CoM 2014.

Arsenic Metallurgy and

the Environment

Mineralogical Study of Arsenic Species from Roasting

Rajan Pandher, Jorge Oliveira,

and Arthur Barnes

Introduction

• “The devil is in the details”

• Neutral or sub-stoichoimetric roasting of sulphide

concentrates to remove arsenic

• Small scale testwork- results and challenges

• Mineralogical identification of critical products

• Proceed armed with improved understanding

• Final piloting results

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Background: A challenging objective

• The concentrate under investigation was typical of a hypogene copper sulphide,

rich in chalcopyrite, but containing a substantial amount of what was initially

believed by the client to be enargite.

• The arsenic content was well above the maximum level tolerated by smelters,

and was considered unsaleable at the levels produced

• The sulphur content was slightly below the norm for a clean copper concentrate,

placing some restrictions on the degree of roasting that was permissible.

• To ensure the roaster product would be accepted by toll copper smelters, it was

desired to retain 20% S as sulphide in the roaster calcine.

• The target terminal arsenic level was 0.4%, necessitating almost 90% arsenic

removal in order to achieve this goal.

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Background

• Arsenic is a common contaminant in copper sulphide ores, and the most common arsenic bearing mineral is enargite, Cu3AsS4. (=Cu12As4S16)

• Other economically significant copper–arsenic sulphides are tennantite (Cu12As4S13) and its antimony- rich equivalent, tetrahedrite(Cu12Sb4S13) with which it forms solid solutions.

• Because enargite and tennatite contain a very high proportion of copper values they cannot be removed form the chalcopyrite and bornite minerals by selective flotation without sacrificing significant copper recovery.

• Smelting of copper concentrates high in As result in unacceptable levels of As contamination in the copper metal, so smelters impose strict maximum levels of arsenic accepted in concentrates.

• Very high penalties on arsenic are imposed by toll smelters and these can quickly result in a concentrate becoming unsaleable. The accepted solution to removing arsenic from copper sulphides is to use the process originally developed and refined by Boliden, in which the concentrate is roasted with a deficiency of oxygen, resulting in volatilization of labile sulphur and arsenic along with partial oxidation of the iron.

• The target sulphur level in the calcine is usually above 20% to ensure sufficient sulphur is retained to provide a satisfactory energy consumption during smelting of the calcine.

Partial Roasting 101

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Initial Scoping Fluidisation Tests

Testwork Procedure

• Since production of sufficient concentrate for roasting testwork can be challenging for clients, selection of suitable testwork parameters is a vital component in achieving a successful outcome with limited feedstock.

• XPS routinely use TGA (thermo gravimetric analysis) to establish initial roasting windows.

• This is supplemented by FactSage modelling and calculation of the required operating parameters for roasting.

• Initial roasting on a batch or semi-continuous basis in our 2” diameter unit confirms optimum roasting conditions.

• If sufficient feedstock is available, the final stage of testwork involves roasting in the 4” diameter pilot roaster to confirm engineering design parameters suitable for final design.

General Approach

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Two inch diameter continuous fluid bed roaster

• .

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Test Parameters

Parameter Units Low High

Temperature °C 650 725

Bed residence time minutes 15 60

Feed: O2 ratio g/g 8 20

Free space velocity m/s 0.2 0.8

Freeboard SO2 vol % 5 8

Bed: Cyclone mass 70:30 20:80

Ranges used for the 6 scoping runs

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Results

• Arsenic removal: between 77-86% achieved.

• While excellent arsenic removal was achieved in the bed material, the product recovered from the cyclone showed arsenic levels 2-3 times higher than the bed overflow.

• Initially it was suspected that this was due to fine particles of arsenic–bearing sulphide being prematurely elutriated from the bed before they were hot enough to react.

• Logic suggested lowering the free space velocity in order to hold material in the bed longer.

• When this proved unsuccessful, increasing the roasting temperature was the next logical step.

• When this resulted in a further increase in the arsenic concentration in the cyclone, it was decided to resort to scientific methods to solve the mystery.

(Numerical values confidential at client’s request)

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Mineralogical Characterisation

Initial Mineralogical Findings

• Because the calcine assays were at variance with the carefully calculated values based on the feed assays, the concentrate (feed) and calcine (products) including bed overflow , bed dump, cyclone over-and underflows and afterburner solids were all analysed by x-ray diffraction.

• The most unexpected outcome was a conclusive identification of the arsenic –bearing mineral as tennantite, not enargite.

• Analysis specifically for arsenic bearing species using QEMSCAN, identified tennantite as the only arsenic bearing mineral. Further EPMA analysis confirmed the presence of low levels of Fe, Sb and Zn in the tennantite, which uniquely confirmed it as belonging to the tennantite group rather than enargite.

• The client’s initial skepticism was resolved by submitting the products to microscopic examination using a combination of QEMSCAN and EPMA, rather than the optical microscopy used for the preliminary mineral identification used by the client.

• Detailed EPMA results are shown in the next slide.

This resulted in an immediate slight but significant adjustment in the feed: oxygen ration which resulted in an improvement in the metallurgical accounting and sulphur levels in the calcine at the desired level.

X-ray diffraction

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Diagnostic Mineralogy

• Mineral texture and association data indicate that 85% of the tennantite is associated with

the 3 major copper sulphide minerals, chalcopyrite, bornite and chalcocite.

• 47% of the tennantite associated with chalcocite. The tennantite is relatively iron free with

respect to textural mineral associations and compositional chemistry.

