WO2019040995A1

WO2019040995A1 - Mining method and mine

Info

Publication number: WO2019040995A1
Application number: PCT/AU2018/050945
Authority: WO
Inventors: William Mckee; Chantel WESTLEY; Andrew Roberts; Chris BERESFORD; Ellody KEARNEY
Original assignee: Dampier Salt Ltd
Current assignee: Dampier Salt Ltd
Priority date: 2017-08-31
Filing date: 2018-08-31
Publication date: 2019-03-07
Anticipated expiration: 2020-02-29

Abstract

A method of mining gypsum includes: in-situ leaching gypsum in a harvesting bed, such as a leach pad 5, in an existing gypsum resource with a leach liquor and dissolving or otherwise washing gypsum and producing upgraded gypsum in the harvesting bed. The method also includes harvesting or otherwise removing upgraded gypsum from the harvesting bed and transporting upgraded gypsum directly as a marketable product to a port or other location.

Description

MINING METHOD AND MINE

TECHNICAL FIELD

The present invention relates to a method of mining gypsum. More directly the present invention may be described as a method of mining and processing gypsum. The present invention also relates to a gypsum mine.

BACKGROUND ART

Dampier Salt Ltd. is a company in the same group of companies as the applicant.

Dampier exports bulk solar salt from operations at Port Hedland, Dampier and Lake MacLeod. The northern sites produce saturated brine for salt growth through seawater concentration. The Lake MacLeod operations extract brine from the Lake MacLeod Evaporate Basin. The evolution of this lake system has resulted in a geology rich in gypsum (i.e. hydrated calcium sulphate), forming one of the largest deposits in the world. Lake MacLeod operations are ca. 80km from Carnarvon, with four operational areas; salt crystallisers, wet salt processing and stockpiling (main buildings and services), gypsum operations (processing and stockpiling) and combined gypsum and salt loading facilities (Cape Cuvier Port Facility).

The gypsum-mining lease is primarily Lake MacLeod. The lake is comprised of three main formations; Ibis Gypsite, Texada Halite and Cygnet Carbonate. The gypsum ore body is a geographically extensive formation immediately below the lake surface. The gypsum ore body has four layers of similar mineralogical and chemical composition, but distinctive stratigraphic characteristics. The underlying Texada Halite (ca. 5m thick) marks the lower extent of the ore body. Below this formation is Cygnet Carbonate. This layer wedges-out at the shoreline to form a hydroseal, which is thought to regulate brine density in the underlying aquifer.

The gypsum ore is soft (standard penetration testing or SPT N values of 19-30) and brine can be present 20-30cm below the surface depending on basin hydrology and inflows. The gypsum particle size distribution is broad and largely uniform, with sand to silt and clay-size particles. The exception is a predominance of gravel-sized particles (4-90mm) comprised of prismatic and pyramidal crystals in the second layer. The method of extraction that has been used at the Lake MacLeod operations comprises the following steps:

(a) floating a dredge with a cutter drum on the lake and extracting gypsum ore for wet classification by cyclones to reject fines and produce a gypsum slurry;

(b) stockpiling the gypsum slurry, usually off the lake bed, and

(c) heap leaching the stockpiled gypsum slurry with water to reduce the salts, mainly sodium chloride (NaCl), concentrations introduced by brine in the lake.

The heap leaching can take up to 16 weeks before a satisfactory reduction in salts is reached within the gypsum.

An alternative method that has been applied in lieu of a dredge, is to extract the raw gypsum from the lake bed by mechanical excavator and truck to the heap leaching pad for heap leaching.

The above description is not an admission of the common general knowledge in Australia or elsewhere.

SUMMARY OF THE DISCLOSURE

Dampier has carried out research and development work on mining and processing of gypsum. The present invention is an outcome of the research and development work.

The invention is based on a realisation that an in-situ leaching (which can also be described as in-situ washing) method within harvesting beds of the Lake MacLeod operation is an effective option for mining gypsum ore.

