The present invention relates to a method and a system for a long-term management of the tailings generated during the bauxite mining process. According to the method and the system, tailings can be replaced in a permanent manner in the same location where it was mined out as a part of the ore. Bauxite ores contain alumina and are commonly deposits mined out off the ground. Such ores are often found under a shallow (normally less than 15 meters) layer of soil (called overburden). The mining method considered to be the most suitable for this type of bauxite deposits is the strip-mining method. This mining method comprises cyclical operations carried out in sequence: vegetal suppression, overburden removal, ore removal and transportation, ore beneficiation, land rehabilitation. Since it allows rehabilitation to occur soon after the ore has been mined, the strip-mining method will commonly have a low environmental and visual impact.

Figure 1 illustrates the main operations of a commonly known bauxite mining method. During the beneficiation step, as the ore is enriched with regard to alumina in a processing plant, tailings with high contents of liquid are generated and commonly stored in dams or reservoirs. One problem for mining companies is that these dams or reservoirs may increase in dimensions year by year due to difficulties in finding sustainable deposit solutions.

The present invention relates in particular to improving the above mentioned state of the art of mine tailings management, by implementing a novel method and system for backfilling of the tailings, and by this method and system essentially returning the tailings back to the same location where they were originally mined in a sustainable and permanent manner. Technical Backqround

The present invention is in particular suitable for operation in accordance with the specific ore of the Applicant’s Paragominas mine in Para of Brazil but may also be applicable for other ores. The present invention relates to depositing tailings that are a byproduct of bauxite ore beneficiation, involving grinding and grain size classification. These are mechanical processes and not chemical processes and, as a consequence, the byproduct is bauxite inert tailings.

Preferably, the mechanical benefication of bauxite ore can take place in close proximity to the mine, before transport of the beneficiated product to an alumina refinery. The distance between the mine and the refinery may sometimes be hundred kilometers or more. Red mud is a byproduct of the refinery.

Bauxite inert tailings vs ‘red mud’ "Bauxite (inert) tailings" describes the byproduct generated in the bauxite ore mining and beneficiation processes through the continuous stages of comminution associated with the process of desliming. In short, bauxite tailings are the byproduct of a mining process.

While “bauxite tailings” are inert i.e. they pose no risk to human health or to the environment, “red mud” is environmentally hazardous due to its alkalinity.

Summary of the Invention

The object of the present invention is a more efficient and environmentally friendly method for the long-term management of the tailings generated during the mining of a bauxite ore and a corresponding system for carrying out the method. By the present invention the environmental impact of the mining activity is significantly reduced. These and further advantages that can be achieved by the present invention are detailed by the accompanying claims.

According to claim 1 there is provided a method for the long-term management of a mine where mining a bauxite ore in a continuous steady-state manner comprising steps related to the mining part which includes vegetal suppression, overburden removal, bauxite ore removal from a mine pit and transporting the ore to a processing facility;

-where bauxite ore is beneficiated into an enriched bauxite fraction for alumina production in a refinery, and a tailing fraction where the tailing fraction is to be stored or deposited,

-and where a land recovery part is performed in the mined area and includes rehabilitation by the following steps,

-the said tailings fraction is dried to a solid content of 60% or more,

-and where dried tailings fraction is filled back into the mined pit on a permanent basis,

-before or in connection with the said land recovery step.

Short Description of the Figures

In the following, the invention shall be further described by examples and figures where;

Fig. 1 illustrates the main operations of the state-of-the-art strip-mining process,

Fig. 2 shows a photo of RP1 tailings dam,

Fig. 3 shows tailings desiccation process,

Fig. 4 shows the final configuration of the Paragominas Tailings Master Plan prior to the Tailings Dry Backfill Project, Fig. 5 shows the proposed approach taken at the mine pits.

Detailed Description

With reference to Fig. 1 , a mining method comprises cyclical operations, carried out in sequence: vegetal suppression, overburden removal, ore removal and transportation, beneficiation, land rehabilitation.

