In previous posts we discussed how electric pulse fragmentation works and why it has selective properties. This information provides a grounding to understand how these properties of electric pulse fragmentation can provide advantages in a variety of processing environments. This post focuses on the advantages of treating incinerator bottom ash (IBA) with electric pulses.
IBA is the by-product of municipal waste incineration – or the burning of household refuse. Anything that cannot be burned will become part of the incinerator residue, IBA is essentially a solid fused mass, similar to a rock, containing the burned remnants of whatever was in the incinerated waste such as metals, stone, glass, and ceramics, etc.
In some cases IBA can be used for construction, for example in breeze/cinder blocks, or as infill in road or pipe bedding. Depending on local laws, the IBA (and metals within) can be put back into the ground, however in places with stricter environmental legislation such as Switzerland, this contaminated material cannot be used in construction, and there are penalties to landfilling metal bearing waste.
The goal is to recover as much metal and other raw materials as possible from IBA. Traditional crushing reduces all of the IBA to the liberation size of the smallest metals allowing their recovery. This gives excellent recovery of metal, however the ‘mineral’ fraction of the IBA, the more rock like parts such as the fused slag and brick, ceramics, glass, etc, are now also crushed and are unable to be recycled, meaning more material must be landfilled or otherwise put back into the ground.
This problem requires disaggregation of the IBA without significant crushing of the material: We want to recover the metal while leaving the slag, glass and other phases as large as possible for later re-use. That’s were EPF comes in.
The discharge is preferentially attracted to areas with high permitivity contrast (PC), for example, a gold nugget within a rock, or a piece of metal within IBA. In this instance, the discharge will travel through the material (Situation I, below). The converse is also true and if the highest permitivity contrast is between the target particle and the surrounding water, the discharge will travel around the particle at the particle-water contact instead of through it, causing minimal breakage (Situation II & III).
Particles with metal are broken more than those without metal: IBA particles containing metal will preferentially have internal discharges, and be broken more, liberating the contained metals, while more homogeneous IBA particles and those without metals will have the discharge travel around the particle and experience only minimal or ‘surface’ breakage.
The end result is that in addition to liberating metals, the glass, ceramic and particles without metals remain larger, more intact, and potentially recycleable, creating an additional stream of decontaminated inert material that can be recycled or reused as a construction aggregate, without putting more metal into the ground.
At our plants, we produce four product streams: ferrous, non ferrous, inert and slag, and we recover down to 2mm and below in each of these streams. All but the slag can be recycled in Switzerland, and we roughly half the amount of material that must go to landfill. We’re proud of our achievements and are doing our part for the environment.