1 The SEM and EDS element mapping of Chinese nickel sulphide ore

(c) (d)

(e) (f)

(g) (h)

(a) J(talc) (b)

Fig. S1 SEM image(a) of samples and EDS element mapping((b)-(i)) and EDS result(j).

2 Methods

The experimental flowsheet is shown in Fig. S2. The experiment was based on the procedures of

roasting and leaching.

Fig. S2 Flow sheet for the two-stage roasting–leaching process

3 XRD patterns at different roasting time

Fig. S3 shows the XRD patterns of roasted clinkers obtained by roasting ores with sulfuric acid

for different times at an acid-to-ore ratio of 1.1:1 and a roasting temperature of 295°C. Table 1 shows

the mineralogical phases at different times.

(i) (j)

Inte

nsity

(Cou

nts)

10 20 30 40 50 60 70 80 9083-0080> Cu(SO4)(H2O) - Copper Sulfate Hydrate

81-0019> Szomolnokite, syn - Fe(SO4)(H2O)

76-0364> Mg(HSO4)2(H2O) - Magnesium Hydrogen Sulfate Hydrate

74-1329> Dw ornikite, syn - NiSO4(H2O)

73-1057> FeSO4 - Iron Sulfate

73-0147> Talc - Mg3(Si2O5)2(OH)2

72-1259> MgSO4 - Magnesium Sulfate

72-0544> Kieserite - MgSO4H2O

72-0090> Chalcocyanite - CuSO4

70-2091> Fe2(SO4)3 - Iron Sulfate

20-0790> Ni-Si-O!H2O - Nickel Silicate Hydrate

Theta(deg)

10min

20min

30min

40min

60min

90min

120min

150min

Fig. S3 XRD patterns at different roasting time at 295°C

Table S1 Phases identified in the XRD patterns at different roasting time at 295°C

10

min

20 min 30 min 40 min 60

min

90

min

120

min

150

min

talc[Mg3(Si2O5)(OH)2] √ √ √ √ √ √ √

ferrous sulfate(FeSO4) √ √ √ √ √

magnesium sulfate(MgSO4) √ √ √ √ √

copper sulfate(CuSO4) √ √ √ √ √

ferric sulfate[Fe2(SO4)3] √ √ √

szomolnokite[Fe(SO4)(H2O)] √ √ √ √

magnesium hydrogen sulfate

hydrate[Mg(HSO4)2(H2O)]

√ √ √

dwornikite[NiSO4(H2O)] √ √ √

copper sulfate

hydrate[CuSO4(H2O)]

√ √ √

nickel silicate

hydrate[NiSiO3(H2O)]

√

kieserite[MgSO4(H2O)] √ √

It can be seen from Fig. S3 and Table S1that the main phases in the specimen obtained at a

roasting time of 10 min are Fe(SO4)(H2O), Mg(HSO4)2(H2O), NiSO4(H2O) and CuSO4(H2O) and that

the diffraction peaks of talc are obscured by other diffraction peaks. Talc phases, NiSiO 3(H2O) and

MgSO4(H2O), appear in the specimen obtained with a roasting time of 20 min, indicating that part of

the Ni is hosted in silicate. Compared to a roasting time of 20 min, the NiSiO3(H2O) phase disappears

after roasting for 30 min. However, this may result from its low content and other strong diffraction

peaks may obscure its diffraction peaks. The main phases in the specimen roasted for 40 min are talc,

Fe(SO4)(H2O), FeSO4, MgSO4 and CuSO4. Roasting for 60 min, 90 min, 120 min and 150 min results

in the Fe(SO4)(H2O) disappearing while Fe2(SO4)3 appears. The diffraction peaks of NiSO4 cannot be

identified because they are hidden by the strong diffraction peaks of other phases.

3.2.2 DTA-TG analysis

Fig. S4 shows the DTA-TG curve obtained from heating a sulfuric acid and ore sample from 0 to

1000°C using a Diamond 6300 (PerkinElmer, America) thermal analysis instrument at a heating rate of

10°C/min.

Fig. S4 The diagram of DTA and TG for sulfuric acid and ore

It can be seen from the DTA curve that there are nine endothermic peaks, located at 111.68°C,

195.05°C, 234.91°C, 310.11°C, 531.46°C, 684.12°C, 740.01°C, 869.12°C and 940.95°C, and three

exothermic peaks, located at 169.48°C, 210.09°C and 259.72°C, respectively. The first endothermic

process, below 111.68°C, is the removal of free water, accompanied by a weight loss of 2.20%. In the

temperature range of 130.78°C to 321.51°C, there are three exothermic peaks corresponding to the

three endothermic peaks. In this stage, sulfuric acid begins to react with ores, releasing SO 2 (or SO3)

and steam. The initial react temperature is 130.78°C with a corresponding weight loss of 3.54%. The

reactions between sulfuric acid and the ore can be summarized as (1) to (21).

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

The reactions continue with increasing temperature until the sulfuric acid is exhausted. After that,

the metal sulfates begin to decompose into metal oxide and sulfur trioxide. In this process, four

endothermic peaks at 531.46°C, 684.12°C, 740.01°C and 869.12°C are produced. The last endothermic

peak at 940.95°C is mainly attributed to the decomposition of unreacted talc to release structural water.

The reactions can be expressed as Eqs. (22) to (29).

(22)

(23)

(24)

(25)

(26)

(27)

(28)

(29)

4 Thermodynamic analysis

Fig. S5 Free energy values for corresponding reactions in first stage roasting

Fig. S5 shows the Gibbs free energy values of Eqs. (1) to (21) that take place in the first stage of

roasting (using HSC Chemistry 6) from 0-500°C.

Fig. S6 Free energy values for the corresponding reactions in the second roasting stage

Fig. S6 shows the Gibbs free energy values of Eq. (22) to (29) in the second roasting stage

(using HSC Chemistry 6) from 0-1000°C. According to Fig. S6, it was expected that some metal

sulfates decompose at particular temperatures, although the actual decomposition will be dependent

upon the SO3 partial pressure. A suitable second stage roasting temperature was explored to promote

the decomposition of ferric sulfate, while ensuring that nickel sulfate and copper sulfate would be

thermodynamically stable. This could thermodynamically explain the extractions of different

components in the second stage roasting process.