In broad terms, the invention is a method of mining gypsum that includes in-situ leaching gypsum in a harvesting bed in an existing gypsum resource with a leach liquor and dissolving or otherwise washing "contaminants" from gypsum from the harvesting bed and producing upgraded gypsum in the harvesting bed.

The in-situ leaching method of the invention further includes harvesting or otherwise removing upgraded gypsum from the harvesting bed and transporting upgraded gypsum directly as a marketable product to a port or other location. The term "contaminants" is understood herein to mean contaminants in relation to gypsum as a marketable product. The term "contaminants" includes by way of example chlorides in brine. Compounds that are "contaminants" in the context of gypsum may be valuable products in another context.

The invention may include:

(a) forming the harvesting bed in the gypsum resource, which may be a in a lake, such as Lake MacLeod;

(b) in-situ leaching gypsum in the harvesting bed with the leach liquor and producing upgraded gypsum in the harvesting bed; and.

(c) removing upgraded gypsum from the harvesting bed as the marketable product.

The method may include forming in-situ leach pads in the harvesting bed.

The method may include forming in-situ leach pads in the harvesting bed by digging or otherwise forming drainage channels for each liquor that has passed through the in-situ leach pads in the harvesting bed and thereby forming perimeters of the in-situ leach pads. With this arrangement, in use, at least some of the leach liquor that passes through the in-situ leach pads drains into the channels.

The method may include pumping drained leach liquor from the channels.

The leaching step may include supplying the leach liquor to an upper surface of gypsum in the harvesting bed and draining leach liquor that flows downwardly through the harvesting bed from the bed.

The term "harvesting bed in an existing gypsum resource" is understood herein to mean any suitable section of a gypsum resource that is a naturally-formed resource - as opposed to a heap or other formation that is artificially constructed.

For example, the harvesting bed may be a section of a lake, such as Lake MacLeod.

In the context of Lake McLeod, the term "harvesting bed" is understood to mean a section of the lake that is in the form of a bed of the lake that has successive layers of gypsum that have been deposited as a result of successively providing new supplies of brine to the bed. The water in each new brine supply evaporates and gypsum precipitates from the brine and deposits as a lower layer and then NaCl precipitates from the brine and deposits as an upper layer, leaving a residual liquor. The residual liquor is drained away and the NaCl layer is removed and forms a NaCl product. The gypsum layer remains in the harvesting bed. In this context, it is noted that gypsum precipitates first from brine as water evaporates from brine and NaCl precipitates after gypsum. After the NaCl is removed, a new supply of brine is supplied to the harvesting bed and the above cycle is repeated. The cycle of brine supply, evaporation, and NaCl removal is repeated until the harvesting bed is essentially a series of gypsum layers.

The method may include recycling leach liquor that has drained from the harvesting bed to the same or another harvesting bed.

The leach liquor may include ground/bore water.

The leach liquor may include fresher/cleaner water than ground/bore water.

The leaching step may include a first stage of leaching with ground/bore water and a second stage of leaching with fresher/cleaner water to provide to clean the gypsum to a required level.

The fresher/cleaner water may be produced from ground/bore water by reverse osmosis or any other suitable option for purifying ground/bore water.

The method may include successively removing layers of the processed upgraded gypsum.

More particularly, the method may include:

(a) leaching gypsum in the harvesting bed with the leach liquor and producing upgraded gypsum in at least an upper layer of the harvesting bed;

(b) removing upgraded gypsum from the upper layer of the harvesting bed as a marketable product;

(c) leaching gypsum in the harvesting bed with the leach liquor and producing upgraded gypsum in at least a new upper layer of the harvesting bed;

(d) removing upgraded gypsum from the new upper layer of the harvesting bed as a marketable product; and

(e) repeating the above steps.

The upper layer(s) may be any suitable depth.

By way of example, the upper layer(s) may be 250mm-lm deep.

The method may include allowing time for the upgraded gypsum to dry after a leaching period to a predetermined extent before removing the upgraded gypsum. The applicant has found in trials that in-situ leaching takes about 6-8 weeks - in contrast to the current dredging/stockpile/leaching method that typically takes 12-16 weeks to produce comparable purity gypsum.

