Research on Suitable Mining Technology for Placer Titanium Mines in Vietnam

Vietnam has a potential reserve of placer titanium of around 650 Mt. The reserve lays in sandy beaches of the country from Thanh Hoa to Binh Thuan province. Placer titanium mines in Vietnam are different in geological condition, the scale of production, mining method, and environmental protection solutions, etc. Some of them are operating with inappropriate mining technology; therefore, have caused environmental issues and ineffective operation. Consequently, it is essential to have criteria for selecting appropriate mining technology to approach the sustainable development for the mines. Based on categories of geology, the scale of production, mining characteristics, requirements of safety, land reclamation and environmental protection, and mining effect, authors defined the criteria and method for selecting adequate mining technology for the placer titanium mines in Vietnam which aim for better safe, effective and sustainable. The categories to select the appropriate technology for coastal titanium placers in Vietnam including 1feasible mining scheme, 2mining procedure, 3requirement volume of water, 4slope stability, 5environmental rehabilitation and reclamation, and 6economic effect. The result of the application for Nam Suoi Nhum placer titanium mine in Bình Thuan province shows that the most appropriate scheme of this mine is the mining technology of using hydraulic jets, pumping and hauling by pumping mud, processing by spiral screws, and discharging by mud pumping.


Mining technology
To date, coastal titanium placers are extracted in 23 mines from Ha Tinh to Binh Thuan (Tab. 3).
Around the world, titanium placers are now mined either by dry mining or wet concentration methods ( The dry mining scheme is the one that uses mechanical equipment of hydraulic excavator-truck, self-elevating scapers, bulldozer traps or wheel loader to excavate and transport to primary processing equipment and discharging sand. This technology is normally applied to dry deposits or to seams that are above the water level (Bignaraj M. et al., 2019; Greg J., 2009). The wet method scheme uses hydraulic equipment to excavate, such as hydraulic jets, dredgers, and excavating pumps to get ores, transporting them to primary processing equipment and discharging sand. This technology is applied to watery deposits or the seams that are under the water level ( Nowadays, coastal titanium placers mines in Viet Nam are using the two following mining and processing schemes:

Mining using excavators (wheel loaders) and trucks, in combination with the fixed processing complex
Ore is loaded by excavators or wheel loaders to trucks for transportation to the fixed processing complex that is located Submission date: 06-03-2020 | Review date: 22-09-2020 in the centre of the mine, and the mixture of sand and water is pumped into the primary process system. The concentration ore collected from the primary process is reprocessed in the concentration processing component to obtain the required concentration ore (of approximately 85% of heavy minerals). (Fig. 3, where: 1 -Feeding pump of run-off mine ores, 2 -Tank for collecting run-off mine ores, 3 -Screen for separating refuse, 4 -Feeding pump for primary processing component, 5 -Feeding tank for primary processing component, 6 -Primary processing component, 7 -Tank for waste sand, 8 -Pump for removing waste sand, 9 -Feeding tank for the secondary processing component, 10 -Feeding pump for the secondary processing component, 11 -Tank for dividing into flows, 12 -Secondary processing component, 13 -Water pump).
Although this technology of mining and processing offers high output, its disadvantages are a mass investment, non-conformance to small and complicated mines, and encountering difficulties in land reclamation, especially when the distance between the fixed processing complex and the mining areas is considerable. To deposits with the grades are less than 2% of heavy minerals, this technology is ineffective.
Mining using hydraulic jets and pumps, in combination with raft-mounted mobile processing complex The ore bodies are broken by hydraulic jets (if necessary) and pumped to the raft-mounted mobile processing complex of the mine for separation into three kinds of products (tailings, intermediate ore, and concentration ore). The intermediate ore from the primary processing is pumped to the secondary processing to get the concentration ore. The qualified concentration ore (the grade of heavy minerals is higher than 85%) is pumped to the store. Waste sand is pumped to the waste dump in the mined-out areas.
This technology of mining and processing is used for sediments below the underground water level (see Fig. 4, where: 1 -Ore sand suction pump, 2 -Feeding tank with separation mechanism, 3 -Primary processing component, 4 -Intermediate processing pump, 5 -Tank for pumping to the intermediate processing component, 6 -Tank for dividing into flows, 7 -Intermediate processing component, 8 -Tank for pumping to the secondary processing components, 9 -Feeding tank for the secondary processing components, 10 -Tank for dividing into flows, 11 -Secondary processing component, 12 -Tank for pumping waste sand, 13 -Pump for removing waste sand, 14 -Water pump). The technology is currently applied in most of titanium placers mines in Vietnam.

