MINERÍA la mejor puerta de acceso al sector minero MINERÍA / OCTUBRE 2023 / EDICIÓN 553 79 Abstract This paper shows that acid mine effluents can be treated directly through Reverse Osmosis (RO) membranes, meeting the highest national and international water quality standards. This is an innovative use of membrane technology for the treatment of acid mine water. Newmont believes that access to clean and safe water is a human right, and reliable water supplies are critical for hygiene, sanitation, livelihoods, health, and the environment. As water is also fundamental to mining operations, it recognizes the need to use water efficiently, protect water resources and collaborate with the different users in the basin where we operate to effectively manage this shared resource. The need to develop treatment processes for acid waters arises from the fact that the technological options currently available to reduce sulfates (SO4) in this type of water are limited; mining effluents can have several thousand milligrams per liter (mg/L) of sulfates, which need to be treated and reduced to a range between [250 - 1000] mg/L so that they can be discharged into the environment. It is under this framework and committed to technological development that through the corporate department of Water Management and Treatment, both laboratory and pilot tests were conducted for the development of this process to treat acid water directly through RO membranes, achieving a reduction of sulfates to values within the required discharge range. Normally acid mine water has a first stage of sulfate precipitation with lime slurry (calcium hydroxide) prior to being treated by Reverse Osmosis (RO), this first stage generates calcium sulfate (CaSO4) also commonly known as gypsum which generates incrustations in the RO membranes and reduces the efficiency of the process. This paper demonstrates that taking advantage of the high solubility of metals and sulfates in acidic waters, they can be reliably and stably treated by RO membranes, thus avoiding RO membrane failure problems due to gypsum scaling. The methodology used to demonstrate the feasibility of this process was the installation of a pilot plant at Newmont Yanacocha's mining unit with a capacity of 5 m3/h, which operated for two years, where acid effluents were treated directly through RO membranes, achieving a constant operation without gypsum scaling, reaching a high treatment efficiency. Within the technological arrangement of the whole water treatment process, Reverse Osmosis (RO) is considered as the central process, which is accompanied by Ultrafiltration (UF), High Density Sludge (HDS) and Sand Filters (SF). The three main considerations for the feasibility of the process are: Maintain at all times a pH less than 3 in the feed solution to avoid iron precipitation on both UF and RO membranes, which is achieved by the addition of sulfuric acid if required. Ensure that there are no metals contained in the pipes that could precipitate when interacting with the high pH solution, this before performing an alkaline membrane cleaning, which is achieved with an adequate rinsing with permeated acid solution prior to this cleaning stage. Ensure that there are no metals contained in the pipes that could precipitate when interacting with the oxygen contained as a result of the air injection during the washing cycle, which is achieved with an adequate rinsing with permeated acid solution prior to this cleaning stage. The limitations of this process are associated with the ratio between sulfate and calcium concentrations that may be required to reduce the recovery of RO membranes, which during the tests remained in the range of 70 - 75 %, with maximums of 80 %. This innovative technological development allows Newmont to have one more water treatment alternative to implement within its operations where required and also allows other mining operations to adopt this treatment if required.