In the alumina production process, the alkali content in "secondary water" is an unavoidable key indicator. It is not only related to whether production is compliant, but also directly affects the cost and efficiency of enterprises. Many practitioners are confused: what range should the alkali content in secondary water be controlled within to be the most appropriate? Today, we will discuss this topic in detail from three dimensions: industry standards, actual production, and cost balance.

The industry has clear standards, with well-defined classifications.

From the perspective of alumina industry regulations, the alkali content in secondary water has clear "pass lines" and "warning lines":

- **Qualified (alkali-free)**: Alkali content below 30 mg per liter. Meeting this standard means the secondary water has high recycling value and complies with the basic requirements of the industry.
 
- **Alkali-containing**: Alkali content ranging from 30 to 50 mg per liter. At this level, although the secondary water does not reach the "seriously excessive" threshold, there are signs of alkali liquor loss, requiring attention to adjustments in the production process.
 
- **Alkali leakage**: Alkali content exceeding 50 mg per liter, which constitutes obvious waste of alkali liquor. This not only increases raw material consumption but may also affect the stable operation of subsequent processes, and must be handled promptly.

The lower the better? Cost is a hurdle to overcome.

Many people believe that "the lower the alkali content in secondary water, the better" — and theoretically, this is true, as it means less alkali loss and higher resource utilization. However, in actual production, "lower" often translates to "more expensive."

The alkali content in secondary water mainly depends on the diameter and height of the separation chamber:

- To control the alkali content below 20 mg per liter, or even below 10 mg per liter, the separation chamber needs to be made larger in diameter, which directly increases the manufacturing cost and installation difficulty of the equipment.

- Moreover, pursuing extremely low alkali content will lead to a stepped increase in costs, which may exceed a reasonable input-output ratio.

Around 25 mg: the balance point between ideal and economy.

Based on industry practices and cost accounting, we recommend controlling the alkali content in secondary water at around **25 mg/L**. This value offers two key advantages:

1. **Compliance Assurance**: Significantly below the 30 mg/L threshold, it eliminates risks of "alkali carryover" or "alkali leakage," ensuring full compliance with production standards. 
2. **Economic Efficiency**: Avoids excessive enlargement of the separation chamber, minimizing alkali loss while controlling costs. This balances production efficiency with resource conservation.

Demister: Don't overestimate its auxiliary role.

Some may ask, "Can using a better demister reduce the alkali content?" 

It is undeniable that the performance of a demister has a certain impact on the alkali content in secondary water, but it is only an auxiliary factor. What truly plays a decisive role is the design of the separation chamber’s diameter and height. Rather than over-relying on the demister, it is better to optimize the core parameters of the separation chamber—this is the key to controlling the alkali content in secondary water.

Conclusion

Controlling the alkali content in secondary water during alumina production is a "balancing act" — it requires not only adhering to the bottom line of industry standards but also keeping a clear account of costs and benefits. An alkali content of around 25 mg/L may not be the "lowest" option, but it is the "optimal solution" that better fits actual production needs. 

If you are still struggling with secondary water indicators, you might as well start with the design of the separation chamber to find the "balance point" that suits your own production line. After all, what is suitable is the best.