Adding lime in the process, apart from the formation of calcium sulfate compound in the neutralization stage with sulfuric acid,if have the negative effect on the precipitation process of metals from the leach solution?
One other reason why lime is beneficial in cyanide leaching is when activated carbon is used for gold recovery. The presence of calcium ions encourages the take-up of gold onto the carbon.
To remove the gold (elution), caustic soda is often used to reverse this process (ZADRA process).
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Lime (calcium oxide, CaO) and caustic soda (sodium hydroxide, NaOH) are commonly used alkaline reagents in cyanide leaching operations, particularly in gold and silver mining. Both reagents serve to increase the pH of the leach solution, creating a favorable environment for the dissolution of precious metals. However, there are differences in their performance and considerations for their use:
### Lime (Calcium Oxide, CaO):
1. **Alkalinity Source:**
- Lime provides alkalinity to the leach solution by reacting with water to form hydroxide ions (OH⁻). This increase in pH facilitates the dissolution of gold and silver.
2. **Carbonate Formation:**
- Lime may contribute to the formation of carbonate ions (CO₃²⁻) in the leach solution, especially if carbon dioxide is present. Carbonate formation can affect the stability of the solution.
3. **Slaking Process:**
- Lime is typically added as a slurry (slaked lime or milk of lime), and the slaking process is essential to ensure proper dispersion and reactivity.
4. **Cost-Effective:**
- Lime is often considered more cost-effective than caustic soda, making it a preferred choice in many cyanide leaching operations.
5. **Precipitation of Impurities:**
- Lime can assist in the precipitation of certain impurities, such as heavy metals, which may be present in the ore or formed during leaching.
### Caustic Soda (Sodium Hydroxide, NaOH):
1. **Alkalinity Source:**
- Caustic soda provides alkalinity by dissociating into hydroxide ions (OH⁻) in solution. It directly contributes to the increase in pH.
2. **No Carbonate Formation:**
- Unlike lime, caustic soda does not contribute to carbonate formation, which can be advantageous for maintaining a stable leach solution.
3. **Ease of Handling:**
- Caustic soda is a liquid and is generally easier to handle and feed compared to lime, which is often delivered in the form of dry powder or slurry.
4. **pH Control:**
- Caustic soda allows for more precise control over pH due to its direct contribution of hydroxide ions without the complicating factor of carbonate formation.
5. **No Slaking Process:**
- Unlike lime, caustic soda does not require a slaking process, simplifying the handling and addition process.
### Considerations:
1. **Cost:**
- Lime is typically more cost-effective than caustic soda, which may influence the choice of reagent based on economic considerations.
2. **Solution Stability:**
- Caustic soda may offer better solution stability by avoiding carbonate formation, which can lead to precipitation and scale formation.
3. **Impurity Precipitation:**
- Lime may be preferred if impurity precipitation is desirable, but this needs to be balanced against potential carbonate-related challenges.
4. **Handling and Safety:**
- Caustic soda, being a liquid, may have advantages in terms of ease of handling and safety compared to lime dust.
Ultimately, the choice between lime and caustic soda depends on factors such as cost, solution stability, impurity precipitation requirements, and ease of handling based on the specific conditions and goals of the cyanide leaching operation. It is essential to consider the specific characteristics of the ore, the desired leaching conditions, and economic factors when selecting the alkaline reagent.
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