Challenges to Treat Complex Zinc Concentrate and Latest Technical Development

The complex zinc concentrate containing high Pb, Cu, SiO2, Na and K, including the Zn-Pb bulk concentrate, cannot be treated in the fluidized bed roaster. The Imperial Smelting Process (ISP) is the main technology to treat this type of concentrate. However, ISP consumes a large amount of coke, which is not only expensive, but also has high CO2 emission. Several metallurgical processes, which can treat the type of complex zinc concentrate and have low CO2 emission, have been reviewed and compared. Recently, a new technology “Flash Burner Roasting Process” has been tested successfully at the industrial scale suspension roaster, and the tests demonstrate that this technology can treat the complex zinc concentrate containing high contents of Pb, Cu and SiO2. The thermodynamic modelling for this process has been done. The industrial test results, the modelling results and the further development are presented in this paper.


Existing technologies
The fluidized bed roaster cannot treat the complex zinc concentrate containing high Pb, Cu and SiO2 due to the accretion formation in the roaster.There are several technologies that could treat those complex zinc concentrates, such as Imperial Smelting Process, Outotec atmospheric direct leaching process, Albion process, PBOX process, pressure leaching process, and pelletised concentrate fluidized bed roasting process.

Imperial smelting process (ISP)
Imperial Smelting Processes Ltd (ISP) began development of a new smelting process for zinc in 1943 in UK.In 1947, an experimental blast furnace was built and by 1950, reach a capacity of 2 t per day [1] .The first breakthrough was the gaseous zinc produced at the smelting temperature, could be recovered by an absorption in a liquid lead spray.One major problem was that some of the zinc produced, re-oxidized to ZnO, making the process inefficient.The second breakthrough was to increase furnace top temperature to prevent this re-oxidization.The third breakthrough for this process was that this Zn blast furnace could simultaneously produce lead and zinc metals.
Since then, 20 ISP plants have been built and commissioned.The typical ISP flowsheet is shown below in Figure 1.However, due to high coke costs, most of ISP plants were shut down in the early 2000, and only 6 of them are still in operation.They are Baiyin, Shaoguan, Dongling, Huladao in China, Hachinohe in Japan, and Miasteczko in Poland.

Outotec atmospheric direct leaching process
In the 1990s, Outokumpu's zinc and technology companies developed an atmospheric zinc concentrate direct leaching process to be able to use a wide variety of sulfidic concentrates in zinc production.
The first laboratory tests before the piloting were performed at Outotec (former Outokumpu) Research Centre (ORC) in Pori with several zinc concentrates used at Kokkola zinc plant [2] .The grain size for most of the zinc concentrates was in the range of 15 to 25 μm.After these laboratory tests the piloting in 1991 with a 10 m high pilot reactor did not give sufficient leaching recovery but showed that the direction of choosing the atmospheric leaching alternative was the best one.So, the piloting was continued at Kokkola zinc plant in a 20 m high pilot reactor, which gave good leaching recovery.
Commercial-scale start-up took place at Kokkola zinc plant in 1998 and three years later another expansion was started using the same technology [2] .The modernization of Boliden Odda plant was finished in autumn 2004 and this technology has also been applied there.The Outotec Zinc Concentrate Direct Leaching (DL) Process equipped with atmospheric leaching reactors of about 900 m 3 having similar height as the big pilot reactor into the existing operations.Schematic picture of the Outotec Direct Leaching reactor is shown below in Figure 2.

Albion process
The Albion Process technology was developed by Glencore in 1994 and is patented worldwide [3] .It is a combination of ultrafine grinding and oxidative leaching at atmospheric pressure.The feed to the Albion Process is a concentrate containing base or precious metals, and the Albion Process is used to oxidise the sulphide minerals in the concentrate and liberate these metals for recovery by conventional means.Two plants treat a zinc sulphide concentrate and are located at San Juan de Neiva in Spain (4,000 tpa zinc metal), and at Nordenham in Germany (18,000 t/a zinc metal).A summary of the key operating data for the two Glencore Albion Process TM plants is presented in Table 1.

