Ct 109 notes: Separation efficiency, Smelter Contracts, and simple
calculations
High price
Low price
N.S.R
.
Concentrate grade
Fig 5:Effect of metal Price on NSR
Changes in metal price, smelter terms etc affect NSR. If metal price increases, then
optimum grade will be lower allowing high recoveries to be attained..
Smelter contracts
Concentrates are sold under contract to “custom smelters” at prices based on metal
markets such as the London Metal Exchange. Smelter are like ZimAlloys, Zisco, ZimPlat
etc.
The smelter, having processed the concentrate disposes it to the consumers.
Assays are performed to determine the value of concentrates.
Example: Simplified Smelter Contract:
Material
Tin concentrates assaying to less that 15% Sn to be free from deleterious
impurities not stated and to contain sufficient moisture as to evolve dust
when unloaded at our works
Quantity
Total production of concentrates.
Valuation
Tin less than 1 unit per dry tonne of concentrates at the lowest official
London Metal Exchange prices.
On the 7th market day after completion of arrival of each sampling lot into
our works.
Treatment Charge ₤385 per dry tonne of concentrate
Moisture
₤24 per tonne os moisture
Penalties
Arsenic ₤40 per unit tonne
Lot Charge: ₤175 per lot sampled of less tha 17 tonnes
Pricing
Delivery
Free to our works in regular quantities, loose in tipping lorry, or in any
other manner acceptable to both parties.
Worked example:
A tin concentrator treats a feed containing 1% tin and three possible combinations of
concentrate and recovery are:
High grade
63% tin at 62% recovery
Medium grade
42% tin at 72% recovery
Low grade
21% tin at 78% recovery
Determine which of these combinations of grade and recovery produce the highest
separation efficiency
Assume: that tin is totally contained in the Cassirite (SnO2) which when pure contains
78.6% tin.
Recovery = 100Cc/f, = 100 x C x concentrate grade/feed grade
For high grade
62 = C x 63 x 100/1 therefore C =9.841 x10-3
S.E. = 100Cm(c-f)/(m-f)f
= 0.984 x 78.6 x (63-1)/(78.6-1) x 1
= 61.8%
Similarly for medium grade, 72 = 100 x C 42/1
C = 1.714 x 10-2
S.E. =
1.714 x 78.6 x (42-1)/(78.6-1) x 1
=
71.2%
Low Grade concentrate: 78 = C x 21/1; C = 3.714 X10-2
S.E
= 75.2%
Highest Separation efficiency is achieved by production of a low grade (21% tin)
concentrate and at high recovery(78%).
NSR = Payment for contained metal –(Smelter charges + Transport costs)
Assuming that we work with the same tin ore above, and that the ore is free of arsenic,
And that the cost of transportation to the smelter is ₤20 per tonne of dry concentrate and
Tin price per tonne = ₤8500 (LME)
NSR can be calculated from the smelter contract:
For the medium grade tin ore, Smelter payment is ₤8500 x 17,14 x (42-1)/100 000,
= ₤59.73
Weight of 1 tonne medium grade ore is 17.14 kg
Smelter treatment charge = ₤ 385 x 1.714 x10-2 = ₤6.59
Transport cost = ₤20 x 1.714 x10-2
= ₤0.34
NSR for processing 1 tonne = ₤ 59.73 - ( 6.59 - 0.34) = ₤52.80
Production of lower grade concentrate incurs higher smelter and freight charges but
increases the payment of the contained metal due to higher recovery
Breakdown of costs and revenue for treatment of cost and revenues for treatment
1 ton of mined ore (1%tin)
Contained value = ₤ 85
mining
processing
Concentrate (72%
recovery) Contained
value ₤61.20
Transport and smelting
Cost ₤40
cost₤8
Tailing Contained
value ₤23.80
Effective cost
₤(61.20-52.80) = ₤8.40
Payment of ₤ 52.80
This balance between milling costs and metallurgical efficiency is critical to a
concentrator treating ore of low contained value where it is critical that milling costs be
as low as possible. Reagent costs are typically around 10% on a large copper mine but
energy costs may contribute over 25%. Grinding is b7y far the greatest energy consumer
and has the greatest effect on metallurgical efficiency. Grinding is essential for liberation
of the minerals both it should not be carried out any finer than is justified economically.
The effect of fineness of grind on NSR and grinding costs can be shown:
Net Smelter return
₤/tonne
NSR - grinding costs
Grinding costs
Grind size, microns (80% passing)
Economic efficiency
Economic efficiency compares the actual NSR per tones milled with the theoretical
NSR
Example;
Calculate the economic efficiency of a tin concentrator treating an ore grading 1% tin
producing a concentrate grading 42% at 72% recovery under the given smelter contract.,
tin price ₤8500 per tonne.
As calculated before, NSR = ₤52.80
Assuming perfect milling, 100% recovery of tin would be achieved into a concentrate
grading of 78.6%(ie pure cassiterite).
Weight produced from 1 tonne concentrate = 12.72 kg.
Transport cost = ₤12.71 x 20/10000 = ₤0.25
Treatment charge = ₤385 x 12.72/1000 = ₤4.90
Valuation = ₤17.72 x ( 78.6-1) x 85000 /100 000 = ₤83.90
NSR = ₤(83.90 - 0.25- 4.90) = ₤ 78.75
Economic efficiency = 52.80 x 100/78.75
= 67.0%