Acid mine drainage (AMD) and other acidic metalliferous effluents are commonly treated by the mining and metallurgical industries by lime neutralization. Upon neutralization, metals precipitate out of the effluent as hydroxides. This neutralization produces voluminous hydroxide sludges with low solids content (frequently < 5%). Despite recent improvements to the traditional neutralization method, it is estimated that as much as 6.7 million cubic metres of sludge are produced annually in Canada. In addition, the Canadian mineral industry is faced with questions related to the long term stability of AMD treatment sludges, and their environmentally acceptable disposal.

There is a need to develop standard sampling, handling and characterization protocols for AMD treatment sludges. A systematic method of assessing sludges with respect to their chemical, physical and leaching characteristics is necessary for estimating the sludge stability and for making informed decisions for disposal.

This report summarizes work that has been carried out in three areas:

• a site survey and sampling campaign of AMD treatment sludges at 11 Canadian mine sites;

• a detailed characterization of the collected sludges including physical, chemical, mineralogical and thermal analyses; and

• the leaching of the sludge samples using two distinct tests in association with a review of current hazardous waste regulations.

This report provides a data bank of lime treatment sludge characteristics which has been applied here in the discussion of sludge stability. Furthermore, this information may be used to assist operators, researchers and regulators in the development of improved treatment processes, effective disposal methods and appropriate regulatory tests for lime treatment sludges. These data may also be used to compare treatment operations and to forecast sludge related issues arising at treatment plants.

Sludge sampling

Sludge samples were collected from 11 Canadian mine sites (seven base metal, two uranium, one gold and one coal) from December 1995 to March 1996. Background information on AMD production, sludge production and disposal as well as the overall treatment process was compiled for all sites. Wherever possible, both fresh (i.e., end of pipe) and aged (i.e., pond core samples at depth) sludge samples were collected to study the effects of natural sludge aging.

Sampling plans were developed prior to the collection of sludge cores to ensure a representative composite sample of aged sludge. The number of samples was based in large part on the volume of disposed sludge, while the sampling stations were defined after a review of site-specific characteristics including pond dimensions and patterns of disposal within the pond. A commercially available hand corer with extension was used to reach depths of up to 6 m and is recommended for collection of cores in shallow water and/or sampling through an ice cover in winter.
Sludge characterization

Physical characterization and leaching tests were performed on the wet samples. The remaining analyses (chemical, mineralogical and thermal) were done using the freeze-dried material.

The pH values for the sampled sludges were alkaline and ranged from 8.2 to 10.8. In most cases aged sludges showed a lower pH than their fresh counterparts. Eh values ranged from 58 to 315 mV with the aged sludges commonly recording the lower values.

Denser sludges, generally produced using the High Density Sludge (HDS) process, displayed both smaller median particle sizes and narrower particle size distributions. Many of the sludges produced from conventional or basic treatment processes exhibited bimodal particle size distributions. In all but one case, the measured particle size was greater for the aged sludge.

The solids content of the sludges ranged from 2.4% to 32.8%. In almost all cases at least a 25% increase in solids content was seen from the fresh to the aged material. Based on the samples tested, no correlation was observed between the degree of densification and either the age of the deposited sludge or the presence (or absence) of a water cover on the sludge pond.

Neutralization potential values for the sludges collected ranged from 108 to 725 tonnes CaCO3 equivalent per 1000 tonnes sludge. While low NP values are attractive in terms of plant efficiency, sludges with high NPs have more neutralization capacity which directly impacts on long term sludge stability. Calcium content in the sludges varied from 3.8% to 27%; calcium is present in two main forms, as calcite or gypsum. The metals content of sludge can be viewed as potential recoverable assets or a source of leachable metals. Zinc recovery may be possible for some sludges ([Zn]>14%). Zinc concentrations ranged from 0.019% to over 14.4%. The low concentrations observed for copper and nickel (generally less than 1%) do not justify their recovery. Aluminum ranged from 0.1% to 11%. Copper, arsenic, boron, cadmium, chromium, mercury, lead, and selenium occur only in trace amounts, generally less than 0.01%. Iron ranged from 1.5% to 28% in the sludges.