The invention also provides a gypsum mine that includes an in-situ harvesting bed in an existing gypsum resource, drainage channels for receiving leach liquor that drains from the harvesting bed, and a system for supplying a leach liquor to the harvesting bed.

The harvesting bed may include a plurality of leach pads, with the drainage channels forming perimeters of the leach pads, and with the system for supplying the leach liquor being arranged for supplying the leach liquor to the leach pads.

The invention also provides a gypsum mine that includes an in-situ harvesting bed in an existing gypsum resource, with the drainage channels forming perimeters of the leach pads, and a system for flooding the harvesting bed with a leach liquor.

The harvesting bed may include a plurality of leach pads of gypsum, with the drainage channels forming perimeters of the leach pads, and with the system for flooding the harvesting bed being arranged for flooding the leach pads.

The system for supplying the leach liquor may include an irrigation system.

The mine may include driveways for allowing mining equipment such as harvesters to access the leach pads to remove upgraded gypsum from the leach pads.

The mine may include a system for pumping leach liquor from the drainage channels.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is described further with reference to the accompanying Figures, of which:

Figure 1 is a diagram that illustrates the gypsum mining method and gypsum mine of the invention in general terms;

Figure 2 is a top plan view of an embodiment of a section of a gypsum mine in accordance with the invention;

Figure 3 is a top plan view of an embodiment of a section of a gypsum mine in accordance with the invention that also includes information relating to an embodiment of a method of mining gypsum in accordance with the invention; Figure 4 is a vertical cross-section of the section of the gypsum mine shown in Figure 3; and

Figures 5 and 6 are vertical cross-sections through a part of the section of the gypsum mine shown in Figure 1 that illustrate different stages in an embodiment of a method of mining gypsum in accordance with the invention.

DESCRIPTION OF EMBODIMENT

It is emphasised that the Figures show embodiments of a method of mining gypsum and a gypsum mine in accordance with the invention and the invention is not limited to the embodiments.

Figure 1 illustrates very diagrammatically the gypsum mine and the method of gypsum mining in accordance with the invention in general terms. Figures 5 and 6 illustrate two stages of an embodiment of a mining method in accordance with the invention that can be carried out in the mine shown in Figure 1.

Figures 1, 5, and 6 show a section of a gypsum mine 3 that has at least one harvesting bed in the form of a leach pad 5 of gypsum that is formed in situ in an existing gypsum resource.

The gypsum resource may be, by way of example, a harvesting bed in the Lake MacLeod operation. The gypsum resource may be any other suitable existing gypsum resource.

A drainage channel 7 defines an outer perimeter of the leach pad 5. The drainage channel 7 is dug into the gypsum resource, with the removed material being used to form an outer raised bank/levee 1 1 shown in Figure 1 but not in Figures 5 and 6. The leach pad 5 may be one of a plurality of leach pads 5 in the harvesting bed.

Figures 1, 5, and 6 also show that the mine 3 includes a system 9 for spraying or otherwise distributing a leach liquor onto an upper surface of the leach pad 5.

Typically, the leach liquor is ground/bore water or fresher/cleaner water. More typically, the leach liquor is ground/bore water for a first leach period and

fresher/cleaner water for a second leach period. The purpose of the leach liquor is to reduce the concentration of NaCl and other soluble contaminants in the gypsum to a required a purity threshold that is suitable for a marketable product. The fresher/cleaner water may be produced from ground/bore water by reverse osmosis or any other suitable purification system.

The system 9 may include spray nozzles supported above the leach pad 5 to distribute leach liquor onto the leach pad 5. Other options include, by way of example, sprinklers on the leach pad 5, agricultural booms with outlet nozzles positioned to extend above the leach pad 5, and a motorised sprinkler system that can move over the leach pad 5.

