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A study of pyrite reactivity and the chemical stability of cemented paste backfill

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Title: A study of pyrite reactivity and the chemical stability of cemented paste backfill
Author: Bertrand, Valérie J.
Degree Master of Applied Science - MASc
Program Mining Engineering
Copyright Date: 1998
Abstract: A two-fold study was carried out to 1) characterize the evolution of the reactivity of pyrite in the early cycles of kinetic test leaching, using cyclic voltamperometry, and 2) document the weathering characteristics of various paste backfill mixtures that contain pyritic tailings, when exposed to leaching environments similar to those encountered in mine settings. Pyrite leaching experiments were carried out on 6 different pyrite samples from existing mines. Cyclic voltamperometry was performed on carbon paste electrodes (CPE) containing fine grained pyrite samples on the unleached samples and after leaching periods of 4, 10 and 20 weeks. Pyrite reactivity profiles, supported by scanning electron microscope (SEM) observations and leachate chemistry data showed that minor phases of sphalerite and galena present in the pyrite samples were the most important parameters affecting pyrite reactivity in the initial leaching cycles. Sphalerite and galena were found to effectively retard the oxidation of pyrite in the early leaching cycles. As sphalerite and galena were leached out, an increase in the reactivity of pyrite was observed, followed by a gradual loss of reactivity from precipitate coatings. At a fundamental level, mineral surface characterization by cyclic voltamperometry was found useful in the interpretation of kinetic test data for the prediction of acid rock drainage (ARD) generation. For the backfill weathering study, paste backfill samples of 4 different mines were leached in deionized water (pH 5.5) in flooded and alternating air-flooded environments and in a simulated ARD solution (Fe=500 mg/1, SO4=1.5 g/1 and pH 2.5) for 20 weeks. SEM, solid phase chemistry, paste pH, acid-base accounting measurements and leachate chemistry were also used to document the weathering characteristics of cemented paste backfill (CPB). This study revealed that hydrated portland cement minerals are pH sensitive and highly soluble. Short-term exposures of portland-CPB to circum-neutral water or to ARD solution promoted the dissolution of the binder material, increasing the porosity of the backfill and further infiltration of aqueous solution. Long-term exposure or flooding of CPB was found to promote the precipitation of secondary, expansive minerals such as gypsum in addition to solubilizing primary cement minerals. Detailed chemical analyses and acid-base accounting indicated that the neutralizing potential added to the material by the cement phase is short-lived and the small volumes added are insufficient to neutralize the acid generating potential of the mixture. All ARD solution-leached CPB samples formed an increasingly thick crust of precipitates that, with time, reduced the ability of the CPB to neutralize the ARD solution. [Scientific formulae used in this abstract could not be reproduced.]
URI: http://hdl.handle.net/2429/8893
Series/Report no. UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/]

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