The disadvantages of the current mining technologies
The inappropriate usage of mining technologies has been an impact in mining efficiency, safety and environment protection in the coastal titanium placers in Vietnam, such as: Changing terrain of the region: during the mining and processing, the terrain of the mine area will be changed in altitude, slope, and landscape, etc. These changes create deformations of the coastlines, subsidence of the mine slopes and beaches, and flying sand, etc.
Influence on the ecosystem: Titanium mining and processing impact directly to the soil environment of the coast- al areas. The waste sand volume and backfill areas occupy a small percentage in comparison to the excavated areas. Impact on the water environment: Rivers and lakes surrounding the mining areas are the main water suppliers to the mining and processing. The underground water in the sand is circularly used. However, the mining and processing activities have lowered the local water level, causing salt incursion into water-bearing aquifers.
Loss of mineral resources: The usage of inappropriate mining technology is the reason for natural resources lost and low recovery.
Radioactive pollution: Mining, hauling, and processing of ilmenite can disseminate radioactive substances from accompanying minerals, such as zircon and monazite, containing Thorium, or Uranium. It is worth noting that concentration ores contain a higher grade of radioactive than safety standards.

Feasible mining technologies
Derived from the two above-mentioned mining technologies, the following technologies are feasible for coastal titanium placers in Vietnam. They are: Mining with excavators and trucks, processing by spiral screws, and pumping mud (Scheme No.1) This technology is applied for any productivity of mine that the water supply is not abundant, technological conditions for an internal waste dump are not supportive. An external waste dump which is close to the primary processing station is helpful in this case (Fig. 5).
At the primary processing station, sandy ore is separated from the garbage and gravel by rotation screening or inclined vibrating screening, running off to the sandy mud hole afterwards. Sandy mud is pumped to the ore tank of the processing station, continuously pumped to spiral screws. The levels of spiral screws are assembled depending on the separation feasibility of the raw ore and the grade of the useful heavy minerals. A clutch separator can be installed before spiral screws. However, it requires a stable feeding of ore, and therefore, is rarely used due to difficulties in operation, low recovery and promotion of the secondary processing. The output of the processing component is concentration ore and waste sand. Waste sand is pumped into the waste dump through the mud pipeline system. In case water is circulated, the mining scheme is illustrated in Fig. 6, where: 1 -Mine site; 2 -Excavator; 3 -Truck; 4 -Tank with garbage filter mechanism; 5 -Pumping mud hole; 6 -Spiral scews stations; 7 -Mud pipe- Tab Mining by hydraulic jets, suction and hauling by pumping mud, processing by spiral screws, and discharging mud by pumping (Scheme No. 2) Scheme No. 2 differs from the scheme No. 1 by the combination of hydraulic jets and pumping mud (Fig. 7). This is the popular scheme in coastal titanium placers, especially in areas with abundant water resources and difficulties in directly sucked by pumping mud (see Fig. 8, where: 1 -Hydraulic jets; 2 -Spiral screws station; 3 -Feeding mud hole; 4 -Water pipeline; 5 -Water circulation pumping station; 6 -Water pipeline from filter well; 7 -Pressure water pipeline for hydraulic jets; 8 -Waste dump filter well; 9 -Temporary waste dump of the first mining division; 10 -Waste mud pipeline; Mining by ore pumping, processing by spiral screws, and discharging mud by pumping (Scheme No. 3) This is the simplest mining technology, which is used in small titanium placers in white sand seams or grey sand seams that are not thick (Fig. 9).
In this mining technology, the ore mud pumping is put on the floatation to pump the sandy mud at the lower level and the run-off sand. High-pressure hydraulic jets support to pump into the crest to dilute the mud concentration for a better ore pumping. The ore mud is transported by the pipeline to the primary spiral screws station of the mine site. Discharged sand is pumped into the mined areas by the mud sand pipeline (see Fig. 10, where: (1 -Ore mud pumping hole; 2 -Pumping raft; 3 -Hydraulic jets (if necessary); 4 -Ore sand pipeline; 5 -Spiral screws station; 6-Floatation; 7 -Waste sand pipeline; 8 -Internal waste dump; 9 -Fixed bolts of spiral srcew station; 10 -Backup water hole; 11 -Water auxiliary pump; 12 -Water auxiliary hauling pipeline; 13 -The next mining area).