All the sludges contained sulphate, in some cases greater than 30%. The sulphate content correlated directly with the amount of total sulphur in most of the sludge samples, indicating that all the sulphur present in these samples occurs as sulphate.

Mineralogical analyses of all sludge samples showed a major amorphous phase. Readily leached metal species such as zinc were commonly associated with this phase, which appeared to be effective in scavenging metal species (Al, Cu, Fe, Mg, Na, Ni, Zn) during precipitation. Calcium is present as calcite, gypsum and bassanite; they occur both as individual grains and in the amorphous phase. The amount of calcite may indicate the degree of recrystallization and the increased stability of the sludges. Quartz, silicates, sulphides and iron oxide particles found in the sludges are detrital in origin.

Sludge leachability

AMD treatment sludge samples were leached using two protocols. The Ontario Leachate Extraction Procedure (LEP) uses an acetic acid solution as a leachant while the Modified LEP substitutes a synthetic acid rain for the acetic acid. Acetic acid mimics the organic acids expected to be present in a municipal landfill and assumes co-disposal of mineral processing and municipal wastes. On the other hand, the mixture of sulphuric and nitric acids better simulates the inorganic acids that are likely to come in contact, through acidic precipitation, with sludges disposed in ponds. Generally, less metal was leached from the sludges when they were subjected to the Modified LEP as opposed to the Ontario LEP. Sludge leachability is strongly dependent upon the final leachant pH which is influenced by the choice of leachant and by the neutralizing potential of the sludge. Metal leachability increases with decreasing pH at pH less than about 9.5. The amount of metal leached is also related to the metal concentration in the sludge itself. In general, the aged sludge samples showed an increase in stability relative to the fresh samples as indicated by the leaching results and supported by the mineralogical data and particle size analyses.

AMD treatment sludges are waste products which may be subject to waste management regulations. A leachate extraction test may be used to evaluate if the waste is capable of yielding a leachate which exceeds regulated concentration limits for selected contaminants. When a waste fails the test in relation to the limits specified in a particular jurisdiction, the waste may be classified as hazardous. All but two of the sludge samples subjected to the Ontario LEP passed the test when the leachate concentrations for metals are compared to the regulated limits governing the classification of hazardous waste material in Canadian jurisdictions. A fresh sludge from a base metal operation failed on zinc and an aged sludge from a uranium mine failed on uranium. There are only three jurisdictions in Canada which have a regulated limit for zinc and this sludge would actually fail only in comparison to Québec=s current regulation. It must be noted however, that Québec=s proposed new regulated limits do not include zinc. None of the sludge samples failed when tested with the Modified LEP. The leachate concentrations from both tests were generally at least five times lower than the most stringent of the regulated limits.

Therefore, fresh AMD treatment sludges would not generally be classified as hazardous wastes based on current leaching protocols and regulated contaminant limits. Aged sludges are even less likely to be classified as hazardous wastes. Based on the samples tested, this work has underlined that while sludge stability is an issue, greater emphasis should be placed on sludge disposal and volume reduction.

Numerous leach protocols have been developed to test solid wastes. None of these leaching tests have been specifically designed for evaluating AMD treatment sludge leachability. A thorough review of regulatory and research leach protocols from Canada and the United States is provided. While regulatory leach tests for classification of hazardous wastes in Canada involve the use of acetic acid in the protocol, more appropriate tests, such as the Modified LEP, need to be considered for assessing the leachability of AMD treatment sludges for on-site disposal in a pond environment. Ultimately, the context within which sludge leachability (stability) is measured must be kept in mind when a leach test is applied.

In most provinces and territories, the testing of AMD treatment sludge and its storage/disposal is controlled by site-specific licences or permits based on appropriate legislation. A review of Canadian and American hazardous waste regulations, and other pertinent regulations and guidelines as they apply to the leachability testing of AMD treatment sludges is provided.