The Bige dredging project is important to Ok Tedi Mining Limited's (OTML) operations in minimising the mine’s impact on river bed aggradation, flooding and dieback.
After 16 years of dredging, the river bed downstream of Bige has stabilised and some floodplain forest species are recovering. While positive results are attained for bed aggradation and dieback, it introduces a new environmental risk associated with possibility of acid rock drainage (ARD) developing in the dredge stockpiles. This new risk is managed by the Mine Waste Tailings Project (MWTP).
The project started in 1998 and has since recovered approximately 17 million tonnes per annum of sand and silt. The dredged material is placed in stockpiles alongside the river on what are known as the East and West bank stockpiles. The stockpiles are predominately constructed of sand-and silt-sized participles and have negligible organic and nutrient content. Some sections of the stockpile on the East Bank have reached final design and are awaiting final cover placement and revegetation. Re-vegetation trials have commenced to develop standard rehabilitation protocols that included preferred species, re-vegetation techniques and fertilizer regimes for successful revegetation of the stockpiles.
The primary aim of the revegetation is to stabilise the stockpiles outer slopes and crown through the development of a sustainable vegetation cover.
It has been identified that the finer silt material naturally supports vegetation, and harvesting of the silt material that had accumulated in low points of the stockpiles has resulted in the recovery of over 20,000 tonnes of material that has been reused to cover the sandy slopes of the East Bank stockpile.
In 2010 trials commenced using hydro mulch technology to stabilise areas of the East bank stockpile and provide a matrix for plant growth. The hydro mulch uses a mixture of bonded fibre matrix (chipped sorghum, paper, glues and tackifiers), millet, and a mixture of Phase I tree species seeds such as Acacia auriculiformis and Paraserianthus falcataria. The mixture is sprayed onto the slopes and initial results show a healthy establishment of trees after twelve months. It is anticipated that the outcomes from these trials can be used to determine rehabilitation strategies on future stockpiles surfaces.
Trials on revegetation strategies using mulch and compost, and direct planting of tube stock have also shown success, and strategies to maximise mulch production are being investigated. An industrial tub grinder is now being used to mulch and chip trees and other vegetation. This grinder will progressively chip vegetation cleared from areas where the West Bank stockpiles were extended. The wood chip will be composted and incorporated into the final soil layer to provide valuable organic material.
Mine Waste Tailings Project
In late 2008, the Mine Waste Tailings Project (MWTP) began to address the risk of acid rock drainage (ARD) at Bige and elsewhere in the Ok Tedi and Fly River systems. The ARD risk was a result of increasing sulfur content of the mine’s tailings associated with the mining and processing of skarn ores.
The MWTP consists of three major components with a capital cost exceeding PGK1, 221 million (USD 466 million).
The first component was the construction of the Tailings Processing Plant (TPP) which is effectively a complete flotation circuit added to the back end of the existing copper concentrator. The TPP removes up to 80 percent of the sulfur in the form of mineral pyrite from the copper concentrate tailings.
The TPP produces two products:
- a Pyrite Concentrate (Pcon) which is transported via pipeline to Bige for safe storage in constructed subaqueous pits on the West Bank at Bige and
- a tailings stream with a mean sulfur concentration of less than 1 percent is discharged to the river system.
The second component of the MWTP is a 125 kilometre pipeline which transports the PCon from the mill to Bige and the third component is a series of large storage pits dredged into the Ok Tedi floodplain that allow the PCon to be safely and permanently stored below the permanent water table.
By decreasing the sulfur content of the tailings, the sulfur content of the sediments deposited throughout the river system is gradually decreased thereby decreasing the ARD risk. Since the implementation of the MWTP the sulfur concentration in dredged material has decreased to approximately 1.6 percent in 2013 from around 3.0 percent in 2008. Simultaneously, the Acid Neutralising Capacity (ANC) of the dredged material has increased as a result of additional limestone input into the mine’s waste dumps and through milling of barren limestone. As a result, the dredged sediment geochemistry has changed from being Potentially Acid Forming (PAF) in 2008 to being on average, Non-Acid Forming (NAF) now.
The best measure of the improvements in the geochemical quality of the dredged material is through ANC/MPA (Maximum Potential Acidity) ratio which is a measure of the risk acid formation occurring for a sample. Extensive testing on OTML dredged materials shows that if the ANC/MPA is less than one, there is a very high probability of the sediment being acid forming.
If the value is greater than one, there is less risk of acid formation. The higher the ANC/MPA ratio the lower the ARD risk. The ANC/MPA target for the dredge sediments under the MWTP’s approval is 1.5. This target has been achieved since 2011 and for the year ending 31 December 2013 the ANC/MPA ratio was 1.98.