Parameter
San Juan de Neiva [3] Nordenham Following the success of the first Albion Process TM plant at San Juan de Neiva, Glencore then constructed a second Albion Process plant at the Nordenham zinc refinery.This plant was commissioned in March 2011.The feed to the plant is also finely ground lead/zinc bulk concentrate from the McArthur River Mine (MRM), and the process flowsheet is similar to that employed at San Juan de Neiva.
The oxidative leach consists of an 800 m 3 and a 280 m 3 reactor in series.The 280 m 3 reactor is similar in geometry to the San Juan de Neiva leach reactor, and the 800 m 3 has a live height of 11.5 m and an aspect ratio of 1.5.Oxygen is injected into the base of the both reactors using HyperSparge supersonic oxygen injection lances, and the slurry is again agitated by a centrally mounted dual hydrofoil impeller.An image of the Nordenham Albion Process TM oxidative leach is shown in Figure 3.
The lead residue from the Nordenham Albion Process TM plant is also sold locally to secondary lead producers.

PBOX process
The PBOX Process is developed by Glencore as well [4] .The first PBOX process plant at McArthur River Mine (MRM) is used for rapid and selective oxidation of galena, prior to flotation.Oxidation of the galena mineralogy to lead sulphate allows selective flotation of the remaining sphalerite in a conventional flotation circuit, to produce a higher grade sphalerite concentrate.The lead sulphate residue then remains in the flotation tailings, and is recovered to produce lead bullion through the Mt Isa lead smelter.This process, termed PbOX, uses six 800 m 3 tanks constructed from SAF904 steel, shown in Figure 4.The plant was commissioned in 2014 and can process 150,000 to 180,000 tonnes per annum of bulk concentrate, producing up to 150,0000 tonnes per annum of saleable zinc concentrate.The Sherritt Zinc Pressure Leach Process was first commercialized at the Trail, British Columbia, zinc refinery of Cominco Limited in early 1981 [5] .Subsequently, Kidd Creek Mines (now Falconbridge Limited, Kidd Creek Division), in Timmins, Ontario and Ruhr Zink GmbH, Datteln, Germany, have employed the Sherritt Zinc Pressure Leach Process.In each case, the zinc production capacity of existing refineries, using roasting -leaching -electrowinning technology, was expanded without a requirement for increased sulphuric acid production.In the case of the expansions at Cominco and Kidd Creek, conventional zinc concentrates are supplied to the pressure leach circuits.The expansion at Ruhr Zink was designed to treat a blend of conventional zinc concentrates and bulk concentrate.A typical zinc concentrate pressure leaching plant flowsheet is shown in Figure 5.
The application of the Sherritt Zinc Pressure Leach Process to the treatment of lead zinc bulk concentrates was examined in 1977.Bulk flotation concentrate obtained from the treatment of New Brunswick complex sulfide ores was used for the study..
In this process, the feed is pelletized and dried, zinc sulphate acting as the binder for the pelletising.The size of the pellets was fixed at 0.5 -4 mm.The pellets are fed into the rectangular shape roaster, shown below in Figure 6.This roaster consists mainly of a vertical refractory lined shaft.Over 85% of the length, the roaster bottom consists of an air distribution grate.The rest of the bottom is the aperture, feeding the finishing bin for cooling the calcine and completing the roasting by the secondary air.
As the particle size of the pellets is up to 4 mm, the ascension speed of the gas is very high, and the space rate is higher than 2.5 m/s, which is about 3-4 times higher than that in the normal fluidized bed roaster.Also, the roasting temperature is very high and often exceeds 1000 o C due to the big particle.
It was reported that this roaster treated zinc concentrate from Kipushi with 6% Cu and Tsumeb with 10% Pb and 3.5% Cu during long campaigns without causing accretions on the roaster walls.The most typical issue of the complex zinc concentrates is the high Pb content, especially Zn-Pb bulk concentrate.The Pb behaviour during zinc concentrate roasting was studied below.