• Tennantite was only 57% liberated in the Cu concentrate analysed.

Ave: S Fe Cu As Zn Sb

High Sb tennantite 27.5 1.6 45.7 17.2 3.9 6.0

Min 26.9 0.6 44.1 14.5 1.8 2.3

Max 27.9 2.5 47.3 20.1 5.0 9.6

Std Dev 0.3 0.6 0.9 1.5 1.0 2.2

Low Sb tennatite 28.0 1.7 47.6 20.9 3.2 0.5

Min 27.5 0.5 45.0 20.1 1.8 0

Max 28.4 3.5 49.2 21.7 4.8 1.6

Std Dev 0.3 0.8 1.1 0.5 0.9 0.6

EPMA Results on Concentrates: Analyses of arsenic- containing grains

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EPMA Image of Feed The onion skin appearance is typical of the two types of tennantite encountered

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Key Mineralogical changes during Roasting

Mineral % in Cu Concentrate % in Calcine

Bornite 3.3 24.6

Chalcopyrite 37.3 0.1

Mooihoekite 0.0 29.2

Tennantite 14.4 0.3

Pyrite 11.7 1.4

Quartz 2.6 3.0

Garnet 10.2 11.3

Fe oxides 0.5 15.2

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Identifying the high arsenic species in cyclone overflow

Micro-probe Image An almost completely oxidized particle, showing fissuring and porosity. The bright phases are residual pockets of mooihoekite. The darker grey area is an Fe-oxide species. EPMA analysis of this phase identified an iron oxide, with As and Ca present. Mean EPMA analysis of key As-bearing phases is presented

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Analysis of High As Species in Calcine

Species Ca S Si Cu Al Mg Fe As O

Synthetic Iron Oxide 3.8 0.9 0.3 1.2 0.08 1.5 53.4 4.4 21.1

“Aluminosilicate 1” 9.2 0.05 25.9 0.17 9.2 1.2 6.4 2.8 41.8

“Aluminosilicate 2” 2.1 0.25 20.1 0.4 15.3 0.5 3.6 5.3 40.9

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Probe Results on Calcine Minerals

S Fe Cu Zn As Sb Total N=

Synthetic Bornite 28.2 19.6 52.5 0.6 0.06 0.09 100.97 2

Unreacted Bornite 27.1 14.9 57.2 1.0 0.06 0.12 100.75 2

Mooihoekite 1 30.2 25.7 43.4 1.2 0.05 0.02 100.50 2

Pyrrhotite 37.7 59.9 1.5 0.04 0.08 0.00 99.20 2

Mooihoekite 2 27.5 17.1 53.8 1.1 0.05 0.08 99.73 4

Sphalerite 33.2 9.5 3.1 53.6 0.03 0.01 99.22 2

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Metal Deportment • Following diagnostic mineralogy, copper recovery to sulphide phases increased to

91%, while sulphur grades were maintained above 20% and arsenic removal from

sulphides was better than 83% Aura Aranzazu Two Inch Roaster Product Copper Distribution

55% 57% 57% 57% 56% 54% 54%

14%

21% 17% 17% 17%15%

11%

37%

77%

20%20% 20% 15% 20%

24%

6% 4% 3% 3%

6% 6% 7%

3% 4% 4% 3% 4% 3% 3%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Test 1 Test 2 Test 3 Test 4 Test 5 Test 6 Test 7 Test 8

Bed Overflow Cyclone Underflow Bed Dump Afterburner Solids Scrubber Solids Scrubber Solution

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Arsenic Deportment Batch roasting tests

Aura Aranzazu Two Inch Roaster Product Arsenic Distribution

12%8% 11%

6% 6% 5% 7%

13%

6%7%

5% 4%

10% 20%

6%

7%5%

4% 3%

21%

23%11%

38%

23%19%

10%7%

23%

15%

11%

8%

13%

15%

28%27%

25%

40%

55%

40%

52%56%

45% 44%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Test 1 Test 2 Test 3 Test 4 Test 5 Test 6 Test 7 Test 8

Bed Overflow Cyclone Underflow Bed Dump Afterburner Solids Scrubber Solids Scrubber Solution

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Pilot Roasting 4” diameter pilot fluid bed roaster

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Pilot Roasting

• Initial results showed high arsenic levels in cyclone underflow.

• Diagnostic mineralogy indicated that the arsenic bearing

species were the arsenic sulphides realgar (As4S4)and

orpiment (As2S3) not initially present in the concentrate.

• These were linked to a cold cyclone, which resulted in

condensation of the volatile arsenic sulphides.

• After discussions with equipment suppliers, modifications to

the roaster freeboard heaters and improved insulation as well

as extensive preheating resulted in a successful run on the 4”

pilot roaster, producing sufficient material for the design of a

suitable afterburner and arsenic fixation system.

Detailed results withheld at client request

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Conclusions and Recommendations

Conclusions

1. Without diagnostic mineralogy, identifying the cause of the

high arsenic levels in the calcine would have required

much more extensive and prolonged testwork.

2. The detailed mineralogical associations possible using

EPMA allowed a much better understanding of the original

mineral deportment as well as the transformations

occurring during partial roasting

3. The more accurate compound analyses permitted

improved estimation of roasting conditions and prevented

over-or under roasting of the calcines .

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Recommendations

• Diagnostic mineralogy is regarded by XPS as an

indispensable tool in high temperature metallurgy, and is

now an integral part of the test procedure involved in

roasting testwork.

• QUESTIONS?

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