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10. 1. 2023

Separation of ferrous particles from quartz sand (case study)

Initial status

Autoclaved aerated concrete (ACC) - sometimes also called cellular or porous concrete - is a lightweight foam concrete building material enabling the reduction of the weight of buildings and thus the decrease in total construction costs, transport costs of building materials, etc. In addition, aerated concrete blocks stand out with very good thermal and sound insulation properties (thanks to the air cavities - pores - in the concrete mass).

White autoclaved concrete blocks usually contain only raw materials of natural origin (primarily quartz sand, calcinated gypsum, lime and/or cement and water). During the production process, a binding agent (aluminum powder) is added. The condition for obtaining the final brilliant white color is high-quality ground quartz sand.

As a cheap variant or replacement of white aerated concrete, grey aerated concrete is used (in less developed countries and mainly for price reasons). The latter is chemically and mechanically less stable (blocks made of this material are therefore prone to cracking and brittling). The main reason for these problems is the use of fly ash from coal-fired power plants (often containing harmful substances such as lead, cadmium, mercury, and arsenic) in grey aerated concrete, instead of high-quality natural raw materials (i. e., the expensive quartz sand). These harmful substances have a negative effect not only on the microclimate of the resulting building but also lead to problems with the ecological disposal of old grey aerated concrete blocks (power plant fly ash substances must not get into the soil or water sources). On the other hand, white aerated concrete is also harmless from the ecological point of view - any debris can be easily recycled and reused. For this reason, renowned world manufacturers focus primarily on the production of white aerated concrete (e. g. in Germany, the grey aerated concrete is almost no longer used).

Problem

One of the most important world manufacturers of white aerated concrete turned to us with a request to solve a long-standing problem with the removal of iron contaminants from quartz sand that is ground in a ball mill on their processing line. However, the ground sand leaves the ball mill contaminated with fragments of steel grinding balls and also ferrous abrasion dust from the mill and pipe system. In order to separate the fragments of the grinding balls, the manufacturer originally installed a fine sieve at the mill outlet. However, the screen very quickly clogged (due to the high grinding capacity of up to 30 t/h) with small stones contained in the ground mixture (besides, this sieve could not separate smaller ferrous particles or the abrasion dust). That is why even after the installation of the sieve, the iron contaminants kept getting into the pipeline system through the high-pressure pump and continued to the storage silos (where they gradually settled).

Solution

For a long time, the client tried to find a solution that would ensure the reliable removal of iron particles the fine slushy quartz sand. Unfortunately, due to a number of specific factors (e. g. semi-fluid and at the same time highly abrasive material, high processing capacity, open flow system, fast flowing material, big variety of iron contaminants – larger fragments of grinding balls as well as ferromagnetic dust, etc.) any of the standard magnetic separators could not be applied. In addition,   renowned separator manufacturers did not want to go into the difficult, lengthy and uncertain development of a completely new type of a magnetic separator.

The client could solve the problem by applying a magnetic separator called trunnion magnet - this system consists of a blind drum (trommel) with a magnet sector through which the processed material flows. However, the processing capacity of 30 tons per hour would require a very large trunnion magnet system, the price of which would be in the order of several hundred thousand euros (and even this solution would not be perfect either, as the very abrasive material would be in direct contact with the drum shell and with a high probability it would wear the blind drum relatively quickly. And the subsequent production shutdown and the financially and time-consuming repair of the expensive trunnion magnet would result in other significant costs...).  

The client could also solve the problem by changing the production technology - however, such a change would be even more expensive than the purchase of the trunnion magnet.

Therefore, the manufacturer of the aerated concrete welcomed the approach of the company SOLLAU that (exactly according to his requirements and existing production technology) elaborated a completely new type of magnetic separator – a plate magnet with automatic cleaning to be suspended above chutes. After the installation of the magnetic separator DND-SFX a significant improvement in the purity of the processed material was achieved (by more than 60 %), as the SOLLAU magnetic separator reliably removes most of all iron contaminants (i. e., both fragments of grinding balls and microscopic iron dust).

Reasons for the application of DND-SFX

  • high efficiency of magnetic separation (removal of small as well as larger ferrous particles)
  • fully automatic operation without the interruption of the material flow during the separator cleaning (24/7)
  • high resistance to abrasion (as the magnetic system is not in direct contact with the processed material)
  • easy and quick separator installation (without the need for major modifications to the existing production line layout)
  • low acquisition costs (compared to competing systems such as the trunnion magnet)

Financial benefits of our solution for the client

The drastic improvement of the material purity resulted in the extension of the service intervals as well as of the overall lifetime of the following key technological devices:

  1. High-pressure pump - a significant extension of its lifetime
  2. Pipelines for the transport of the cleaned material - reduction of pipe wear led to 100 % extension of the pipeline cleaning and service intervals
  3.  Storage silo for the processed material - a significant service interval extension with a simultaneous reduction in the demands on regular maintenance (as a result of a decrease of 75 % in the ferrous deposits accumulated at the bottom of the storage containers)

Another financial benefit for the client is the sale of the separated iron waste material to specialized collection centers of secondary raw materials.

And even for the final user of aerated concrete products the removal of ferrous contaminants is of great importance. In the past the iron particles used to react with air humidity and created more or less unremovable rusty stains on the rendered walls with time (and even after repainting the metal contaminants reacted with humidity again...).


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