MRM zinc concentrate has 3.76% Pb, and 5-6% SiO2.The total content of (Pb+Cu+SiO2) is 10-11%.This concentrate is ultra-fine, and the particle size P80 is 7 μm.This MRM zinc concentrate cannot be treated in the fluidized bed roaster alone and needs to be blended with other zinc concentrate to control the total content of (Pb+Cu+SiO2) in the blended feed.
During the fluid-bed roasting of zinc concentrates, agglomeration/accretion and fusion of the bed sometimes occur when the Pb content in the zinc concentrate is high.The lead silicate has long been suspected as the source of the trouble.Recently I investigated this and summarized it below.
Jorgensen from CSIRO in Australia analysed the accretion from an industrial zinc fluidized bed roaster, and their investigations had not found the presence of lead silicate in fused bed accretions from an industrial fluidized bed roasting plant [7] .
Jorgensen and his team in CSIRO conducted further lab scale tests which the galena was added to the zinc calcine under the fluidized bed roasting condition, and found that the Pb mineralogy in the accretion are mainly monobasic lead sulphate (PbO.PbSO4) under fluidized bed roasting condition of zinc concentrates, but the lead silicate was not found in the accretion/agglomerates [7] .
The Pb compounds in the accretions are shown below: The main one is monobasic lead sulphate (PbO.PbSO4), the minor ones are anglesite (PbSO4) and dibasic and tetrabasic lead sulphate (2PbO.PbSO4 and 4PbO.PbSO4), no lead silicate (PbO.SiO2) in the accretion/agglomerates were detected.Those lead sulphate compounds high density, and are difficult to be removed out by air in the fluidized bed roasters, so they form accretion in the roaster bottom to block the tuyeres.
Recently, the test results conducted in XPS, a Glencore company, confirm that Pb is carried in Pb sulphate, Pb sulphide (galena), and fine textures composed of Zn sulphide/sulphate and Pb sulphide/sulphate.
In both these roasting tests of Kidd Creek zinc concentrate and MRM zinc concentrate, Pb exists in the calcine and the sinter/accretion as Pb sulphate, and there is no lead silicate detected.
The Gibbs energies of formation of PbSO4, and PbO.SiO2 (or PbSiO3) are compared as below: The Gibbs energy of formation for PbSO4 is much lower than that for PbSiO3 under the temperature from 600 o C to 1100 o C. Therefore, this confirms that PbSO4 is more stable than lead silicate PbSiO3 thermodynamically and the Pb prefer to form PbSO4 under the roasting temperature range.
The thermodynamic modelling for the MRM concentrate roasting process was done using FactSage.The modelling shows that the PbSO4 is formed, but the PbSiO3 is not formed.This modelling validates the XPS test results.

Zn-Pb bulk concentrate tested in pressure leaching process
The pressure leach process was also tested for MRM Zn-Pb bulk concentrate at the lab scale in BGRIMM in 2020.

Pressure leaching tests:
Under the condition of leaching temperature: 150℃, acid zinc ratio: 0.9:1, oxygen partial pressure: 0.5MPa; stirring speed: 550rpm; leaching time 2h, the zinc leaching rate can reach up to 97.84%,The iron leaching rate is 27-35%; the end point acidity is about 28g/l.

Flotation tests of pressure leaching residue (or sulphur concentrate):
The bubble performance of the leaching pulp is good, the flotation effect is good, the concentration of the control pulp is about 0%, the flotation time is 6~8min, at room temperature or 80℃.The medicament can obtain sulfur concentrate with 80% sulfur.
During the flotation process, the sulfur element is mainly distributed in the concentrate, and the lead element is distributed in the tailings.Distribution of sulfur: sulfur is mainly distributed in the concentrate, while the zinc and iron are also mostly distributed in the concentrate.Distribution of lead: ~34% of lead is distributed in sulfide concentrates, and ~66% of lead is distributed in middle mines and tailings.Distribution of silver: silver is mainly found in flotation concentrates and flotation tailings.

