Treatment of Cyanide in Gold Mine Tailings with Ferrous Sulfate

Introduction

Gold mine tailings often contain high levels of cyanide, which is highly toxic and poses a significant threat to the environment and human health. The improper disposal of these tailings can lead to the contamination of soil, water sources, and air. Therefore, effective treatment methods for removing cyanide from gold mine tailings are crucial. Among various treatment options, ferrous sulfate has emerged as a commonly used and cost-effective reagent. This article will delve into the use of ferrous sulfate for treating cyanide in gold mine tailings, covering aspects such as reaction mechanisms, operating conditions, practical applications, and advantages.

Reaction Mechanisms

Formation of Ferrocyanide Complexes

Ferrous sulfate (FeSO₄) contains ferrous ions (Fe²⁺). When ferrous sulfate is added to gold mine tailings containing cyanide, the ferrous ions react with free cyanide ions (CN⁻) in the tailings. The primary reaction is the formation of ferrocyanide complexes, which can be represented by the chemical equation: Fe²⁺ + 6CN⁻ → Fe(CN)₆⁴⁻. This reaction is the initial step in the process of using ferrous sulfate to treat cyanide - containing tailings.

Generation of Prussian Blue

Under certain conditions, when an excess of ferrous sulfate is added to the cyanide - containing solution, a further reaction occurs. Cyanide is converted into an insoluble precipitate known as ferric ferrocyanide, which is commonly called Prussian blue. The chemical reaction for the formation of Prussian blue is complex and can be simplified as follows: after the formation of ferrocyanide complexes, additional ferrous ions react with Fe(CN)₆⁴⁻ to form Fe₄(Fe(CN)₆)₃. This insoluble precipitate is beneficial as it effectively reduces the concentration of free cyanide in the tailings, making the tailings less toxic.

However, it should be noted that the reaction is not always straightforward. Prussian blue can exist in different forms under various solution conditions. One such form is “soluble Prussian blue,” represented by MFeⅢ(FeⅡ(CN)₆) (M = K or Na), which forms a colloidal solution with water. Additionally, precipitation and oxidation reactions involving ferrous hydroxide also play a role in the overall process.

Operating Conditions

pH Value

The pH value of the solution significantly affects the reaction between ferrous sulfate and cyanide. The optimal pH range for the reaction is typically between 5.5 and 6.5. In this pH range, the reaction between ferrous ions and cyanide is the most rapid and thorough. When the pH is too low (below 4), the ferrocyanide ions become unstable. They can react to form pentacyano - iron (II) complexes (Fe(CN)₅H₂O)³⁻, which are then rapidly oxidized to ferricyanide ions (Fe(CN)₆³⁻). On the other hand, when the pH is higher than 7. the insoluble Prussian blue can decompose, forming ferrocyanide ions and various insoluble iron oxides, which is unfavorable for the removal of cyanide.

Dosage of Ferrous Sulfate

The dosage of ferrous sulfate needs to be carefully controlled. It should be determined according to the content of cyanide in the tailings and the quality of the water. If the dosage is too low, it may not be possible to completely remove the cyanide. Conversely, if the dosage is too high, it will not only cause waste but may also introduce new pollutants. Through experiments, it has been found that the optimal molar ratio of Fe to CN⁻ is 0.5. This ratio ensures efficient removal of cyanide while minimizing the use of ferrous sulfate.

Mixing and Sedimentation Time

Adequate mixing is essential to ensure that ferrous ions and cyanide can fully contact and react. Sufficient mixing time allows for a more homogeneous distribution of reactants in the solution, promoting the reaction rate. After the reaction, an appropriate sedimentation time is required. This time is beneficial for the formation of stable precipitates and the reduction of cyanide concentration in the effluent. The specific mixing and sedimentation times can vary depending on the actual situation, such as the concentration of cyanide in the tailings and the equipment used for treatment.

Practical Applications

Case Study of a Gold Mine Tailings Treatment Project

In a certain gold mine tailings treatment project, a combined process of ferrous sulfate and lime was adopted. First, an appropriate amount of lime was added to the tailings water to adjust the pH value to the appropriate range (usually 5.5 - 6.5). This step helps to promote the transformation and precipitation of cyanide. Subsequently, ferrous sulfate was added to the water, and through stirring, the ferrous ions reacted fully with the cyanide to form Prussian blue and other precipitates. Finally, after precipitation and filtration steps, the purified wastewater was obtained. The treated tailings met the relevant environmental standards, reducing the environmental risk significantly.

Combination with Other Reagents

Ferrous sulfate is often used in combination with other reagents to improve the treatment effect. For example, it is commonly used in conjunction with high - molecular flocculants such as polyacrylamide. Polyacrylamide can enhance the aggregation of precipitates, making the sedimentation process more efficient. This combined treatment process not only effectively removes harmful substances in the tailings but also reduces the treatment cost and improves the treatment efficiency. By optimizing the dosage and addition sequence of different reagents, better treatment results can be achieved.

Advantages of Using Ferrous Sulfate

Cost - effectiveness

Ferrous sulfate is relatively inexpensive compared to some other reagents used for cyanide treatment. Its wide availability in the market makes it an attractive option for gold mining companies. Using ferrous sulfate can significantly reduce the cost of tailings treatment, especially for large - scale gold mines that produce a large amount of tailings. This cost - effectiveness is crucial for the sustainable operation of gold mining enterprises.

Simplified Treatment Process

The treatment process using ferrous sulfate is relatively simple. After adding ferrous sulfate to the tailings and adjusting the appropriate reaction conditions, the subsequent separation and precipitation steps are relatively straightforward. In some cases, the wastewater treated with ferrous sulfate does not require complex pre - separation steps before proceeding to the next treatment process, which saves reaction units and simplifies the overall treatment process. This simplicity also makes it easier for operators to control and manage the treatment process.

Challenges and Future Perspectives

Environmental Impact of By - products

Although ferrous sulfate treatment can effectively remove cyanide from gold mine tailings, the by - products generated during the process, such as certain iron - containing precipitates, may also have potential environmental impacts. For example, if not properly disposed of, these precipitates may release iron ions or other substances into the environment over time. Future research is needed to explore more effective ways to handle these by - products to minimize their environmental footprint.

Optimization of Treatment Conditions for Different Tailings

Gold mine tailings can vary significantly in composition and properties from one mine to another. The current optimal treatment conditions for ferrous sulfate, such as pH value, dosage, and reaction time, may need to be further optimized for different types of tailings. More in - depth research is required to develop a more flexible and adaptable treatment process that can be applied to a wider range of gold mine tailings, improving the overall efficiency and effectiveness of cyanide treatment.

In conclusion, ferrous sulfate is a valuable reagent for treating cyanide in gold mine tailings. By understanding its reaction mechanisms, optimizing operating conditions, and exploring practical applications, it can play a crucial role in reducing the environmental impact of gold mining activities. However, continuous research and improvement are still needed to address the challenges associated with this treatment method and to make the gold mining industry more sustainable.