Treatment of green liquor dregs

Abstract

A method is disclosed for treatment and utilization of green liquor dregs. The method comprises obtaining green liquor dregs originating from a pulp process. The green liquor dregs are dried at a temperature of 95 to 120 °C, preferably at 100 °C, to obtain dried green liquor dregs. The dried green liquor dregs are calcined for 1 to 4 hours, preferably for 2 hours, at a temperature of 800 to 1200 °C, preferably at 800 to 1000 °C, to obtain calcined green liquor dregs. The calcined green liquor dregs are used to control the pH of a flotation process in mining industry.

Description

[0001] TREATMENT OF GREEN LIQUOR DREGS

[0002] FIELD OF THE INVENTION

[0003] The invention relates to treatment of green liquor dregs, and particularly to a method for treatment of green liquor dregs.

[0004] BACKGROUND ART

[0005] Green liquor is residue produced in pulp production. Green liquor dregs originate from recovered cooking chemicals of pulp. Additionally green liquor dregs may contain organic wood-based matter and metal compounds such as aluminium compounds. Currently, there is no definitive solution to the problem of utilization of green liquor dregs. Instead, green liquor dregs or sludge are still being disposed as waste.

[0006] SUMMARY

[0007] The following presents a simplified summary of features disclosed herein to provide a basic understanding of some exemplary aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key / critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to a more detailed description.

[0008] According to an aspect, there is provided the subject matter of the independent claims. Embodiments are defined in the dependent claims.

[0009] One or more examples of implementations are set forth in more detail in the description below. Other features will be apparent from the description, and from the claims.

[0010] DETAILED DESCRIPTION OF EMBODIMENTS

[0011] The following embodiments are exemplary. Although the specification may refer to "an", "one", or "some" embodiment's] in several locations, this does not necessarily mean that each such reference is to the same embodiment's], or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. Furthermore, words "comprising", "containing" and "including" should be understood as not limiting the described embodiments to consist of only those features that have been mentioned and such embodiments may contain also features / structures that have not been specifically mentioned. Green liquor dregs originate from the chemical recovery cycle where effective cooking chemicals sodium hydroxide and sodium sulphide are regenerated from black liquor. Green liquor dregs are the insoluble materials in the green liquor after it has been produced by dissolving the molten smelt from the recovery boiler in weak wash in a pulp process. Green liquor dregs consist of water, typically 40-60 %, unburned carbon (char), organic matter and insoluble impurities, mostly calcium carbonate, sulphides, and iron and other metal compounds which must be removed from green liquor. Dregs are typically produced at the rate of 4 to 6 kg per ton of unbleached pulp. Green liquor dregs may be in a form of sludge, slurry, and / or precipitate.

[0012] Burnt lime is typically used as a pH adjuster in flotation process in mining industry.

[0013] In the present invention, instead of burnt lime, green liquor dregs are used as a pH modifier in a mining industry flotation process. Green liquor dregs or green liquor sludge are a strongly alkaline type of waste material produced in the forest industry. In Finland about 100 000 tonnes of green liquor dregs are produced every year.

[0014] The present invention enables the mining industry to use calcium oxide produced from green liquor dregs obtained as a waste in the forest industry instead of calcium oxide refined from the primary raw material. Utilizing the green liquor dregs in the mining industry enables recycling the green liquor waste material.

[0015] The present invention discloses a method for utilizing calcined green liquor dregs as a substitute for calcium oxide in flotation.

[0016] The use of a calcined green liquor dregs as a pH modifier for flotation process enables to avoid the disposal of green liquor dregs as waste and the dissolution of potentially harmful substances. The green liquor dregs are mostly used as a pH regulator in the flotation processes, especially in concentration of sulphide minerals.

[0017] The present invention provides advantages both to the mining and forestry industry. Flotation of the green liquor dregs solve at least two problems. The largest unused fraction of waste from the forest industry can be recovered as a substitute for burnt lime in the flotation enrichment process of the mining industry. The mining industry is able to replace primarily used expensive burnt lime with calcinated green liquor dregs. At the same time, most of carbon dioxide emissions from the production of burnt lime can be avoided.

[0018] Advantages of using a green liquor dregs include that a huge amount of waste generated in the forest industry can be utilized in the mining industry. The mining industry is able to give up an equivalent amount of primary raw materials that is, mining and refining limestone to produce burnt lime. The generation of carbon dioxide in the production of burnt lime can be avoided (0.95-1.35 tonnes of COz per tonne of burnt lime).

[0019] Utilization of green liquor dregs for use in the mining industry is highly profitable. The use of green liquor dregs for the mining industry is much easier compared to other potential applications.