Sintering -leaching process
This process is proposed to treat the complex zinc concentrate that the fluidized bed roaster cannot treat.The Zn-Pb bulk concentrate is desulphated in sinter machine, similar to that in the sinter machine in the ISP plant.The differences are mainly: a) The limestone and silica are not added in the feed as flux for the SiO2/CaO and SiO2/Fe ratios control, as the sinter is not treated in the ISP, but is crushed, grinded, and leached.
b) It is also not needed to control the ratio of Zn/Pb in the feed, which is different for ISP.
c) The sinter in this process does not require big size, or the strength is not required as strong as that for ISP.
d) The sulphur content in the sinter should be below 0.5%, so that the leaching process can handle this sinter easily.

Suspension roasting -leaching process
The industrial scale tests were conducted in a suspension roaster in Glencore plant in Spain in 2023, shown in Figure 7.The flowsheet of this plant is shown in Figure 8.
The concentrate from the feed bins is fed by the conveyor and paddle feeder onto the bottom of the two drying hearths.The concentrate is rabbled across the first hearth into a centre drop-hole, across a second drying hearth and out to a storage bin.Drying air is controlled to 100 -300 o C by mixing hot gas from the roasting hearths with tempering ambient air.The final moisture content of fee after drying is less than 0.5%.
The dried concentrate passes to a ball mill where it is ground to 99% -150 mesh.The combined air and feed stream is blown into the upper chamber of the roaster.Upon entering the roaster, the concentrate undergoes very rapid spontaneous combustion of its sulphides.The temperature at this point is in the order of 950 -1200 o C. Approximately 40% of the calcine falls onto the first of two roasting hearths where retention time and the rabbling action of rotating arms are used to reduce the sulphide content of the calcine.
The remaining 60% of the calcine is carried out of the roaster with the combustion gas and enters the waste heat boiler.Calcine remaining in the gas stream after boiler dropout and hot cyclone separate is removed by ESP.The calcine recovered from the gas cleaning system is returned to the roasting hearths.The roasting tests with MRM zinc concentrate trial at the suspension roaster have been conducted successfully.
The assays of MRM zinc concentrate and particle size are shown in Tables 4 and 5, respectively.MRM zinc concentrate has 3.76% Pb, and 5 -6% SiO2.The total content of (Pb+Cu+SiO2) is 10 -11%.This concentrate is ultra-fine, and the particle size P80 is 7 μm.
In the period of 18/June/2023 to 21/June/2023, the suspension roaster treated 90 -92% MRM zinc concentrate and 8-10% Waelz oxides.The Waelz oxides were introduced in the feed blend to keep the high production but decrease the temperature from 1100 o C to 850 -900ºC.The sulphide content in calcine was <0.3%.The content of (Pb+Cu+SiO2) in the feed reached to 9 -10%.
Since 22/June/2023, the suspension roaster treated 100% MRM zinc concentrate for 12 days.The roaster operated very stable during those 12 days without any issue observed.The content of (Pb+Cu+SiO2) in the feed reached to 10 % -11%.The roasting temperature was controlled between 850 -900ºC.The sulphide content in the calcine was well controlled below 0.3 -0.4%.Feed bypassing the ball mill was set around 25% of the total feed rate.
The industrial test results in the suspension roaster are shown in Table 7.