The crushed run-of-mine (ROM) of Paragominas, which feeds the beneficiation plant, will typically present a moisture content between 9 % and 12 % and the following particle size and chemical characteristics:

Granulometry 81 % to 90 % passing sieve 3” (76 mm)

32 % to 38 % passing sieve 20# (0.85 mm)

25 % to 32 % passing sieve 400# (0.038 mm)

Chemical quality

Usable AI203 - Ranging from 36.90 % to 41.38 Reactive Si02 - Ranging from 7.77 % to 10.67 %

Total AI203 - Ranging from 48.12 % to 50.68 %

Total Si02 - Ranging from 9.78 % to 12.76 %

Total Fe203 - Ranging from 9.60 to 12.69 %

Total Ti02 - Ranging from 1.79 % to 2.06 % Loss on ignition - Ranging from 24.27 to 26.27 %

Paragominas' bauxite beneficiation circuit includes three main classification steps to separate the coarser fraction, with the highest gibbsite content, from the finer fraction, where most of the kaolinite is present. The first step is carried out on sieves, where the thicker fraction becomes product after grinding. The passing fraction feeds a pair of in series cyclone batteries. The underflow of the cyclones feeds a ball mill and becomes product, while the fines are pumped to the third classification stage, with 2 cyclones. The coarser fraction of this last stage also becomes product, while the overflow is the tailings.

Paragominas’ tailings will typically present the following particle size and chemical characteristics:

Granulometry

100 % passing sieve 20# (0.85 mm) 90 % to 98 % passing sieve 400# (0.038 mm)

Chemical quality

Usable AI203 - Ranging from 18.02 % to 19.83 %

Reactive Si02 - Ranging from 20.29 % to 21.97 %

Total AI203 - Ranging from 38 % to 42 % Total Si02 - Ranging from 21.69 to 23.69 %

Total Fe203 - Ranging from 12.10% to 14.94 %

Total Ti02 - Ranging from 1.90 % to 2.10 %

Loss on ignition - Ranging from 18 % to 20 %

Since the ore beneficiation basically consists of particle size classification and does not include any chemical process, one can note that, apart from being finer, tailings do not differ much chemically from the ROM. As a result, Paragominas tailings are classified as chemically inert. Also, after desiccation occurs at the dams, tailings can reach a moisture content between 18 % and 40 %. Prior to the Tailings Dry Backfill Project, the Paragominas Tailings Master Plan established the final and permanent disposal of tailings in dams. Figure 2 shows a photo of the Paragominas RP1 , the tailings storage dam currently in operation.

RP1 dam was designed so that the geometry of the quadrants, the positioning of their decant systems and the spacing between spigots allow for the adequate drying of the tailings. RP1 total area is approximately 300 ha and it contains 142 spigots spaced between 75 to 100 meters from one another.

RP1 dam decant system is composed of 4 spillways, which have the purpose of driving out rainfall water and water released from the tailings thus helping in the drying of the tailings. The spillways are connected to transfer channels, which in turn will flows to water clarification basins.

RP1 dam was designed based on the following geotechnical characteristics of the tailings:

• Specific gravity of the tailings (Gs) = 2.68 · Average solids content at disposal (by weight) = 35 %

• Final solids content (by weight) = 60 %

• Density of the tailings at disposal = 1.27 t/m ³

• Density of tailings after desiccation = 1.60 t/m ³

Currently, the disposal method used in RP1 consists of the disposal of thickened tailings, with average solids content of 35 %, in layers of approximately 50 cm that are later exposed to solar drying, allowing tailings to reach 60 % solids contents. Disposal alternates between the four quadrants to allow enough sun exposure time for the tailings to desiccate. Figure 3 shows tailings desiccation process.

