinc Sulfate Production Process

Removal of Fe²⁺ and Mn²⁺ by Ozone Micro-Nano Bubble Technology in Zinc Sulfate Production Process

In the production process of zinc sulfate, the presence of iron and manganese can seriously affect the purity of the product. Traditional chemical treatment methods are costly and prone to introducing new impurities. However, the cutting-edge technology combining micro-nano bubble technology with ozone oxidation offers a new possibility to solve this problem. This article will delve into the effect of this technology on the removal of Fe²⁺ and Mn²⁺ in the production of zinc sulfate and the related influencing factors.

01 Problem Background

In the production process of zinc sulfate crystallization, the presence of iron and manganese acts as a "stumbling block", significantly reducing the purity of MgSO4·7H₂O products. Traditional chemical treatment methods, such as using strong oxidants like sodium persulfate or hydrogen peroxide to oxidize Fe²⁺ and Mn²⁺ in the zinc sulfate solution and then removing them through precipitation and filtration, can solve the problem to some extent, but they are costly and prone to introducing new impurity ions. This undoubtedly poses a huge challenge for zinc sulfate production enterprises.

Ozone, as a strong oxidant, performs well in removing organic and inorganic compounds from drinking water and wastewater. It is simple and convenient to oxidize, does not introduce impurities, and is convenient for subsequent operations. Micro-nano bubble technology, as a cutting-edge technology at home and abroad, is widely used in multiple fields such as medicine, environment, and mineral processing. Micro-nano bubbles have unique characteristics such as long existence time, high interface ζ potential, and high mass transfer efficiency. Moreover, after breaking, they can generate hydroxyl radicals with stronger oxidation performance than molecular ozone. Combining ozone with micro-nano bubbles is expected to provide a new solution for the removal of iron and manganese in zinc sulfate production.

02 Technical Principles

Micro-nano bubbles generally refer to bubbles with a diameter less than 50 µm but larger than the nanoscale. Due to their small size, they exhibit many characteristics distinct from ordinary bubbles. They have a longer existence time and can remain in water for a longer period, thus having more time to react with substances in the water; they have a high interface ζ potential, which gives them a stronger adsorption capacity when in contact with other substances; they have a high mass transfer efficiency and can more rapidly transfer components from the gas to the water. Additionally, when micro-nano bubbles burst, they generate hydroxyl radicals with stronger oxidation performance than molecular ozone, further enhancing their oxidation capacity.

In the current research, it has been found that combining micro-nano bubble generation devices with other technical means, such as coagulation sedimentation processes and strong oxidation technologies, can effectively enhance the pollution removal capacity of micro-nano bubbles. In this study, ozone was combined with micro-nano bubbles, taking advantage of the strong oxidation property of ozone and the unique characteristics of micro-nano bubbles, to act on the process water with high concentrations of Fe²⁺ and Mn²⁺ generated during the production of zinc sulfate, with the aim of removing iron and manganese. This combination not only fully leverages the advantages of both, but also provides an innovative approach to addressing the iron and manganese issues in zinc sulfate production.

03 Experimental Design

The process of micro-nano ozone oxidation of iron and manganese is as follows: The raw solution in the oxidation tank flows and circulates in the micro-nano generator, where oxidation reactions occur. O₃ is prepared by an ozone-oxygen integrated machine, and oxygen is supplied by an oxygen generator. It enters the reactor through a pipeline connected to the micro-nano generator and is transformed into micro-nano bubbles. CaCO₃ is directly added to the oxidation tank. Meanwhile, cooling water pipes are used to cool the interior of the micro-nano generator to control the reaction temperature. For each experiment, 10L of raw solution is added to the oxidation tank, maintaining the gas source input flow at approximately 2L/min, and corresponding valve connections and parameter settings are made according to different gas source conditions. Every 15 minutes, about 10mL of sample solution is taken for analysis of Fe²⁺ and Mn²⁺ concentrations, and CaCO₃ is added to the oxidation tank. After the Fe²⁺ and Mn²⁺ in the raw solution are completely oxidized, alkali is added for precipitation, followed by filtration treatment.

04 Analysis of Experimental Results

In terms of the influence of gas sources, the experimental results show that using ozone as an external gas source, the oxidation effect of Fe²⁺ and Mn²⁺ is significantly better than that of air and oxygen. The oxidation-reduction potential of ozone is 2.07V, second only to fluorine, while the oxidation-reduction potential of dissolved oxygen is only 1.23V. The oxidation-reduction potential difference between O₂ and Mn²⁺ is 0.22V, and that between O₃ and Mn²⁺ is 1.06V. This indicates that the ability of dissolved oxygen to oxidize Fe²⁺ and Mn²⁺ is much lower than that of ozone. Additionally, the solubility of ozone is much greater than that of oxygen. At normal temperature and pressure, the solubility of ozone in water is 13 times that of oxygen. Therefore, using ozone as an external gas source for the oxidation of Fe²⁺ and Mn²⁺ is more ideal, and the concentrations of Fe²⁺ and Mn²⁺ can be reduced to 0.5mg/L and 0.1mg/L respectively, meeting the requirements for zinc sulfate production.

Temperature also has a certain impact on the removal effect of iron and manganese. Without cooling, the temperature rises at a relatively fast rate, with the highest temperature around 24℃. With cooling, the higher temperature is around 18℃. Under cooling conditions, the oxidation rate of iron and manganese is relatively fast, with Fe²⁺ completely removed in about 16 minutes and Mn²⁺ completely removed in about 70 minutes. Without cooling, the oxidation rate of iron and manganese is relatively slow, with Fe²⁺ completely removed in about 20 minutes and Mn²⁺ completely removed in about 80 minutes. This is because higher temperatures have a significant impact on the stability of micro-nano bubbles and the solubility of ozone, causing a large number of micro-nano bubbles to burst, accelerating the self-decomposition rate of ozone, reducing the solubility of ozone in water, and ultimately leading to a decrease in the oxidation rate.

05 The Role of CaCO₃

In the reactions of Fe²⁺ and Mn²⁺ with ozone, H⁺ ions are present in the oxidation products. Adding alkaline substances can promote the oxidation reaction. CaCO₃ is selected as the added material due to its low cost, low solubility, and no introduction of impurities. In the intermittent addition experiments of CaCO₃, the results show that the addition of CaCO₃ significantly promotes the oxidation of Mn²⁺. Without adding calcium carbonate, manganese is completely oxidized within 75 minutes. When CaCO₃ is added at a relative dosage of 0.44 g/L every 25 minutes, manganese is completely oxidized within 60 minutes. When the addition intervals of CaCO₃ are 10 minutes and 15 minutes, the oxidation rates are basically the same, and the oxidation time is approximately 50 minutes. The faster the addition frequency, the greater the increase in oxidation rate.

At the same time, the experiments also found that the addition of calcium carbonate reaches saturation after a certain limit. The intermittent addition effect is better than one-time addition, and less CaCO₃ is consumed. When CaCO₃ is added at a relative dosage of 0.44 g/L every 15 minutes, the total effective addition of CaCO₃ is 17.6 g, and the complete removal of iron and manganese takes about 50 minutes. When 80 g of CaCO₃ is added at one time, the complete removal of iron and manganese also takes about 50 minutes. The former CaCO₃ dosage is only 22% of the latter. This indicates that intermittent addition of CaCO₃ is a more economical and efficient method.