[0020] In the present invention, prior to the flotation process, the green liquor dregs are dried at 95-120 °C, preferably at 100 °C. The dried matter is calcined for at least 1 hour, preferably for 1-4 hours, more preferably for 2 hours, at 800-1200 °C, preferably at 800-1000 °C, and the calcined product is optionally ground to break up large particles into smaller particles. The optionally ground calcined green liquor dregs are used, optionally dissolved in water or mixed with burnt lime, to control the pH in the grinding or conditioning stage or in the flotation cells of the flotation process. Alternatively, the use of calcined green liquor dregs in solid form as a powder for the pH control, is also possible.

[0021] The calcined green liquor dregs are used to control the pH of a grinding stage and / or a conditioning stage, and / or the pH in a flotation cell of the flotation process at a pH value of 12, or at any value higher than a natural pH of an ore slurry.

[0022] In copper mineral flotation, the target pH value of the copper ore slurry in the conditioning stage of the flotation process is typically 12.

[0023] The calcinated green liquor dregs together with water forms a saturated solution having a pH of 14, which is particularly suitable for pH control in copper enrichment. In addition, sodium sulphide contained in the green liquor dregs acts as a depressant to increase the selectivity of the flotation of sulphide minerals, and thereby improves the quality of the concentrate. The calcinated green liquor dregs may replace both burnt lime and sodium cyanide as the depressant in flotation of sulphide minerals.

[0024] For example, in the calcining step, calcium carbonate decomposes into calcium oxide and carbon dioxide. The calcium oxide thus formed is then mixed with water to form a saturated aqueous solution of calcium hydroxide which is used in the pH control of the flotation process of sulphide minerals in the mining industry.

[0025] Calcinated green liquor dregs may be used in concentration of copper containing sulphide ores, or in concentration of nickel containing sulphide ores, for example. Also, calcinated green liquor dregs may be used as pH regulator in the flotation of any multi metal oxide or sulphide ore containing valuable metals. The aim is then to increase the pH from its natural pH value to value high enough for achieving a good selectivity in the concentration process. The flotation process may be a flotation process of an ore containing sulphide and / or oxide minerals. The ore containing sulphide and / or oxide minerals may be an ore containing copper, nickel and / or any valuable multi metal minerals.

[0026] Converting the green liquor residue to a sustainable product by the pre- sent invention is advantageous as regards to material utilization in circular economy.

[0027] It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

CLAIMS1. A method for treatment and utilization of green liquor dregs, the method comprising obtaining green liquor dregs originating from a pulp process; drying the green liquor dregs at a temperature of 95 to 120 °C, preferably at 100 °C, to obtain dried green liquor dregs; calcining dried green liquor dregs for 1 to 4 hours, preferably for 2 hours, at a temperature of 800 to 1200 °C, preferably at 800 to 1000 °C, to obtain calcined green liquor dregs; using the calcined green liquor dregs to control the pH of a flotation process in mining industry.

2. A method according to claim 1, wherein the method comprises grinding the calcined green liquor dregs to decrease the particle size of the calcined green liquor dregs, before they are used to control the pH of the flotation process in the mining industry.

3. A method according to claim 1 or 2, wherein the calcined green liquor dregs are dissolved in water, before they are used to control the pH of the flotation process in the mining industry.

4. A method according to claim 1, 2 or 3, wherein the calcined green liquor dregs are dissolved in water to obtain an aqueous solution with a pH value of 14, wherein the aqueous solution is used to control the pH of the flotation process in the mining industry.

5. A method according to claim 1, 2 or 3, wherein the calcined green liquor dregs are mixed with burnt lime, before they are used to control the pH of the flotation process in the mining industry.

6. A method according to claim 1 or 2, wherein the calcined green liquor dregs are used in solid form to control the pH of the flotation process in the mining industry.

7. A method according to any of the preceding claims, wherein the flotation process is a flotation process of an ore containing sulphide and / or oxide minerals.

8. A method according to claim 7, wherein the ore containing sulphide and / or oxide minerals is an ore containing copper, nickel and / or any valuable multi metal minerals.

9. A method according to any of the preceding claims, wherein the drying of the green liquor dregs is carried out by using hot air.

10. A method according to any of the preceding claims, wherein the calcined green liquor dregs are used to control the pH of a grinding stage and / or a conditioning stage, and / or the pH in a flotation cell of the flotation process.

11. A method according to claim 10, wherein the pH is controlled to be at a pH value of 12, or at any value higher than a natural pH of an ore slurry.

12. Use of green liquor dregs for controlling the pH of a flotation process in mining industry, wherein the green liquor dregs are obtained, originating from a pulp production process; the green liquor dregs are dried at a temperature of 95 to 120 °C, preferably at 100 °C, to obtain dried green liquor dregs; dried green liquor dregs are calcined for 1 to 4 hours, preferably for 2 hours, at a temperature of 800 to 1200 °C, preferably at 800 to 1000 °C, to obtain calcined green liquor dregs; the calcined green liquor dregs are used for the controlling of the pH of the flotation process.

13. Use of green liquor dregs for controlling the pH of a flotation process in mining industry according to any of the preceding claims.

14. Calcined green liquor dregs produced by the method according to any of the preceding claims 1 to 11.