It can be appreciated from Figures 1, 5, and 6 that the leach liquor flows downwardly through the leach pad 5 and washes/leaches soluble materials such as chlorides from the gypsum, thereby purifying, i.e. upgrading, the gypsum. The arrows in Figure 5 show the downward flow of leach liquor during a leach stage of the method. As is also shown in Figure 5, at least some of the leach liquor that flows through the leach pad 5 flows outwardly into the drainage channel 7. The remaining leach liquor flows downwardly to the water table, which typically is quite close to the surface in the case of Lake MacLeod.

Typically, the mine 3 also includes a pump and other drainage system components (not shown) that remove leach liquor from the drainage channel 7.

The mine 3 may also include a system (not shown) for recycling drained leach liquor from the drainage channel 7 to be used again in the same or a different leach pad 5.

In use, the leaching stage runs until gypsum in at least an upper layer, typically 150-900 mm (basically a shallow cut), of the leach pad 5 reaches a purity threshold that is suitable for a marketable product. Based on trials carried out by Dampier, a suitable time period may be as short as 2 weeks, with drainage time to follow this time period.

At this point in time when the upper layer reaches the purity threshold, the supply of leach liquor to the leach pad 5 is stopped and the upper layer of purified gypsum is allowed to drain and dry. The upgraded gypsum at this point at the end of the leach stage is the layer shown as the circled regions in the upper section of the leach pad 5 shown in Figure 6.

When the upper layer has dried to a threshold dryness to allow harvesting, a harvester 13 such as a continuous salt harvester or other suitable excavating equipment (see Figure 1) removes the upper layer of upgraded gypsum from the leach pad 5 and transfers the material directly into a trailing transport vehicle 15 that transports the upgraded gypsum as a marketable product to a port or other location. In this regard, the mine 3 includes a network of driveways (not shown) that allow mining equipment such as excavators to access the leach pads 5 as described above.

When the upper layer is removed, the supply of the leach liquor is re-started and the remaining gypsum in the leach pad 5 is leached further. This leaching stage continues until a new upper layer reaches a purity threshold that is suitable for a marketable product.

The above-described drying and removal steps are then repeated to remove this next layer of upgraded, i.e. marketable, gypsum. These leaching, drying, and removal steps are repeated on successive layers of the leach pad 5 until it becomes impractical, for example, because the water table has been reached, to continue.

The above described mine and mining method have a number of advantages over the current dredging/stockpile/leaching method that has been used at the Lake McLeod operations, including the following advantages.

1. 300 mm layers, can be removed with a harvester 13 such as a salt harvester that is a substantially cheaper option than the original methods.

2. The single handling of gypsum, i.e. removing upgraded gypsum directly from the leach pad 5 onto the transport vehicle 17, to a port results in further cost advantages compared to original methods.

3. The drainage channel 7 and pumping system remove leach liquor from the leach pad 5 and this makes it possible to achieve low levels of chlorides through effective leaching.

The section of another embodiment of a mine in accordance with the invention shown in the top plan of Figure 2 has the same basic features of a leach pad 5, drainage channel 7, and driveways (identified by the numeral 17) shown in Figure 1.

The same reference numerals are used in Figures 3 and 4 to describe the same features as shown in Figures 1 and 2.

Whilst not shown, the Figure 2 mine also includes the other features, such as a pumping system, described in relation to Figure 1.

In addition, the mine shown in Figure 2 is suitable for mining gypsum in accordance with the mining method described in relation to Figures 1, 5 and 6. Figure 2 also shows a gypsum mine 3 on a larger scale than that shown in Figure 1, although noting that Figure 1 only shows a section only of a mine.

In Figure 2, the mine includes 8 leach pads 5 grouped together in a "cell" that is bounded by a bank 19.

The bank 19 protects the group of leach pads 5 from external flooding. The bank 19 also provides an opportunity for flood leaching the group of leach pads 5.

The leach pads 5 in the cell are defined by drainage channels 7, which are common to adjacent leach pads 5.

The cell also includes a single main discharge outlet 21 for leach liquor that drains from the leach pads 5 into the drainage channels 7.

