Spain is the world's leading producer of mussel shell. Until recently, the thousands of tons of shells that were produced annually were an unbearable hindrance that ended in a set of controlled landfills or, in the worst case, uncontrolled, constituting a serious problem for the environment.
One solution to this environmental problem is the transformation of these mussel shells into calcium carbonate. This calcium carbonate serves as raw material for numerous industrial processes: obtaining quicklime (CaO), cement industry, paper industry, glass industry, stabilization of road bases, feed manufacture, fertilizers ... It is, in definitive, to take advantage of the values of calcium carbonate, of which the shell of the mussel is composed by 90%. The rest is silica and phosphates.
In the treatment plant, the first step is to remove the saline content of the raw material (shell of mussels and other bivalve molluscs) by means of a fresh water wash. The washed product is burned in a countercurrent rotary oven, at a temperature between 400 and 500 ° C, to remove moisture and organic matter. Finally, the product is milled to adapt the granulometry of the product to the requirements of each application.
The result of this process is the obtaining of 94% pure calcium carbonate.
The gases that come out of the calcination oven are at a temperature of about 200 ° C and contain approximately 4 g/Nm³ of total organic carbon and a high moisture content (16% vol.). The organic compounds emitted are, mainly, oils and sulfur compounds. This causes a serious problem of odors (defined as "burned hair odor") that makes its treatment essential if you want the process to be viable. In addition, these gases contain a large amount of dust.
Due to the low concentration of organic compounds and the high flow rate of gases to be cleaned (around 20,000 Nm³/h), the most suitable technique for this application is regenerative thermal oxidation (thermoreactor). But, before introducing the gases into the thermoreactor, it will be necessary to eliminate the dust they contain by means of a bag filter that supports temperatures of up to 240 ºC. The gases emitted have a total organic carbon content lower than 10 mg/Nm³ and are free of odors.
Bag filter and thermoreactor for cleaning 18,000 Nm³/h of gases coming from a plant for calcinating mussel shell.