To better understand sludge production and behaviour a study was completed at CANMET to assess the effect of process parameters during AMD treatment and sludge aging on metal leachability and sludge density.

Several process parameters (recycle rate, agitation rate, flocculant usage and ferric-ferrous iron ratio) were examined through continuous pilot plant testing. The sludge produced was characterized through physical, chemical, mineralogical and metal leaching analyses to ascertain optimal conditions to produce dense, stable, lime treatment sludge. In addition, batch tests were completed to study the effect of sulphate concentration in the AMD stream. The method of neutralization was examined to observe its effect on sludge production.

This study found recycling can affect several treatment related factors and can impact on long term sludge stability, as well as intermediate concerns such as sludge density. Increased recycling, while beneficial in terms of plant efficiency and reagent costs, increased the heavy metal content of the sludge and led to greater metal leachability. As well, sludge recycling in a high hydrodynamic shear environment was found to be detrimental to the formation of large, compact particles as particle abrasion and particle growth processes occurred counterproductively.

Slow, controlled neutralization was shown to increase settling rates by five-fold. This has significant implications with respect to clarification requirements. Through supersaturation control, as with staged-neutralization, settling rates can be further increased by 50%. Furthermore, lime consumption can be reduced through selection of a controlled or staged method of neutralization, reducing reagents costs by 10-15%.

Results suggest that a greater proportion of ferric iron in the raw water enhances sludge density and settleability. However, the amount of zinc leached from the sludge increased with increasing ferric iron concentration in the raw water.

The addition of excess lime to HDS-type treatment sludges resulted in lower metal leachability; however, the physical properties of the sludge were adversely affected by this potential remediation technique.

An investigation into sludge aging found that when the aging time was increased the degree of metal mobility from sludge samples declined, while the proportion of calcite and gypsum increased. This was accomplished through a process of dissolution and recrystallization of the sludge components. This supports the theory presented in Part I of this project (3.42.2a), that the calcite and gypsum are the >final= phases of the aged sludge and in the long term (i.e. millennia), the sludge may transform into a carbonate rock with a minor iron oxide phase (Zinck et al.1997). Oxidation rims of goethite and magnetite which formed around sulphide particles present in the aged sludge suggest that long-term codisposal of sludge and tailings may be beneficial for both waste management and metal leachability.

Further work should focus on:
* relating these aging tests with controlled field tests;
* modeling the data contained herein to optimize lime treatment systems;
* investigating metal leachability and sludge aging for the codisposal environment; and
* examining the long term environmental stability and mechanisms responsible for metal mobility from ettringite and desautelsite compounds.