In acetylene plants, carbide lime is generated as a byproduct. Carbide lime is further processed to make it useful in other industries and applications. Most acetylene manufacturing plants using the wet acetylene generating process, install a carbide lime treatment plant to process the raw material. Six main types of such plants can be manufactured depending on your requirements. These are expanded upon below.
Purpose of Treatment Plants
In the wet acetylene generation process, the solid content makes lime slurry too dilute for economical transportation. The solid content must be increased using one or more of the following methods:
- Lime Ponds
- Membrane Filtration
- Centrifugal Action
- Drying Lime Ponds
Types of Carbide Lime Treatment Plants
The following types of treatment plants are used to process carbide lime to make it commercially viable:
- Lime Ponds: Most lime ponds have been constructed as excavations with earthen-like perimeters. This is no longer considered a best practice. It is now preferable to have concrete-walled ponds. The process is defined below:a. As carbide lime slurry is pumped into a pond, the solids settle with time and water rises to the top.
b. After prolonged settling and natural evaporation, a solids content of 50% or more can be obtained.
- Decanting: Using this process, a consistency of 30 to 40% solids content can be achieved, depending on the time available for the settling process. The process is as follows:a. Decanting systems are normally a series of connected tanks, which receive the slurry from the acetylene generators.
b. The tanks allow the solids to settle while excess water is removed.
c. As the solids settle to the bottom of the tanks, the water accumulates at the top.
d. The water at the top can be decanted to a holding tank or pond for reuse.
- Membrane Filtration: Lime slurry can be concentrated by the membrane filtration process. Lime blocks with as little as 15% moisture can be formed using this process. The details are provided below:a. A filter system with polypropylene plates that operates at pressures of 11-14 bar (1100-1400 kPa, 160-203 psi) is commonly used in the membrane filtration process.
b. Solids are concentrated on the plates in block form.
c. The water that is removed is reused in acetylene generation or suitably treated and discarded.
d. The higher the temperature, the better the filtration and the lower the water content in the block.
- Filtration: a. Slurry can be pumped under pressure through a series of membranes held between plates in a filter pass.
b. This press is opened up periodically to allow the concentrated dry lime to drop into a vehicle for transportation off-site.
c. The water is recycled back to the generators, via holding tanks.
- Centrifugal Action: Slurry can be concentrated up to 60% solids by centrifuge. Mechanical thickeners can obtain a concentration of approximately 40% solids.
Dilute or concentrated slurry can be dried effectively by mixing with quicklime. The surplus water in the carbide lime slurry slakes the quicklime so that the percentage of solids of the resultant mixture is appreciably increased, even to the extent of achieving a commercially dry hydrate. The process is as follows:
a. This is accomplished in a slurry tank with a manually controlled discharge, a quicklime feeder and a mixing tank or hydrator.
b. The quicklime hydration develops considerable heat, which vaporizes some of the water and the volatile impurities of carbide lime.
c. The resultant hydrated lime is completely free from sulfide and other odors, which may be considered objectionable.
Carbide lime slurry can prove to be a valuable product when used in specific processes. For example, sewage sludge can be conditioned efficiently using carbide lime slurry. Hence, when the right process is used to process carbide lime, it can be beneficial to the plant owner. Instead of discarding the carbide lime, it can be sold to organizations that need lime slurry for their plant processes. This helps increase the revenue of the organization. Depending on your plant setup as well as your requirements, the suitable carbide lime treatment plant can be integrated with your acetylene plant.