PASTE & FILTERED TAILINGS
Large deep cracks before next layer during the > 12 years stack life of bauxite tailings TSF
Tailings surface stacking – a longevity review
Jerold Johnson & Brad Bentley of WesTech Engineering , LLC talk about how paste and thickened tailings facilities stand the test of time
Surface stacking has provided plants with increased storage capacity , improved stability , and conservation of water for decades . It has been used around the world in all type of mines , in dry to wet climates . This paper provides a brief technology introduction for producing Paste & Thickened Tailings ( P & TT ), using a paste expert team for each process stage ( production , transport , geotechnical ) and case studies . The longevity of the technology is shown in profiles of three active sites : 1 ) an iron site operating in Inner Mongolia for over 14 years , 2 ) an iron site in Republic of South Africa with about 16 years of operation and 3 ) a bauxite site in Brazil with 15 years of operation . Each of these sites illustrates the benefits from proper design , layout , sizing of equipment , and operation of a surface stack . This is a seasoned technology .
The importance of proper Tailings Storage Facilities ( TSF ) design and management is paramount to reduce the associated risks of tailings storage . More than 20 years ago , surface stacking technology using P & TT was introduced . It brought with it the claimed benefits of increased water recovery , low-risk TSF , increased site capacity , the ability to be used in wet and dry climates , quick access for remediation , and the ability to be used with most any tailings that satisfy some minimum requirements . After twenty years , it is time to evaluate if these claims have been met . This article reviews three sites , each using surface stacking for around 15 years .
Any discussion about P & TT is best started with a reminder of the unique non-Newtonian properties that are the foundation for all these process claims , how and why it works . These unique properties of P & TT begin with the yield stress that provides a relatively non-settling , non-segregating suspension . Producing a desired yield stress , the paste-type thickener dewaters to a significantly higher wt % solids than a high rate thickener ( 10-15 % higher ). This higher underflow density provides rapid drying as the water is drawn to the surface . The deposit has a shallow angle of repose , which will slowly drain any precipitation ( free water ) to not erode the stack and keep the surface exposed to the air for drying .
P & TT technology is unlike high rate thickening in several critical ways . The pastetype thickener mechanism must include properly designed low-profile rake arms with high raking capacity blades and dewatering pickets . The K- factor will be 5-10 times more than with high rates . With the very deep thickening bed in paste-type thickeners , the high rate rake-lifts concepts / benefits do not apply . These are critical features that must be optimized . All suppliers ’ designs are not equal . The high rate thickener underflow is a settling slurry and often
targets a wt %. However , a P & TT thickener must target a yield stress . By targeting a relatively steady yield stress , the underflow is suited for the downstream process stages .
Each process stage in a P & TT system ( dewatering in the thickener , transportation to the TSF , and TSF layout and management ) must be designed to handle the yield stress and viscosity at that stage . This requires experts for each process stage . P & TT expertise is not prevalent like knowledge and experience with slurry . Assembling a paste team best addresses this unique material and the different challenges of each site . Each site can consider different layouts , such as two-stage thickening when there is a distance to the TSF . The high rate thickener would be located at the processing plant and transporting the underflow the several km to the TSF . The smaller volume of the underflow and return paste-type thickener overflow reduces the transportation costs compared to producing the P & TT at the plant and transporting this higher viscosity material to the TSF directly . The management of the TSF is less complicated as the paste-type thickener will have some surge capacity for spigot changing . Also , with short P & TT transport lines , it will use less flush water for line shutdowns . Any emergency dump sites would be at the TSF and not by the plant .
A successful installation must start with the thickener performance . WesTech supplied the paste-type thickener for these three longevity site studies . With almost 15 years of operation , the technology is not new , there has been enough operating time to confirm if the promises have been achieved .
Case study 1 : China
The first site is in the arid climate of Inner Mongolia . This iron mine , Baoto , and the surrounding community receives 2,500 m 3 / h ( 11,000 gpm ) water pumped from the Yellow River some 130 km ( 80.8 miles ) away . This greenfield plant produces 2,100 t / d of tailings . The TSF plan identified a shallow valley about 5 km ( 3.1 miles ) from the processing plant . The tailings system uses two-stage thickening with a 48 m ( 157 ft ) HiFlo™ high rate thickener at the beneficiating facility recovering a majority of the water and reducing the volume to be pumped to the second stage 20 m ( 65.6 ft ) diameter WesTech Deep Bed™ paste thickener at the TSF site . The paste-type thickener design generally uses a smaller diameter thickener compared to high rate thickeners . These thickeners have steeper floor slopes and taller sidewall heights . At this site , the high rate is producing in the low- 50 wt %, the paste-type thickener dewaters
P10 International Mining SUPPLEMENT | JULY 2022