Reference no: EM133111615

MINE 836 Mineral Processing and the Environment - Queen's University

Flowsheet 1:

This flowsheet was used before 1978 and consisted of rather simple circuit arrangements to produce a bulk nickel-copper concentrate that was shipped to smelter at about 60% solids (by wt.). Assume that before this date, the laws on SO₂ emissions were not very strict and that the pyrrhotite rejection capability of the concentrator was limited. In earlier practices, nickel production was the main metallurgical concern which often included the nickel portion of the nickeliferous pyrrhotite. There was no acid plant at the smelter complex to fix sulphur in the form of sulphuric acid, H₂SO₄.

However, the company management had been developing an SO₂ abatement program even before this date due to increasing environmental concerns. This was also encouraged by the government's new incentives through additional funding for research and development programs and tax deduction for related expenses to help identify the best process technology to implement at the concentrator for future SO₂ targets.

Flowsheet 2:

This flowsheet has been adopted in the early '90s. Important changes took place in the pyrrhotite rejection circuit, involving primarily improvements in regrinding and flotation circuits. Magnetic drum separators were installed to focus on regrinding of the pyrrhotite-pentlandite middlings for additional pentlandite recovery to pay for nickel losses that would be inevitable due to pyrrhotite rejection at increased levels. The magnets were introduced even earlier along with the production of high-grade copper concentrate. The copper-nickel separation was carried out in column flotation circuits and the copper concentrate was sold to external customers. This automatically reduced the amount of sulphur input (associated with copper mineral chalcopyrite) to the smelter. An important change was also the installation of an acid plant at the smelter complex.

Flowsheet 3:

This flowsheet represents the more recent changes that took place in the early 2000s in the process to increase the capability of the circuits to reject more pyrrhotite for future requirements in the reduction of SO2 emission levels. Magnetic separators were introduced for the treatment of secondary rougher concentrate to divert more pyrrhotite-pentlandite middlings into the regrinding and pyrrhotite rejection circuit. Pyrrhotite rejection circuit was also upgraded in terms of more automation and installation of larger cells which replaced old 100 cuft cells. As a result of a pilot plant campaign carried out at the concentrator, a multi-stage cleaning circuit was adopted for pyrrhotite rejection that has solely relied on the regulation of pH at 10-10.5 using lime with no additional collector or frother input to the circuit. As far as I am aware, magnetic separators were completely removed from the more recent version of this flowsheet. However, the process efficiency has not changed significantly as far as the rejection of pyrrhotite is concerned.

Tasks & Questions:

Question 1: Using the sampling data and the computer program Bilmat, a metallurgical and water/solid balance has been developed for each flowsheet (see below). A summary of pertinent information consisting of calculated tonnages of mass flows in conjunction with Ni, Cu & S grades readjusted at 5% relative standard deviations and slurry densities for each case are tabulated below. Extend the columns to the right and generate recovery columns for each element. Using these data, determine and construct a cumulative nickel grade/recovery performance curve associated with each flowsheet. This performance curve represents the quality (% Ni, Grade) and quantity (% Recovery-yield) of nickel products obtained in each case. Compare the curves on the same graph and comment on the grade-recovery differences between the flowsheets. The cumulative performance should account for all products contributing to the final nickel concentrate (i.e., smelter feed). Note that for flowsheet 1, this represents streams #2, #4 and # 10 joining to form the smelter feed, #12. For flowsheets 2 and 3, there are four components making up the smelter feed to base the cumulative grade-recovery relationships on in each case.

Question 2: Convert elemental assays (i.e., estimated nickel, copper and sulphur assays) into mineral assays (assuming that Pn + Cp + Po + Gng = 100%) using the following mineralogical compositions (Table 1), which do not show % Fe making up the remainder for the three sulphides since the development of the mineralogical formulas will be based on Ni, Cu and S assays. An example of how mineral equivalents can be obtained from Ni, Cu, S assays is posted in OnQ. Once the mineral contents have been determined for each stream, calculate the distribution (recovery) of each sulphide mineral similar to the case with nickel and copper in section 1. Using a bar graph, show how much pyrrhotite rejection has been accomplished in each flowsheet (i.e., pyrrhotite recoveries corresponding to Po tails in each case). Also, show the corresponding Po grades in each case using a separate bar graph. What can you conclude about the capability of each flowsheet for pyrrhotite rejection?

Question 3: Water recovered from thickeners is utilized in grinding circuits to minimize dependence on freshwater. Considering dry tonnages (DMTPD) and the % solids (wt.) for each stream in the mass balance table for each flowsheet,

i) Determine the amount of water recycled from thickeners (m³/day) in each flowsheet. What is the % recovery of water relative to the water input to thickener feed?

ii) What is the total volumetric flow of reclaimed water (m³/day) going to the grinding circuits as internal recycling? What is the overall % water reclaimed relative to the amount of water in the plant feed?

Question 4: Determine and plot (e.g., using a pie or bar graph) the distribution of sulphur by destination (i.e., among the products of concentrator and smelter) associated with each flowsheet for the following conditions at the smelter complex specified in Table 2 below.

The distribution of smelter products also includes SO₂ emissions from stacks! Regulation 661/85 specified the control order by the government during the period of Flowsheet 2 as 100 kilotonnes of sulphur dioxide per year. Report your results on sulphur balance using the format given below, "Sulphur Distribution". Consider that the company has a 15-day shut-down period for annual maintenance (i.e., 350 days of operation in a year).

The following format is suggested to record the breakdown of sulphur-bearing products in the milling and smelting processes.

a) What was the level of SO₂ emissions (in kilotonnes per year) at smelter during the period associated with Flowsheet 1?

b) Was the company able to meet the limit specified by Regulation 661/85 during the period of Flowsheet 2?

c) What was the capability of the Flowsheet 3 in terms of % reduction in SO₂ emissions era in comparison with Flowsheet 2?

Question 5: Suppose that the next control order reduced the limit to 30 kilotonnes per year. During that period suppose that sulphide sulphur grade averaged 12% (wt.). In the meantime, the tonnage of ore processed also decreased by 15% due to declining ore resources. Pyrrhotite rejection circuit improvements in the flowsheet enabled only a 5% increase in fixing the ore sulphur in the pyrrhotite tails. Assuming that no changes took place in the sulphuric acid plant or the sulphur level of matte and slag in the smelter complex, is the company likely to meet the new limit of SO₂ emission?

Attachment:- Mineral Processing and the Environment.rar