The Chemistry of Limestone Treatment: Limestone is an alkaline agent with the ability to neutralize, or partially neutralize strong acids. The neutralization process occurs when strong acids, in intimate contact with limestone chips, react with Calcium Carbonate (CaCO3, the primary constituent of limestone) to form water, carbon dioxide and calcium salts. The following depicts the neutralization of hydrochloric acid by limestone.
CaCO3 + HCl ==> CaCl2 + CO2
Calcium Carbonate + Hydrochloric acid YIELDS Calcium chloride and Carbon dioxide
Due to the liberation of CO2 (resulting in the formation of carbonic acid) limestone does not have the ability to raise the pH of an effluent stream above 7.0. Due to the solubility of these products even a pH of 7.0 is not usually practical. Considering that CO2 is only liberated as a result of the neutralization of an acidic solution, limestone is not effective in neutralizing alkaline solutions. In fact, limestone will not at all contribute to the neutralization of alkaline materials.
The pH neutralization process occurs as strong acids react with the calcium carbonate in the limestone through intimate contact with small limestone chips. A high surface area is important as is sufficiently long contact time. The reaction is not instantaneous and requires sufficient time. Additionally the acidic solutions must be in intimate contact with the limestone. This leads to one of the biggest problems with the use of limestone as an effective treatment process, the coating of the available limestone surface area with precipitated debris.
One of the byproducts of the neutralization process is calcium salts. Calcium salts tend to be very insoluble in water. This results in the precipitation of salts that deposit on the limestone chips forming very effective coatings. Once coated with precipitated products, the limestone is rendered useless and must be replaced. Other solids and organic materials that are suspended in the waste stream will often come out as a result of mechanical filtration thereby contributing to the coating of the limestone chips.
The chemistry stands, limestone will neutralize strong acids, at least partially. However, in practice limestone cannot be relied upon to neutralize acidic waste streams. The chips are very easily rendered useless from coating by the highly insoluble calcium salts and other products that precipitate out of the effluent stream. Additionally the design of the system cannot inherently guarantee the treatment of the effluent stream. There are no controls that regulate the addition of acidic or alkaline neutralizing agents. The system designer simply hopes that sufficient alkaline products dissolve into solution to achieve the desired treatment.
For these reasons Digital Analysis cannot endorse, encourage, or recommend the use of limestone for the purpose of treatment. Limestone certainly does not hurt the process and it can be argued that it does help. However, if the limestone contact tank is going to displace an active chemical treatment stage or system then we must strongly discourage the use of this method. We will never use, under any circumstances, limestone exclusively. In most states around the nation limestone is not an acceptable treatment method and will not be accepted by the local controlling authority. Limestone is an effective way to line the containment area of a bulk acid storage facility, which safeguards against the accidental discharge of a hazardous chemical. However, as a primary treatment agent it cannot be considered.
Disadvantages of Limestone as a Neutralizing Agent...
- Does not treat alkaline waste streams.
- No controls to regulate or oversee the process.
- No guarantee that the system will treat an acidic stream.
- Limestone chips are easily coated rendering them useless.
- Periodic replacement is necessary; this is a very difficult and expensive job.
- Adds to the suspended solids (TSS) and dissolved solids (TDS) loading.
So what is the alternative to limestone pH Adjustment?
Active chemical treatment systems that use strong acids / strong bases (such as sulfuric acid and caustic) are the safe, proven method for treating acid and basic wastes streams. All of the pH Adjustment systems manufactured by Digital Analysis Corp. utilize active chemical treatment which guarantees the complete pH Adjustment of any industrial process. Our pHASE, batchTREAT and labTREAT family of pH adjustment systems are designed to handle any flow from any source at anytime. Click on the System Example button below to learn more.