Chilean mining legislation requires physical and chemical stability of mining sites. Deposition of tailings at sub-sea environments has not been specifically included in chemical stability guidelines. Only “Sub-Sea Tailings Deposition Evaluation Guideline – prSN/ TR-9432”, published by KREC-Norsk Berdindustri, could be considered an antecedent. This guideline includes specific analytical standards and criteria for impact analysis, potential approaches for the characterization of the sub-sea environment and methods for characterization of materials in tailings facilities.
Sub-sea deposition of tailings is not a common practice in Chile. The practice was developed by Chilean iron mining in different coastal contexts for about 40 years, but was abandoned without knowing the extent of its impact. SRK Consulting (Chile) and the Chilean iron mining industry are developing methodologies to assess solute release rates from tailings deposited at sub-sea environments. Environmental conditions of sub-sea deposition do not correspond to those of continental facilities. Sub- sea environmental parameters affect the thermodynamics reactivity of mineral phases that generate mining drainages and metal and metalloid leaching capabilities. Some of these variables are pressure, temperature, luminosity and irradiance, microbial activity, concentrations and mechanisms of supply of oxidizers, catalysts and nutrients, and chemical composition of seawater.
To determine reaction mechanisms and the release rates of solutes from tailings, Kinetics Tests are usually applied under standardized lab conditions (CANMET, 2009; INAP, 2012). However, the effects of the variability of these mechanisms under sub-sea conditions could not be scaled from the results of these tests. Consequently, a modified Kinetics Test methodology was designed considering: (1) a constant P-T corresponding to the environmental pressure and temperature of the sub-sea site, (2) a rate of renewal of inflow solutions that assimilates constant concentrations of seawater, (3) inoculation of microorganisms and nutrients, and (4) isolation from site luminosity conditions. These conditions were implemented in a stainless steel “Zero Headspace Extractor”. Preliminary tests were performed using distilled water to assess the capability to leach solutes from the kinetics cell, and to modify the chemistry of the resulting solutions. Some tests were implemented considering different renewal times of seawater.