At the end of its useful life, estimated to happen in 2021 , an expansion in storage capacity would take place. This would happen continuously by building new disposal quadrants - named RP2 to RP8 - and by further elevating the dykes. Figure 4 shows the final configuration of the Paragominas Tailings Master Plan prior to the Tailings Dry Backfill Project. The Tailings Master Plan was divided in 11 construction stages - of which the construction of RP1 dam starter dyke was the first and, therefore, concluded. The remaining 10 construction stages would be built in a span of 20 years and provide approximately 126 Mm ³ for tailings disposal in an area of 854 ha. Dykes would be raised to reach heights of up to 14 m. Estimated capital cost to build the remaining 10 stages was USD 800 million.

The main concept proposed for the tailings dry backfill project at Paragominas is to use the current tailings storage facility RP1 for the drying of tailings - achieving at least 60 % solids - and from there mechanically remove and transport the tailings for final disposal at the mine pits.

RP1 dam is a perfect fit for such a task. The internal separation of the dam in four big quadrants will allow tailings removal and disposal to happen simultaneously thus not having great impact on normal operations

The minimum solids content of 60 % was defined to increase productivity in the mechanical removal of tailings from the dams and also in the disposal at the mine pits. Furthermore, this solids content optimizes volume usage on the pits. Tailings removal happens by using mine operation equipment such as wheel loaders, excavators and trucks. Upon being removed from the dams, tailings are transported for disposal into existing pits in a manner similar to what is currently done for overburden disposal. This approach eliminates altogether the need for building new dams or further raising existing dams. Figure 5 shows the proposed approach taken at the mine pits.

As one can note from the Figure 5, an offset of 5 meters was allowed between one tailings layer and the next to allow rainfall water to seep in between the tailings layer - since tailings present a very low permeability - and thus not affect water table recharge. According to the present invention it is shown that it is possible to obtain very low moisture contents of the bauxite inert tailings solely by using solar energy, i.e. where no filtering or separate drying is needed.

By this, a more viable mining can be achieved, and in addition less energy for drying and transport will be needed.

The present method allows the backfilling of tailings with moisture as low as 15%. The low moisture makes it possible to return the mined area to its original topography - high moistures will not allow such method and will therefore result in changes in the topography of the recovered land when compared to the original situation.

Volumetric expansion

When excavating a natural terrain (such as a mine), the soil - that was previously in a certain state of compaction will experience a volumetric expansion. Therefore, it is difficult to fit back into a ‘hole’ everything removed from it.

Since the overburden layer can be considerably larger than the original bauxite layer, this would pose a problem for the backfilling of tailings i.e. the overburden occupies almost all of the available volume.

The solution as proposed by the applicant, addresses this problem by desiccating the tailings to extremely high solids contents (or extremely low moisture). While a minimum solids content of 60 % was established in one mine of the Applicant, the solution has been able to provide solids content of over 80 %.

Tailings at 80 % solids will occupy approximately 4 times less space than tailings at 35 % solids (the original solids content at disposal). This volumetric reduction of tailings - caused by its desiccation - will allow tailings to be backfilled into the mine pits occupying very little space.

Therefore, even when considering the volumetric expansion of the overburden layer, the backfilling can occur and return the land to its original topography. The return of the land to its original topography makes for a more sustainable method of mining since it facilitates the environmental rehabilitation and diminishes the aesthetic impact of the mine.

Further benefits of the invention

While this method may perhaps seem obvious to the skilled person, the sheer volumes of tailings generated in mining - in the mentioned mine approximately 4.5 million tons of tailings per year - make it extremely difficult to find a solution for drying, transporting and backfilling the tailings into the pits, and tailings are commonly deposited in dams or the similar.

By the present method savings in logistics operations can be achieved for instance by exploiting free lorry capacity in return from the bauxite beneficiation plant towards the mine, for the transport of tailings for backfill. This will also reduce the C02 footprint of the operations.

Also, the present method allows the backfilling of large quantities of tailings. This is related to the low moisture of tailings due to drying tailings by solar energy.