It can be appreciated that Figure 2 illustrates, by way of example only, how it is possible to form a large-scale mine that comprises a series of basic units of the leach pads 5, larger cells of a group of leach pads 5, and a bank 19 for the cells.

The top plan and vertical cross-section of Figures 3 and 4, respectively show another embodiment of a section of a gypsum mine 3 in accordance with the invention. The same reference numerals are used in Figures 3 and 4 to describe the same features as shown in Figures 1 and 2.

The section of the mine 3 shown in Figures 3 and 4 includes a rectilinear leach pad 5, noting that the leach pad 5 may be any suitable shape and dimensions, that is bounded by inner and outer drainage channels 7a and 7b separated by a boundary wall 23 of gypsum. A leach system 9 supplies leach liquor to the upper surface of the gypsum in the leach pad 5. The leach liquor flows through the leach pad 5 as described above and drains into the inner drainage channel 7a. Leach liquor in the drainage channel 7a is pumped from the channel and either recycled to the leach pad 5 via a stream 27 or rejected in a discharge stream 25. The inner and outer drainage channels 7a and 7b and the intermediate boundary wall 23 operate as a reverse osmosis cell, with the intermediate boundary wall 23 defining a semi-permeable membrane.

With this arrangement, dewatered brine passes into the outer drainage cell 7b from inner drainage channel 7a and leaves a leach liquor with a lower salt concentration in the inner drainage channel 7a to be recycled to the leach pad 5. Many modifications may be made to the embodiments of the present invention described above with reference to the Figures without departing from the spirit and scope of the invention.

By way of example, whilst the invention is described in the context of the Lake McLeod operations, it can readily be appreciated that the invention is not limited to these operations.

Claims

1. A method of mining gypsum includes: in-situ leaching gypsum in a harvesting bed in an existing gypsum resource with a leach liquor and dissolving or otherwise washing gypsum and producing upgraded gypsum in the harvesting bed, harvesting or otherwise removing upgraded gypsum from the harvesting bed, and transporting upgraded gypsum directly as a marketable product to a port or other location.

2. The method defined in claim 1 includes:

(a) forming the harvesting bed in the existing gypsum resource;

(c) removing upgraded gypsum from the harvesting bed as the marketable product.

3. The method defined in claim 1 or claim 2 includes forming in-situ leach pads in the harvesting bed.

4. The method defined in claim 3 includes forming in-situ leach pads in the harvesting bed by digging or otherwise forming drainage channels for each liquor that has passed through the in-situ leach pads and thereby forming perimeters of the in-situ leach pads.

5. The method defined in claim 4 includes pumping drained leach liquor from the drainage channels.

6. The method defined in any one of the preceding claims wherein the leaching step includes supplying the leach liquor to an upper surface of the harvesting bed and draining leach liquor that flows downwardly through the harvesting bed.

7. The method defined in claim 6 includes recycling leach liquor that has drained from the harvesting bed to the same or another harvesting bed.

8. The method defined in any one of the preceding claims wherein the leach liquor includes ground/bore water.

9. The method defined in any one of the preceding claims wherein the leach liquor includes fresher/cleaner water than ground/bore water.

10. The method defined in any one of the preceding claims wherein the leaching step includes a first stage of leaching with ground/bore water and a second stage of leaching with fresher/cleaner water.

11. The method defined in any one of the preceding claims includes successively removing layers of the upgraded gypsum.

12. The method defined in claim 1 1 includes:

(e) repeating the above steps.

13. The method defined in any one of the preceding claims includes allowing time for the upgraded gypsum to dry after a leaching period to a predetermined extent before removing the upgraded gypsum.

14. A gypsum mine that includes at least one in-situ leach pad of gypsum in a harvesting bed, drainage channels that form perimeters of the leach pads for receiving leach liquor that drains from the leach pads, and a system for supplying a leach liquor to the leach pads.

15. The mine defined in claim 14 includes driveways for allowing mining equipment such as harvesters to access the leach pads to remove upgraded gypsum from the leach pads.

16. The mine defined in claim 14 or claim 15 includes a system for pumping leach liquor from the drainage channels.