STUDY OF FLUSHING CYCLE EFFECTS ON PYRITE OXIDATION RATE AS SUPPORT FOR PIT LAKE SCENARIO

• Main Author: Wisnugraha, Gelar
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/70150
• Institution: Institut Teknologi Bandung

In post-mining activities, pit lakes are used as water attractions, fisheries, water supplies, hydroelectric power plants, and so on. The potential use of pit lake water is highly dependent on the quantity and quality of pit lake water. Prediction of water quality in pit lakes generally uses data from the results of geochemical characterization, especially for predicting the quality of water resulting from reactions with rocks in the pit walls. The characteristic data used comes from the results of static tests and kinetic tests to determine the representative leach characteristics. Prediction of pit lake water quality can use software based on the geochemical equilibrium – geochemical equilibrium software to determine geochemical reactions and calculate more diverse leach characteristics. One of the geochemical equilibrium software used in the prediction of water quality in pit lakes is PhreeqC. The geochemical modeling used is inverse modeling and forward modeling. In predicting water quality from pit wall leaching, forward modeling can be used using kinetic reaction rates. The value of kinetic reaction rates is the result of inverse modeling from the results of pit wall leaching tests or scale-up laboratory scale kinetic tests. Kinetic reaction rates can be calculated from mole transfer and kinetic test parameters, especially sample surface area and leaching time/cycle. Kinetic reaction rates in the formation of acid mine drainage (AMD) are for the oxidation reaction of sulfide minerals. The leaching time/cycle determines the content of water and oxygen in the kinetic column. A previous study conducted a pit wall leaching kinetic test to predict pit lake quality by calculating the kinetic reaction rates in continuous (sequential) daily, 3-daily, and weekly cycles. With the same sample, this research was carried out in parallel laboratory-scale kinetic tests on daily, tri-daily, and weekly cycles which were used to calculate kinetic reaction rates for different leaching cycles so that they could represent weathering conditions in the field based on different rainfall events and as supporting data in a pit lake scenario. The rock samples came from the active pit wall of PT. ABC which represents the main lithology that makes up the pit wall and was taken using the grab sampling technique by previous researchers. The three types of samples used are mudstone (PRI 01), carbonaceous claystone (PRI 02), and sandstone (PRI 03). Geochemical characterization of the samples was built to determine whether the rock has the potential to produce acid or not. Geochemical characterization was carried out through static and kinetic tests in the laboratory as well as mineral and element content tests. The kinetic test was carried out using the free draining column leach test (FDCLT) method which is a scale-up from the kinetic test of the pit wall leaching method in previous studies. The watering intervals for the FDCLT method, are daily, three daily, and weekly, are carried out at the same time and for the same number of weeks. The results of the leachate from the kinetic test will be tested for its physical and chemical properties. Geochemical modeling was carried out using PhreeqC software with Inverse Modeling and Inverse Modeling features. Inverse modeling produces many predictions of reaction models in the form of minerals that may react and to choose the most appropriate model, model verification is carried out with forward modeling. The selected model is a model that produces a simulated pH close to the pH of the leachate from the kinetic test results as well as the content of metals or their species. In the selected model, the pyrite mole transfer value will be used to calculate the oxidation rate. Based on the results of geochemical characterization, the PRI 01 sample is classified as non-acid forming (NAF) rock with a NAG pH value of 7.19, while the PRI 02 and PRI 03 samples are classified as potentially acid-forming (PAF) with NAG pH values of 2.19 and 3.08, respectively. The pyrite oxidation rate using the total iron mole approach (8.73×10-12 – 6.44×10-8 mol/m2s) has a lower value than the other two approaches. This is because the iron content in leachate has a greater tendency to precipitate. The oxidation rate of pyrite with the mol sulfate approach (7.61×10-10 – 1.13×10-7 mol/m2s) in leachate has a value close to the oxidation rate of pyrite using the PhreeqC modeling approach (1.45×10-10 – 1, 45×10-7 mol/m2s). Of the three types of leaching intervals, it was found that the three-day cycle had the highest oxidation rate value when compared to the daily and 7-day cycles from the PhreeqC modeling (1.17× 10-9 – 1.45×10-7 mol/m2s) which was supported by the appropriate value pH on the oxidation rate of leachate. In the 3-day cycle, the oxygen consumption in the oxidation reaction of sulfide minerals reaches the highest value due to the diffusion of oxygen and moisture content in "optimal" conditions in the oxidation reaction of sulfide minerals. From a comparison of the oxidation rate of the Phreeqc approach with previous studies, a difference of 1 – 2 was obtained for the order of pyrite mole transfer. This oxidation rate value can be used as a variation of geochemical modeling input (water quality prediction) in pit lake management scenarios by considering the conditions being modeled (worst scenario, most possible scenario, base scenario).