CHEM-E6155 Minerals engineering
project work
Spring 2017, Aalto University
Design Project Cases
Background
A problem that is currently being faced by mines around the world is the decrease
in the valuable grades in extracted ores as a result of site exploitation. Consequently,
companies are looking into alternative solutions to maintain their production goals.
Among these alternatives is to further process their scavenger tailings or treat the
minerals in the tailing dumps that were discarded in the past but are now
economically attractive.
One of the main reasons why valuables are not reported to the concentrate is the
lack of liberation. To overcome this, further grinding is required. Obviously, this
represents an additional investment and an increase in operating costs associated
with the installation of new mills.
Nevertheless, our Company is evaluating the possibility of installing a ball mill
capable of liberating valuables in a sufficient extent for them to be further
concentrated. Your team’s task is to design a mill (or mill series) for the processing
of the tailings stream in preparation for the concentration step. The expected service
life of the mill will be 15 years and a salvage value of 5% is forecasted.
Your report is expected to contain the technical details of your design and justify
why do you consider this to be the most suitable. A questionnaire containing the
pertinent information should be prepared for potential suppliers, in case our
company decides to proceed with the project. Based on your estimation of costs
and potential revenue due to additional liberation, suggest whether this is a sound
investment.
Case 1. A copper ore operation in Chile
Mineralogy
The tailings produced from the current concentration process have an average grade
of 0.12% Cu. About 97% of the Cu in the tailings is found in the form of
chalcopyrite, with the rest being a mixture of enargite, chalcocite, covellite and
bornite. The main component of these tailings is quartz, which also contains up to
1% pyrite and about 1% other miscellaneous minerals such as molybdenite and
arsenopyrite. The d80 particle size of the tailings is of 157 microns (Fig.1a)
Fig.1a. Particle size distribution of Cu-containing tailings
Capacity
The planned throughput for the tails enrichment plant is of 130 000 tons of ore per
day.
Economic drivers
The current smelter contract reads as follows: Deduct from the copper assay 1 unit
and pay for the remainder at 95% the LME price for higher-grade copper. A penalty
of 5 units will be paid for Cu grades lower than 20%. The minimum acceptable Cu
grade is 15%. Molybdenum will be paid at 90% the LME price. Refining charges are
220 US Dollars per ton of payable Cu.
Note: For simplicity, in this particular case assume that 45 kg of Mo are associated
to every ton of payable Cu.
Background information
In a flotation cell at laboratory scale under scalable conditions, the correlation
between particle size, grade and recovery in Fig.1b was found.
Fig.1b. Grade and recovery as a function of particle size for the treatment of copper
ore tailings
Case 2. An enrichment operation for a South-African PGMcontaining ore
Mineralogy
The ore being treated by the current plant contains platinum group metals (mainly
Pt and Pd with traces of Ph) in a cumulative grade of 5 g/ton. The enrichment
process produces a concentrate containing 150 g/ton of PGMs with a reported
recovery of 88%. The vast majority of the ore is chromite (>90%), the rest being
mainly feldspar, magnetite and olivine with traces of other minerals. The particle size
of the tailings present a d80 of 128 microns (Fig. 2a).
Fig 2a. Particle size distribution of PGM-containing tailings
Capacity
The new enrichment plant should process up to 280 000 tons per day of ore.
Economic drivers.
The smelter contract reads as follows: Smelter payment terms for Pt and Pd will be
86% of the average London Metal Exchange price of the day. Treatment and
sampling charges add up to a total of US$185/ton of dry concentrate. A grade
penalty of US$100 per dry ton of concentrate delivered to the smelter shall apply if
the grade of concentrate is below 150g/t PGM’s, US$150 for grades below 130 ppm
and US$200 for grades below 110 ppm. The minimum acceptable grade for the
smelter is 100 g/ton of PGM’s.
Note: For simplicity, assume that the ratio of Pt to Pd in the concentrate is of 4:1
Background information
In a flotation cell at laboratory scale, the following correlation between particle size
and recovery was found:
Fig.2b. Grade and recovery as a function of particle size for the treatment of PGM
tailings