Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

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Leach circuit residence time testing (2 replies and 1 comment)

N
NathanT
6 years ago
NathanT 6 years ago

I am wondering if anyone has done any residence time testing on any leach circuits or tanks and has any advice they could supply.  We are planning to do a residence time test on our leaching circuit shortly and have ordered Azorubine (basically powdered red dye) to be used in the test. 

 

We ordered enough dye (250 kg) to dose the entire circuit (9100 cubic meters) to approximately 50 ppm.  It will be split over several tanks so we will never see 50 ppm coming out but our colorimeter has no issues reading down to 1ppm.    The dye is readily soluble in solution to 120 grams / L. 

We have tried two trials on one tank 394 cubic meters in size using 8 kg of the product with a bit of mixed results.  We were able to read and get a decent looking curve for the retention time; the results were however a bit different in test to test.  One indicated a median retention time of 50 minutes the other 75 minutes.  The testing conditions were not entirely the same however given that one test (shorter time) the dye was added as a powder straight into the downcomer of the tank and the other the dye was added as a ‘slurry’ straight into the tank.  I will mention that the slurry did not mix well in the pail (stuck to the sides and clumped like flour in too little water) and took a while to get out.

 

We are planning to just dump the 250 kg into the front tank of the circuit and sample the back end and a few of the larger tanks periodically.  Should we perhaps consider getting several totes or a small tank to solubilize the dye before adding it to the tanks?   It would take a bit longer to add but I think there might be a benefit to having the dye already in solution as opposed to mixing in the tank. 

Any advice or suggestions are appreciated there is not any real defined procedure I could find for this type of test.  The data will be very useful to us as we increase throughput in the plant and to help target proper cyanide concentrations for the kinetics.

 

Thanks

J
John R
6 years ago
John R 6 years ago

Which phase do you wish to measure the residence time for.  Traditionally, in a gold leach circuit it is the solids which are of interest.   If this is the case, zirconium flour (zirconium silicate) is often used.  It has approximately the same size distribution and SG as many ores and is easily detected by XRF.

John R

 

 

N
NathanT
6 years ago

We do wish for the solids but given that everything is in slurry both the solids and liquids should have the same residence time (not accounting for small variances in the thickener density) or we would have a buildup of solids in the tanks..

b
Robert
6 years ago
Robert 6 years ago
1 like by David

Putting into solution prior to introducing the tracer is a good practice. The time to dissolve may result in loss of resolution of the tracer peaks. The time for tracer dissolution can be more problematic if the tracer sits for some time and becomes more resistant to dissolution, i.e., watch storage practice.

You have not mentioned location. There has been some past work in this area using nuclear and non-nuclear tracers, e.g.,

Dagadu, C.P.K., et al., Determination of Malfunctions in Gold Processing Tanks by RTD Modelling, Rsch. J. Appl. Sci., Eng. Tech., vol. 4,2012, 262-268.

http://maxwellsci.com/print/rjaset/v4-262-268.pdf

Radiotracer technique was used to measure the Residence Time Distribution (RTD) of material in 4 leach tanks at AngloGold Ashanti, Iduapriem gold processing plant. The investigation was conducted to determine the mean residence time and possible flow malfunctions in the tanks. The liquid phase of gold ore slurry was traced using 131I radiotracer which was instantaneously injected into the feed stream of the first tank. RTD curves were generated using sampling data collection method to monitor the radiotracer at tank outlets. The measured RTD data were treated and the Mean Residence Times (MRTs) of material in the tanks determined. For each tank, the experimental MRT calculated from the method of moments exceeded the corresponding theoretical MRT. The experimental curves were simulated using perfect mixers-in-series with exchange model. Results of the simulation indicate exchange of flow between life and stagnant volumes at a very slow rate leading to increase in the MRTs.

 

Grosulak, K., Barrick thiosulfate leaching of double refractory ore, BS Honors Thesis, Univ. Nevada – Reno, 2017.

https://scholarworks.unr.edu/bitstream/handle/11714/1881/Kehley_Grosulak_Honors_2017.pdf?sequence=1

Barrick has partnered with the University of Nevada, Reno’s Chemical and Materials Engineering Department in order to examine the thiosulfate leaching process. Barrick requests the Leaching Team to optimize the thiosulfate leaching process for double refractory ores. In addition, the Leaching Team analyzed the thiosulfate leaching process compared to the traditional cyanide process. They found that the profit of the thiosulfate leaching process was similar to that of the traditional cyanide method. Furthermore, it was found that after three hours of thiosulfate leaching, 63 weight% of gold was recovered of the original pre-autoclave ore. The goal, always, is to try to achieve maximum recovery of gold. Although the percent recovered after three hours was not perfect, it provides a strong starting point for next year’s Capstone Team.

 

 

Stegowski, Z., et al., Determination of flow patterns in industrial gold leaching tank by radiotracer residence time distribution measurement, Nukleonika, vol. 55, 2010, 339-344.

https://pdfs.semanticscholar.org/5616/10f0de4de7429ed131353adbf4e59ca9593f.pdf

The carbon-in-leach (CIL) process is one the most efficient methods of gold recovery from gold bearing ores. The efficiency of the leaching process greatly depends on the flow structure created by mechanical agitation (in some cases air agitation) in the leaching tanks. Residence time distribution (RTD) measurement was conducted in the CIL section of a gold processing plant in order to determine the flow structure in the first tank using the 131I radioactive tracer. The shape of the experimental data revealed that the flow behaviour in the tank was close to an ideal mixer. Modelling of the experimental data, however, revealed that the tank was not behaving as a single perfect mixer, but consisted of two mixing zones. The flow structure in the tank was best described by the “perfect mixers with exchange” model consisting of two mixing zones. The model allowed the determination of flow parameters including the mean residence time, flow rate and volumes of the mixing zones.

 

Zwolak, G., et al., Barriers to best practice: variability in measurement practices across the gold processing industry, ALTA, 2017.

http://www.orica.com/ArticleDocuments/421/ALTA%202017%20GPM%20-%20Orica.pdf.aspx?Embed=Y

We are still to come across a measurement of the actual residence time in the tanks; by-passing and back-mixing are important factors, especially with ever-decreasing residence time due to increases in throughput(4) (other simple measurements like tank dipping can also assist). This is especially important, when evaluating process performance through batch tests, which can lead to overestimation of expected recoveries. This measurement can also be of assistance, when the flow is split between two trains of CIL tanks – unmeasured and uncontrolled split and differences in actual mean residence times can lead to unexpected differences in performance between the two trains. Other very simple measurement can also lead to improved troubleshooting, e.g. tank dips can assist in measuring actual volume of the tanks and also can lead to more timely maintenance of the agitators.

 


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