GOLD PROCESSING
for these projects were designed around the Leachox process .
As with most gold ore deposits , Kibali consists of a covering of weathered oxide ore , underlain by a layer of transitional ore . Under the transitional ore was the primary sulphide ore , which constituted most of the orebody and was refractory in nature .
Randgold designed the process flowsheet for Kibali to first treat the oxide ore by a conventional CIP method with Aachen Reactors installed to increase dissolved oxygen levels , negate cyanide consuming species in the ore and maintain a higher level of gold recovery . When transitional ore was introduced , the reactors had to work harder to maintain maximum recovery . However , the plant was designed to treat most of the orebody in a separate circuit where the gold was contained in refractory sulphides .
To this end a flotation plant and concentrate ultra-fine grind circuit was installed prior to the Aachen Reactors . Combined with ultra-fine grinding , the reactors partially oxidised the sulphides , thus liberating the gold particles to be leached . The Aachen Reactors also negate passivation to assist the gold leaching process .
Finally , Aachen Reactors were also installed in combination with the air / SO 2 cyanide destruction process to make the plant tailings safe for depositing in the tailings dam .
In all , there were 18 Aachen Reactors installed at Kibali , and , eventually , Aachen Reactors were installed or retrofitted at all of Randgold ’ s operations .
The company concluded : “ The licence and supply of Aachen Reactors to the client means that they are monitored and maintained by Maelgwyn in a continuous good working order . This , with the continued support by the Maelgwyn technical team , including metallurgical laboratory development and process troubleshooting services to the whole operations , means that the excellent working partnership between Maelgwyn and Barrick Gold has continued into this next decade .”
Recognition in real time
Real-time sensors have been used in the mining industry for decades , but their application in the gold space has lagged other market segments .
Scantech International Pty Ltd ’ s Henry Kurth ( Chief Marketing Officer & Minerals Consultant ) and Dr Luke Balzan ( Chief Technology Officer ) say this is changing , with new ore and waste quality digitalisation solutions tailored specifically for measuring grade in gold ores .
“ Metal recoveries can reduce by 10 % or more due to feed quality variability alone ,” Kurth and Dr Balzan explained . “ Measuring and controlling feed quality , therefore , has major benefits .”
Some major gold companies no longer accept
sub-optimal performance and are measuring feed quality variability , they said , developing a better understanding of its controls and consequences , and introducing systems to manage and improve it .
Moisture and fragmentation measurement have been available for many years and have demonstrated numerous benefits . There are also numerous benefits to adopting some sensing technologies for real time conveyed gold ore measurement .
“ Sensors used prior to primary crushed conveyed flow are not able to provide high quality , representative measurement of bulk ore and waste ,” Kurth and Dr Balzan said . “ Sampling of coarse blasted rock , ore stockpiles , or material in shovels and trucks is impractical and expensive and gold ’ s high nugget effect results in significant sampling error for results available many hours later .
“ Sub-samples for duplicate or triplicate assays often contain high variability so they are averaged in the expectation that this value is closer to the truth : but is it ?”
Primary crushed material on a conveyor can be representatively and continuously measured over small tonnage increments in real time using high specification prompt gamma neutron activation analysis ( PGNAA ). Recent advances , as well as ongoing incremental improvements , have resulted in the GEOSCAN GOLD being used successfully on gold ores , according to Kurth and Dr Balzan . Production units are installed with three of the top 10 global gold miners , one of which is due to publish a paper on its application in March 2022 .
PGNAA analysers measure a suite of elements over each 30-second increment of conveyed flow , including proxies for gold where available , as well as direct measurement of gold over 5-10 minute increments at precisions suitable for
Scantech ’ s GEOSCAN GOLD measures elements in primary crushed rock in conveyed flows at < 100 t / h to > 10,000 t / h irrespective of belt speed , particle size , mineralogy , dust , moisture , or segregation / layering , the company says
decision making ( as low as 0.2-0.3 ppm Au achieved ), the pair said . Decisions can be made for ore blending changes , bulk increment diversion and feed forward control . The measurement data is also used for metal accounting and ore reconciliation .
They continued : “ Measurement data can generate additional benefits , some of which are being implemented : n “ Immediate feedback to mining operations helps confirm grade delivered against expected quality so schedules can be adjusted in real time ; and n “ Feed forward control can be used to identify quality parameters that affect processing in real time : o “ Ore hardness for mill feed control ; o “ Mineralogy and deleterious content for reagent dosing control to optimise recoveries , ( eg clays , talc , sulphides , carbonates , carbonaceous , etc ); and o “ Sulphur content for diverting sulphide ore to flotation and oxide ore to leaching , segregating acid-generating waste from clean waste , smelter feed chemistry control , etc .”
Moisture measurement using through belt moisture ( TBM series ) microwave transmission technology measures free moisture to precisions typically better than 0.5 % moisture as a standalone system or integrated with GEOSCAN , according to Scantech . Such measurements in run-of-mine ore prove useful in ore reconciliation and metal accounting as dry tonnages can be easily and reliably
32 International Mining | NOVEMBER / DECEMBER 2021