“In minerals processing plants there are two distinct classes of samplers, namely metallurgical and process control. It is important that a distinction between these two classes of samplers and their applications is established, as the aim of all samplers is to provide a small sample that is as close to 100% representative of the original process stream from which it was taken” says Richard Rule, Director at eDART Slurry Valves (Pty) Ltd
Metallurgical or metal accounting samplers
Rule continued “Slurry sampling is a well-defined science and there are numerous university courses offered that cover these required principals. It is with good reason that a lot of emphasis is placed on the specifications for metallurgical samplers in the mineral processing plants. Metallurgical samplers are used for accurate metal accounting purposes, for example to determine the quantity of metal present in a concentrate, to be sold to a smelter or to measure for a contaminant in the product in order to determine penalties”.
One of the many requirements for a sampler to qualify as a metallurgical sampler is that the sampler cutter moves across the entire process stream when taking a sample cut.
This technique of cutting across the entire stream has been proven to result in a sample with the highest possible representivity, hence the name of cross stream samplers. These samplers are generally large and sophisticated pieces of process equipment that condition the stream prior to presenting it to the cross cutter. In modern large flotation concentrators it is common to find a cross stream sampler on the plant feed and final tail streams of the plant. Moving the cutter across a large process stream at a fixed constant rate will produce a large sample, which often requires a secondary cut before one produces a manageable sample size. When a secondary sampler is required, and here it is common for the “Vezin” type sampler to be used for this purpose, the secondary sampler has to also conform to the rule of cutting the entire stream in order that the final sample qualifies as a metallurgical sample.
However this still leaves many other requirements for samplers on the minerals processing plants that are not being serviced by these metallurgical samplers for a number of reasons.
By the very nature of the cross stream type samplers the stream has to be de-pressurised, before being sampled, this eliminates all metallurgical samplers from (pressurised) pumped streams.
In the following situations, where a sample is required from a pressurised pipe, or a plant does not have the space or budget to install a metallurgical sampler, where a density sample is required to calibrate a densitometer, or get an indication of the metal in a minor stream in order to make a process adjustment.
This is where process control samplers have now come into their own.
Process control samplers
The aim of process control samplers is to give the process control team information in order to make informed process control decisions, or to calibrate process control instrumentation.
“One of the easiest ways to improve profitability in modern minerals processing plants is to detect and speed up the response to changes in the process. The causes for process variations are too numerous to detail but include ore mineralogy, changes in reagent control , variation in flotation air and level control, changes to grind and equipment wear or failure, It’s in this realm that the process control samplers are most effective” said Rule.
“It is still very common to find ‘manual sampler cutters’ used on minerals processing plants. Here an operator simply takes a manual cut through the process and then uses this to make process control decisions. This technique is on the opposite end of the scale from the metallurgical sampler and is the worst possible technique to take a sample, as they are so vulnerable to a myriad of sampling and human errors”.
Rule added “eDART’s first process control samplers, viz: the poppet sampler was designed simply to replace the manual sampler cutters as it provides a more consistent sample, with a fixed volume and which can be timed as required , it is also free from human errors during the sample taking”.
The aim of the process control sampler is to give the plant metallurgist or plant operator sufficient information to be able to make an informed decision about controlling their plant. The process control sampler is often used in a “policeman” role where is used to monitor the metal in a minor process stream before the next stage of processing.
For example a dedicated calibration sampler should be installed with every densitometer on a processing plant. Control of processing plants is complicated by the fact that you have a number of varying process parameters.
eDART supplies a range of process control samplers that include primary and secondary samplers .These samplers are suitable for batch (or fixed volume) as well as continuous sampling as required by on stream analysers, for example, XRF, Optical Mineral Analysis and Particle Size Analysis. The primary samplers are divided into two groups pressurised (or pumped) and non-pressurised or (gravity).
eDART offers the full range of Gravity Sampler Assemblies or GSA’s from 150 NB to 900 NB. The cutter can be optimised to reduce the volume of sample produced if required. These are suitable for continuous and or batch requirements.
For the pressurised streams eDART has two ranges of Centre Pipe Samplers to suit requirements. The innovative recirculating Centre Pipe Sampler eliminates the requirement for a discard stream and pump, because it internally recirculates the stream to provide a fresh sample to the secondary sampler at all times.
eDART secondary samplers are variants of the popular poppet type sampler. eDART bought the original poppet sampler from Mitech, who had been producing them for over 25 years, and subsequently developed the thief, coarse, corrosive, viscous and dry powder variants.
The poppet provides a compact robust and reliable sampler to produce a sample on a periodic basis. It produces 125 ml of sample per cycle making it ideal to produce a composite or shift sample as it will take a sample every three minutes to produce 20 litres of sample over an 8 hour shift.
Foskor, Process Engineer, Robert Lukinda Mulumba, commented “All the samplers from eDART that we have installed are working well. I am happy with their performance and will be ordering a further range of samplers from eDART”.
Flotation plants in general are considered to be a “black box” whereby accounting samples are mainly used to quantify the effects of any changes or upsets to the circuit. Although our accounting samples form the basis our metallurgical evaluation, they more often than not lack the detail which is required to fully quantify the effects of any changes to the circuit. For this detail we rely on our process control samples” said Brian Whitehead, Plant Superintendent – Technical – Northam Booysendal.
Whitehead continued “Through our process control samples sourced from eDART, we are able to continuously monitor the metallurgical performance for each of our individual flotation circuits. We have since been able to improve on our response time in identifying and correcting circuit inefficiencies thereby minimising production losses arising from substandard conditions. The metallurgical performance evaluation has also formed the basis of our optimisation strategy allowing us to select the optimum product/equipment/configuration for our circuit “.
“Currently a large amount of our process control samples are still collected manually on our plant which increases the probability for human error. This poses a significant risk as poor quality information can result in poor quality decisions. It is more dangerous to base decisions on false information than to not have any information at all. To minimise the risk of human error we have opted for automated poppet samplers for our critical process control samples. As an example; our online grade analysers are calibrated using poppet samplers” Whitehead concluded.
“The modern minerals processing plant now has a wider and a more informed choice of slurry samplers to meet all of their requirements, including budget. We believe we offer the best sampler solutions which can accommodate all specific requirements” said Rule.
For further information please contact: Richard Rule, Director, eDART Slurry Valves (Pty) Ltd Tel: 011 8236620 email: mailto: email@example.com