Mining approach
It is critically important in defining the appropriate mining approach in surface mining, especially in coastal titanium placers in Vietnam. Suitable mining technology does not only ensure the effective and safe mining operation but also support sand discharge and reclamation, partly contributing to the natural recovery and environmental protection. The following methods are proposed to the coastal placers titanium mines in Vietnam.

Longitudinal mining order
The mining procedure is alternated in the longitudinal axis of the mine, from the first block to the last one (Fig. 11).
Horizontal mining order By this mining order, the procedure is alternated in the horizontal axis of the mine (Fig. 12).

Mining order en éventail
This mining order is applied for complex-shape mines, alternately for en éventail blocks (Fig. 13).

Requirement of water
The required volume of water and the ratio of water recovery Either in dry mining or wet concentration methods, water is used to transform the titanium placers to mud sand, transporting them to the spiral screw raft. The required annual volume of a mine for the planned output is defined after the formula Q = Aq.q.[1+(100% -Kth)], m3/year. In which: Aq Fig. 9. The flowchart of the mining technology using ore pumping, spiral screw processing and mud pumping Fig. 10. The flowchart of mining by ore pumping, processing by spiral screws, and discharging mud by pumping Rys. 9. Schemat blokowy technologii wydobycia z wykorzystaniem pompowania rudy, przeróbki ślimakowej i pompowania płuczki Ryc.10. Schemat blokowy wydobycia przez pompowanie rudy, przetwarzanie za pomocą ślimaków spiralnych i odprowadzanie mułu przez pompowanie -Annual ore of output, m3/year; q -Index of water consumption in mining, m3/m3; Kth -Ratio of water recovery, %.

Water consumption
In above-mentioned mining technologies for coastal titanium placers in Vietnam, there is only one phase of the scheme No. 1 (using excavator and trucks) that does not use water. Other phases of this scheme (primary processing and sand discharge) and all phases of schemes No. 2 and No. 3 use waterpower. The necessary volume of water to maintain the mining operation depends on many factors, such as clay content in the ore, the moisture of ore, the ratio of water recovery in the mining, and the mine output, etc.

Slope stability
Due to the complexity of impact factors to define the slope stability, such as underground water, evaluation methods, and sliding surface position, Slope/W (Geoslope) software is used to calculate the slope stability in mining at coastal titanium placers with bench height of above 90 m (Fig. 18). The relationship between the slope stability coefficient and face angle in accordance with the bench height of 90 m in coastal titanium placers in Vietnam is illustrated in Fig. 19. Fig. 20 illustrates the procedure of mining, discharge, and reclamation in blocks for coastal titanium placers in Vietnam.

