JultikaUniversity of Oulu repository

Abstract

Sodium hydroxide also known as NaOH is well known and extensively used reagent in mining and mineral processing. Processed ores generally contain a large amount of minerals. Ore minerals contain for example sulfate and sulfide. These sulfide minerals are just one group of minerals that occurs in ore. A common way to dissolve sulfides is to use acidic conditions (acid leaching). However, due to challenging mineral structure and preg robbing ores, the best leaching results can be achieved by using alkaline conditions (alkaline leaching). NaOH with sufficient amount of oxygen is an effective substance to use in alkaline leaching.

In this thesis the aim is to find out the optimal NaOH dosage for sulfide containing concentrate to dissolve the metals from sulfide matrix. NaOH is an expensive reagent and that is why the dosage optimizing is one of the fundamental questions when it comes to the process planning, budgeting, and profitability. Sulfide dissolution is measured through the oxidation level of sulfides. Oxidation level of sulfide can be measured as sulfur oxidation when all sulfur of the ore is in sulfide form. Oxidation level of sulfur correlates with the oxidation products, like sulfate and thiosulfate. Also, the ratio between thiosulfate and sulfates can be used to see if the oxidation level is high enough or how well the process is balanced.

The goal of this thesis is to provide an information on the basis of which the process can be adjusted to improve the cost-efficiency of the process. Knowing the optimal dosage will help the company to perform in the best possible way. The use of right dosage of chemicals reduces the environmental impact of the mining industry. In the best-case scenario dosage optimization will reduce reagent consumption and enables cleaner and more efficient processes in the future.

The research material and results of this thesis were gathered during the industrial pilot. The pilot consisted of three continuously operating leaching reactors each with a volume of 2,7m3. Consumption of sodium hydroxide was monitored in various ways in order to get as accurate consumption estimate as possible. Based on sulfur content of the feed, the other reagents, such as oxygen, can also be adjusted to achieve the best oxidation rate. The optimal dosage of the sodium hydroxide can be estimated by comparing the sulfur oxidation rate and the concentration of the formed dissolution products.