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SAG Mill Feed moisture sample (13 replies and 2 comments)
Obtaining representative sample will necessitate a conveyor sample. Are you planning on obtaining several such samples in order to understand moisture in open pit and underground mining locations and possibly seasonal changes?
Thank you for the response. That is the other objective. The main objective is to determine the moisture content of sag mill feed for daily report. Do you think we need to take sample everyday or just take sample for several days and use the moisture number for calculating the sag dry throughput? Taking sample everyday for moisture will be effecting to our production stability i.e. SAG down.
Consider additional details around the mine, e.g.
What is the climate like - do you see seasonal precipitation
Are parts of the orebody wetter than others
What time does ore spend in stockpiles
If you brainstorm potential causes of variability it will assist in making decision re using "a" moisture number.
I worked at an operation where they always used 4 % as moisture & it was impossible to really determine where the number had come from in long ago history.
The other side of addressing the question is:
What will you use this moisture number for
What is the value of the sample?
Is it in control system
In met accounting
The moisture number is mostly will be used for the daily calculation of sag dry feed throughput. We are on the commissioning phase currently and would like to have a better number of throughput daily reports using the moisture number. I used to work in the same operation with "re-using" moisture number.
Thank you for the sharing, much appreciated.
I've seen where operations have used the moisture content of fines as that's where much of the moisture variation lies. However, it's a lot of work to obtain a representative sample, particularly on a daily basis.
If I'm correct the moisture data (feed) will be required for met accounting daily whilst the plant is fully operational. I believe the normal procedure is grab samples off the conveyor belt (sampled by the plant operator) maybe every hour or every 2 hours. If you are taking in 10,000 tons/hr feed daily than a representative sample per day or shift should be taken hourly. I believe also that your feed grade will also be obtained from this sample?
I was thinking the same thing grab a sample from the sag conveyer feed for the moisture sample daily and use the moisture number to calculate the dry tonnage in met accounting report. I had experience in my previous company on this.
Have you considered installing a mechanical falling stream sampler at the discharge of the conveyor belt?
I would strongly advise not to be taking grab samples from a moving conveyor belt as this is quite a danger to the operator's safety. I have seen overhead cross belt sampler retro-fitted to a number of mill-feed conveyors - although not ideal for estimating mineral contents they may be acceptable for moisture.
Perhaps your geologists can help with moisture estimates if you take moisture samples from the ROM ore and attempt to correlate them with the stop belt samples (when you have an opportunity to take them). Do you have dust suppression adding water during crushing - this may be an additional complication.
How large a sample did you collect at last operation? Given the issues of collecting a representative sample and the associated difficulties, the justification of this sample would seem difficult.
Is moisture viewed as being a major source of error in met balancing / met accounting?
If you are going to use the % moisture to calculate dry tons, taking one sample on the conveyor would be ideal. The sampling point is very critical, if you have spray water for dust suppression the sample has to be taken after the spray water point.
To take a sample on the conveyor it has to be stopped and locked out, and a sample should be taken within a 1 meter distance. And everything has to be collected including the fines. You will need something to measure the 1m distance on the belt. Your sampler has to ensure that the sample container for collecting the sample is free of water and that the sample is sealed after collection.
The best location to take the sample is just after the belt weigher. Like that you have the moisture content of the weighed material. The moisture measurement (at least the wet sample mass) has to be performed as soon as possible, specifically if there is spray water, which is superficial moisture subject to evaporation.
The quantity of sample to take depends on the moisture distribution. If the moisture content varies a lot with the particle size, the necessary quantity of sample to reach a representativeness level can be high. Conversely, if it is homogeneous, it can be small.
When the mass of sample Ms (in kg) has been determined to reach a confidence level, the length of belt L (in m) along which the sample has to be taken can be calculated knowing the feedrate F (in kg/s) and the belt speed V (in m/s): L = V * Ms / F.
If moisture content depends on the particle size, the length has to be at least 6 times the largest particle size and the coarsest particle selection has to follow the rule of the centre of gravity.
The sampling frequency depends on the time variability of the moisture content of the feed. A variographic study can be necessary to determine this variability. Such a study can also help in the choice of the mean moisture content to use in case of no sampling (to avoid SAG shut-down) and when it has to be checked.
Error on the moisture content estimate (or on the dry content) can contribute as a large part of the metal mass uncertainty during metal accounting. Never neglect it.
I think the mechanical falling stream sampler at the discharge conveyor belt might the good solution for us to take the samples regularly.
I agree we still have to commit on the safety first and we would not recommend to get the samples when the belt is running. We need to take the sample when the belt is stop. Do you think we need to take the samples regularly say weekly, biweekly?
The last time we collected the samples around 1000kg (this was also for the distribution size analysis) and it was in December 2012. Do you think we need to take another sample for this moisture or just use what we already have?
Thank you for the advice. This procedure is excellent and we will apply for the next moisture sampling. The same question will arise, do we need to have regular sampling for moisture or just once and use this number for the calculation?
Yes, I do agree with you. I think we can prepare for this belt cut sampling for the moisture.
I have done this on several occasions when I was working as a Met Tech. We used the safest operating procedure at the time by grabbing a sample from the discharge conveyor feeding the SAG. We did this at the start of every, shift for Met accounting purposes.
Once you have established a moisture database through multiple belt samples, you should nominate a moisture content that you are confident will not overstate your throughput and just stick with it, or if you have wet and dry periods through the year, make adjustments seasonally. Sampling mill feed is difficult, time (and production) consuming, potentially dangerous, and rarely representative. Variations in your material balance can be reconciled mathematically, so sampling to "perfection" is unnecessary.
We are looking at the best practice to take the moisture sample of SAG Feed. The alternatives are on the belt conveyor or at the stockpile. Does anyone have any experience with this sampling practice? Which one will be more practicable and more representative sample? I prefer to take from the belt conveyor; however, it will be shutting down the SAG Mill.