**3. Leaching of the SPL individual constituents by H2SO4 solution**

The leaching process starts with the dissolution of the water-soluble compounds of the SPL (namely, NaF, NaCN, Na2CO3, and NaAlO2) in the H2SO4 solution rather than leaching in water followed by the acid. However, leaching of these four compounds in water is possible but it is very slow and requires large vessels.

Leaching reactions of the above-mentioned water-soluble compounds with H2SO4 are presented by Eqs. (11) to (14). See reactions R1 to R4 in **Table A.1**.

$$2\text{NaF}(\text{s}) + \text{H}\_2\text{SO}\_4 \rightarrow \text{Na}\_2\text{SO}\_4 + 2\text{HF}(\text{g}) \tag{11}$$

$$\text{Na}\_2\text{CO}\_3\text{ (s)} + \text{H}\_2\text{SO}\_4 \rightarrow \text{Na}\_2\text{SO}\_4 + \text{H}\_2\text{O} + \text{CO}\_2\text{(g)}\tag{12}$$

$$2\text{NaCN}\left(\text{s}\right) + \text{H}\_2\text{SO}\_4 \rightarrow \text{Na}\_2\text{SO}\_4 + 2\text{HCN}(\text{g}) \tag{13}$$

$$\text{AlNaAlO}\_2\text{ (s)} + 4\text{H}\_2\text{SO}\_4 \rightarrow \text{Al}\_2(\text{SO}\_4)\_3 + \text{Na}\_2\text{SO}\_4 + 4\text{H}\_2\text{O} \tag{14}$$

On the other hand, the graphite present in SPL is the only compound that does not react with acids (e.g. H2SO4), alkalis (e.g. NaOH) or acidic Al3+ solution. However, the reactions of the three other insoluble compounds present in the SPL (namely, NaAlSiO4, Na3AlF6, and CaF2) are explained below.

1.The NaAlSiO4 dissolves in aqueous H2SO4 solution and produces the intermediate product NaAl3(SO4)2(OH)6 according to Eq. (15)

$$\text{3NaAlSiO}\_4 + \text{3H}\_2\text{SO}\_4 \rightarrow \text{3SiO}\_2(\text{s}) + \text{Na}\_2\text{SO}\_4 + \text{NaAl}\_3(\text{SO}\_4)\_2(\text{OH})\_6 \tag{15}$$

NaAl3(SO4)2(OH)6 dissolves in excess H2SO4 [39] according to Eq. (16).

$$\text{2NaAl}\_3(\text{SO}\_4)\_2(\text{OH})\_6 + 6\text{H}\_2\text{SO}\_4 \rightarrow \text{3Al}\_2(\text{SO}\_4)\_3 + \text{Na}\_2\text{SO}\_4 + \text{12H}\_2\text{O} \tag{16}$$

By multiplying Eq. (10) by 2, adding it to Eq. (15), and dividing the result by 3 gives the net result presented by Eq. (17) (similar to that reported by [40]:

$$2\text{NaAlSiO}\_4 + 4\text{H}\_2\text{SO}\_4 \rightarrow \text{Al}\_2(\text{SO}\_4)\_3 + \text{Na}\_2\text{SO}\_4 + 2\text{SiO}\_2 + 4\text{H}\_2\text{O} \tag{17}$$

2.The cryolite (Na3AlF6) does not react with H2SO4 spontaneously; it has a high +ΔGR. However, it reacts spontaneously with concentrated NaOH solution to produce NaF and the intermediate product NaAl(OH)4 according to Eq. (18):

$$\text{Na}\_3\text{AlF}\_6(\text{s}) + 4\text{NaOH} \rightarrow \text{NaAl}(\text{OH})\_4 + 6\text{NaF}(\text{aq})\tag{18}$$

However, both resulting products (NaF and NaAl(OH)4) need to be leached with or neutralized by H2SO4 according to Eq. (11) (for NaF) and according to Eq. (19) for NaAl(OH)4:

$$2\text{NaAl}(\text{OH})\_4 + 4\text{H}\_2\text{SO}\_4 \rightarrow \text{Al}\_2(\text{SO}\_4)\_3 + \text{Na}\_2\text{SO}\_4 + 8\text{H}\_2\text{O} \tag{19}$$

An alternative to this two-step leaching process expressed by Eqs. (16) and (18), the Na3AlF6 can be leached with an acidic Al3+ solution comprised of Al(OH)3 and H2SO4, which was found to be more effective than leaching with an acid only or an alkali only [41, 19]. This acidic Al3+ solution can be prepared according to Eq. (20):

$$\text{2Al(OH)}\_{3}(\text{s}) + \text{3H}\_{2}\text{SO}\_{4} \leftrightharpoons \text{Al}\_{2}(\text{SO}\_{4})\_{3}(\text{aq}) + \text{6H}\_{2}\text{O} \tag{20}$$

and the reaction of Na3AlF6 with the above solution gives

$$2\text{Na}\_3\text{AlF}\_6(\text{s}) + 2\text{Al}(\text{OH})\_3(\text{s}) + 3\text{H}\_2\text{SO}\_4 \rightarrow 3\text{Na}\_2\text{SO}\_4 + 4\text{AlF}\_3 + 6\text{H}\_2\text{O} \tag{21}$$

However, the Al2(SO4)3 (or acidic Al3+) solution is already produced by Eqs. (14) and (17) presented above. Here, the Al2(SO4)3 has an *amphoteric* character, i.e. it can both act as an acidic and a basic solution in the aqueous phase. Thus, the Na3AlF6 reacts (spontaneously) with the present acidic Al2(SO4)3 solution to give Na2SO4 and AlF3 according to Eq. (22):

$$\text{2Na}\_3\text{AlF}\_6(\text{s}) + \text{Al}\_2(\text{SO}\_4)\_3(\text{aq}) \rightarrow \text{3Na}\_2\text{SO}\_4(\text{aq}) + \text{4AlF}\_3(\text{aq})\tag{22}$$

3.The reaction of CaF2 with H2SO4 is less spontaneous (very small -ΔGR that decreases with temperature) and gives CaSO4 and HF according to Eq. (23), (which is not required at this stage of leaching):

$$\text{CaF}\_2(\text{s}) + \text{H}\_2\text{SO}\_4(\text{aq}) \to \text{CaSO}\_4(\text{s}) + 2\text{HF}(\text{aq})\tag{23}$$

However, CaF2 can react (spontaneously) with the solution presented by Eq. (19) according to Eq. (24).

$$\text{\bf{3CaF}\_2(s)} + \text{2Al(OH)}\_3(s) + \text{3H}\_2\text{SO}\_4 \rightarrow \text{\bf{3CaSO}\_4(aq)} + \text{2AlF}\_3(aq) + \text{6H}\_2\text{O} \tag{24}$$

But again, CaF2 can also react (spontaneously) with the acidic Al2(SO4)3 produced by Eqs. (14) and (17) to give CaSO4 precipitate and aqueous AlF3:

$$\text{\bf{\bf{\bf{C}}}}\text{\bf{\bf{\bf{A}}}}\text{\bf{\bf{\bf{\bf{2}}}}}(\text{s}) + \text{Al}\_2(\text{SO}\_4)\_3 \left(\text{aq}\right) \rightarrow \text{\bf{\bf{\bf{C}}}}\text{\bf{\bf{\bf{\bf{A}}}}}\text{\bf{\bf{O}}}\_4(\text{s}) + 2\text{Al}\text{F}\_3(\text{aq})\tag{25}$$