Environmental rehabilitation and reclamation
Waste sand dunes are moved by movers, which the height after the mover is equal to the natural height of the area. On the whole moved area, plants which are suitable to the natural conditions, such as a windbreak, pine and acacia are planted. The number of plants is 660 trees/hectare. Plants and trees are taken good care in 3 years till the trees are normally developed. In case if any trees are dead, new trees are planted to assure the density of mining in the rehabilitation and reclamation areas (Decision 38/2005/QD-BNN, 2005).
The cost of environmental reclamation, on average, for the coastal titanium placers in Vietnam is estimated at 45.5 million VND (approximately 2,000 USD) for 1 ha sandy waste dump.
The procedure of mining, discharge and reclamation of blocks in coastal titanium placers in Vietnam is shown in Fig.   Fig. 11. Longitudinal mining order Rys.13. Kolejność wydobywania bloków wachlarzowych 21, where: 1-4 -Mine -out blocks have been recalled; 5 -Block for sand discharge; 6 -Current mining block; 7-9 -the following blocks to mine.

Economic effect
The economic effect is a required factor to evaluate and select appropriate mining methods for coastal titanium placers in Vietnam. Although there are many methods to evaluate the economic effect, however, the basic formula is based on the estimated costs of each specific scheme.
3. Revenue (R): R = An×P, VND/year. Where: An -Annual output of raw ore after excavation, t/year; P -Price of a ton of raw ore, VND/t. After calculation, the mining scheme of maximum of equity (EI > 0) is selected to apply.

The selection of appropriate technology to coastal titanium placers in Vietnam
From the six above-mentioned categories, the selection of appropriate technology of coastal titanium placers in Vietnam are as followed: Step 1: Input data of geological and hydrogeological conditions, quality of sands ore, expected output, the shape of mine and other relevant economic -technological indexes.
Step 4: Evaluate the capacity of water supply to requirement volume water as requested by schemes No. 1, No. 2, and No. 3.
Step 5: Check the slope stability conditions by schemes No. 1, No. 2, and No. 3.
Step 6: Check the conditions for environmental rehabilitation and restoration by schemes No. 1, No. 2, and No. 3.
Step 8: Select the scheme that maximizes EI (EI > 0). This is the appropriate mining technology for the mine.
The flowchart of the selection of appropriate technology to coastal titanium placers in Vietnam is shown in Fig. 22.

Pilot calculation to the coastal titanium placers Nam Suoi Nhum, Binh Thuan, Vietnam
The mine Nam Suoi Nhum is in Binh Thuan province, occupying the following information: the deposit is distributed in marine sediment of Pleistocene; the seam thickness is 78 m; the content of clay is 13 -15%; the deposit is located at unfavorable condition of hydrogeology; the reserve of mine   Ryc.22. Schemat doboru odpowiedniej technologii wydobywczej przybrzeżnego okruchowego tytanu w Wietnamie 6. The equity of schemes No. 1 and No. 2 are 0.11 and 0.42, respectively. Consequently, the mining scheme No. 2 (the mining technology of using hydraulic jets, pumping and hauling by pumping mud, processing by spiral screws, and discharging by mud pumping) is the appropriate technology for the coastal titanium placers in Nam Suoi Nhum.

Conclusion
Vietnam has a potential reserve of coastal titanium placers. However, mines here have not been effective operating, facing challenges in safety, loss of resource and negative impacts on the environment due to inappropriate mining technology.
The article offers 6 categories to select the appropriate technology for coastal titanium placers in Vietnam, including (i) necessary conditions: 1 -feasible mining scheme, 2 -mining procedure, 3 -requirement volume of water, 4 -slope sta-bility, 5 -environmental rehabilitation and reclamation; and (ii) a sufficient condition: 6 -economic effect.
A pilot calculation is applied to Nam Suoi Nhum placer titanium mine in Binh Thuan province, showing that the most appropriate mining scheme is the scheme No. 2 (the mining technology of using hydraulic jets, pumping and hauling by pumping mud, processing by spiral screws, and discharging by mud pumping) because it has maximal equity (0.42 in comparison with the equity of remaining scheme is 0.11).
The above mentioned criteria and procedures can be applied also for other coastal titanium placers in Vietnam and in other mines with similar conditions of geology and hydrogeology.

Acknowledgement
This study is funded by Hanoi University of Mining and Geology through the research project T20-15.