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Waste disposal in the Chinese cement industry: part two

World Cement,

Waste disposal centre concept

There are thousands of hazardous and non-hazardous materials and it is impossible to find one total solution for all waste treatment due to the different specifications of each material. Based on extensive knowledge and experience in the cement industry, the author recommends building a cheaper waste disposal centre based on ‘raw materials grinding process, pyroprocess and cement grinding process’ in an existing cement plant. This could then be benchmarked to other cities or places in order to find the cheapest way to keep the cement industry moving forward in an environmentally-friendly way.

Raw grinding process

In the author’s experience, there are hundreds of waste materials that could be used in the raw mix process, such as, coal gangue, mining waste, Pb slag, ash with higher moisture, furnace slag with high moisture, bigger size slag, Sb slag, waste bauxite and so on. In order to open the topic up for wider discussion, this article will share the author’s experience in using Pb slag to replace iron ore in the production of OPC and coal gangue to partially replace clay in the production of white cement.

Pb slag to replace iron ore

Based on the results of 10 months testing in the research centre, using Pb slag to replace iron ore achieved an ISO 28-day compressed strength increase of 5.3 MPa and standard coal consumption reduction of 1.75 kg/t clinker.

Process measurements

Because the hardness of Pb slag is very high and the chemical component is fluctuating, the following process measurements were taken.

To avoid the reduction of vertical mill output, an old tube mill was used (1.83 m dia. x 7.2 m long) and small modifications were made to grind Pb slag separately. Because the particle size is less than 30 mm, 50 mm ~ 60 mm grinding ball were added in the first chamber of mill and 20 ~ 30 mm grinding ball were added in the second chamber of mill. The final fineness is 0.8% R 0.08 mm. It is necessary to have this degree of fineness to ensure it is more active during the pyroprocess.

It is necessary to be aware of blending. The Pb slag heap is huge. Slag was taken from 20 points and was pre-blended before going into storage, where it was stacked in at least 50 layers stacking and reclaimed for grinding. The final powder produced feeds into the bucket elevator of the raw meal silo to ensure the stable raw meal feed into the preheater cyclones.

Coal gangue to replace white clay

The heating value of coal gangue from Mine B is 1000 ~ 1250 kcal/kg.

Grinding coal gangue in an individual mill and emphasising its blending are very important measures to reduce its influence on the plant operation. The final result is that quantity of the first class white cement was increased to 92% from 80% at the similar cement physical test result.

This is part two of a three-part article written by Shixiang Chen for World Cement’s October issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the October 2015 issue here.

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