Comparison of Different Technologies
All the above technologies can treat the complex zinc concentrate containing high Pb, Cu and SiO2.However, each of them has some advantages and some disadvantages: a) ISP requires large amount of expensive coke.This was the main reason for the shutdown of most ISP plants in early 2000.Also, the sinter machine operation has a long flowsheet and has serious dust emission.It is also difficult to capture the SO2 from the sinter machine.
b) Outotec atmospheric direct leaching process generates elemental sulphur residue that can't be hot filtered to produce elemental sulphur.Therefore, the whole elemental sulphur residue can't be damped easily and needs to be re-processed in furnaces or roasters.This requires significant Capex and Opex.c) Albion process requires ultra-grinding.The Albion reactor is cheaper than the Outotec DL reactor, however, the elemental sulphur residue from the Albion process also needs to be reprocessed as well.
d) PBOX process just separates Pb and Zn from the Zn-Pb bulk concentrate.The zinc concentrate from PBOX still needs to be treated in the fluidized bed roaster or other processes.Also, the concentrate requires ultra-grinding before it is fed into the PBOX reactors.
e) The pressure leaching process has high Capex, through the elemental sulphur can usually be hot filtered, except some Zn concentrates.f) The pelletised concentrate fluidized bed roasting process was shut down for decades.Now it is difficult to re-install such roaster.Also, the pelletising requires acid as binder, and the working environment is difficult due to this.
g) The sintering -leaching process has some issues that is similar to ISP, such as the dust emission issue and the SO2 capture etc.
h) The suspension roaster has small capacity due to the rotating arms for calcine removal physically and the pipeline feeding system.This suspension roaster has 10 t/h of feed rate only.

Future development
to the above disadvantages of those technologies, we are currently focusing on the development of a new technology: Flash Burner Roasting Process, which does not require the expensive coke, does not have elemental sulphur residue issue.The calcine from this flash burner roaster can be treated as the same as the calcine from the fluidized bed roaster.
The concept of the flash burner roaster is based on the industrial tests in 2023, which is presented above, in the suspension roaster in Spain.
The suspension roaster in Spain can treat the complex zinc concentrate, but there are significant disadvantages that limits the scale and capacity.
a) The rotating arms have limited capacity due to the length.This determines the capacity of the suspension roaster.
b) In the suspension roaster, the dried and milled concentrate, and the air are blown into the upper chamber of the roaster via the two pipelines, not via flash burners.Therefore, the mixture efficiency of the concentrate and the air in the pipelines is limited.The flash burners can provide much better mixing between the feed and the air.
In order to solve the above issues, a new flash burner roaster is currently studying and under constructure.The two features are listed below: a) The cylinder shape is changed to rectangle, so that the calcine can be removed out from the roaster bottom by one of the following two methods: Scraper chain conveyor, similar to that for the dust transportation system under the waste heat boiler Bed conveyor, similar to the sinter machine bed, but the grates have no holes or gaps, in order to reduce the air leakage into the roaster.
b) The pipeline feeding system is changed to 2 flash burners, similar to the Kivcet.
The sketch of the flash burner roaster is shown in Figures 9 and 10 below: .Flash burner roaster with bed conveyor

Conclusions
The 6 different technologies for the complex zinc concentrate treatment are discussed and compared.
In order to address the disadvantages of those technologies, a new technology "Flash Burner Roasting Process" concept was tested in an industrial suspension roaster successfully.The industrial tests results showed that the flash roasting process can treat the complex zinc concentrate, and the content of (Pb+Cu+SiO2) in the complex zinc concentrate was up to 11% during the tests.Such complex zinc concentrate is not able to be treated in the fluidized bed roaster.
The flash burner roaster is currently under development.The sketch of the flash burner roaster is presented as well.

Figure 7 .
Figure 7. Flowsheet of Suspension roasting process in Spain

Figure 9 .
Figure 9. Flash burner roaster with scraper chain conveyor

Table 2 .
Gibbs energy of formation

Table 3 .
Pressure leaching tests

Table 4 .
Leach residue flotation tests *The unit of Ag content is g/t.

Table 5 .
Assays of MRM zinc concentrate

Table 6 .
Particle size distribution of MRM zinc concentrate

Table 7 .
Industrial scale test results