**3.2 Sequential decomposition sulfides minerals and carbonaceous matter**

DRGO often contains less than 7% of carbon content, and it is considered to have a more non-uniform structure compared to activated carbon, making it relatively easier to decompose [8]. However, the susceptibility to carbonaceous matter to decomposition by lignin-degrading enzymes should vary depending on the ore. Raman spectroscopy was used to generate **Figure 4**, which shows the difference in graphiticity of carbonaceous matter from various gold ores. The C=C peak for continuous graphitic carbon near 1580 cm−1 (G-band) and the C=C peak for vibration adjacent to the graphite defect

**Figure 4.** *Raman spectra for DRGOs from different mines (modified [41]).*

#### *Biotechnological Approaches to Facilitate Gold Recovery from Double Refractory Gold Ores DOI: http://dx.doi.org/10.5772/intechopen.94334*

near 1320 cm−1 (D-band) was distinguished, and the relative intensity *I*D/*I*G and can be used as an index of the degree of the defect [42]. Relative intensity *I*D/*I*G varies with ore and is an indicator of the preg-robbing ability of the carbonaceous matter. The gold ore from the Paddington mine in Australia has a very high defect compared to carbonaceous matter from the Macraes mine in New Zealand, which has a very aromatic structure. This indicates that the carbonaceous matter from Paddington mine might be more susceptible to lignin-degrading enzymes than the ore from Macraes mine.

The sequential treatment of DRGO has shown significant results, as seen in **Table 1**. Using lignin-degrading enzymes to treat DRGO began with work by Yen et al., [10] who applied the fungi *T. versicolor,* but information about this work is limited because it is a patent. This was followed by Ofori-Sarpong et al. [11] who built open their previous work using *P. chrysosporium* to decompose coals of various ranks [38]. They found that applying the lignin-degrading enzymes directly to the DRGO to decompose both the sulfides and carbonaceous matter improved the gold recovery from 41% - 78% which was slightly less than 81% recovery obtained when the sample was only subjected to sulfide oxidation by chemoautotrophic bacteria. This indicates that the fungal treatment was less effective at oxidizing both refractory materials. Therefore, sequential treatment was used, with the sulfide oxidation by bacteria preceding the carbonaceous matter treatment by *P. chrysosporium,* and they reported a final gold recovery of 94%.

They attempted to further understand the effect of the *P. chrysosporium* on the DRGO by using only the spent medium to treat the DRGO [12]. This was done to check the ability of the lignin-degrading enzymes to oxidize the carbonaceous matter while also avoiding complications like the fungal biomass. This work did not yield as high a gold recovery compared to when the *P. chrysosporium* was cultured with the DRGO, but it did show that it could be viable for DRGO treatment.


*\*1Numbers in brackets mean the gold recovery after 1st step of microbiological oxidation and before 2nd step of enzyme treatment.*

#### **Table 1.**

*Summary of the previous works on bio-treatment of carbonaceous matter in DRGO by lignin-degrading enzymes.*

To this end, Konadu et al. [13] used the spent medium of *P. chrysosporium* to treat a DRGO, which had a Raman spectrum similar to the Rodeo gold ore in **Figure 4**. It was observed that starting with an iron oxidizer, followed by the spent medium of *P. chrysosporium*, help to improve the gold recovery from 24–92%. Although, an alkaline washing step had to be incorporated after the spent medium treatment to remove some of the byproducts of the carbonaceous matter decomposition to allow for the final 15% recovery to attain overall the 92% recovery. On the other hand, using the spent medium before the iron oxidizer appeared to inhibit sulfide oxidation and lead to a final gold recovery of 45% for that sequence.

The changes in the mineral phases were observed by Quantitative Evaluation of Minerals using Scanning Electron Microscopy (QEMSCAN) [3, 43]. The sulfides (yellow color) were observed to be relatively liberated while the carbonaceous matter was associated with illite as seen in by the dark green color (**Figure 5a)**. After the best sequential treatment condition (sulfide oxidation followed by carbonaceous matter treatment by spent medium, DAC) was applied, most of the sulfide minerals were decomposed and disappeared (**Figure 5b**). Additionally, a new mineral phase (C-Si-Al) containing currently unknown amounts of carbon, aluminum and silicon was observed. A backscatter image of two organic carbon-containing particles is

#### **Figure 5.**

*QEMSCAN maps for DRGO (a) and DAC (b): Particles are classified into liberated carbonaceous illite, associated with illite, associated with quartz, and others according to particle sizes. Notable changes from DRGO to DCA are the disappearance of pyrite and formation of larger particles than 100* μ*m which are categorized to C-Si-Al, and formation of larger particles than 100* μ*m which are rich in C, Si and Al (modified [43]).*

*Biotechnological Approaches to Facilitate Gold Recovery from Double Refractory Gold Ores DOI: http://dx.doi.org/10.5772/intechopen.94334*

shown in **Figure 6**. **Figure 6a** shows a particle in which the carbonaceous matter is associated with the illite in the original DRGO while **Figure 6b** shows a residue produced after the sequential treatment was applied (DAC). The sediment-like morphology of particle in **Figure 6b** indicated that it was most likely a product of the carbonaceous matter decomposition.

#### **Figure 6.**

*Backscattering images of (a) DRGO and (c) residue after sequential biotreatment with an iron-oxidizer and spent medium of P. chrysosporium (DAC). Images (b) and (d) are false color renditions of the arrowed particles in (a) and (c), respectively.*

#### **Figure 7.**

*Raman spectra (a) before and (b) after 1 M NaOH washing of the as-received ore, and the solid residues after treated by CFSM (DC), A. brierleyi (DA), CFSM followed by A. brierleyi (DCA), and A. brierleyi followed by CFSM. Numbers beside figures indicate the intensity ratio (ID/IG) for the relative quantity of the defect in all samples with graphitic structures (modified [43]).*

Raman spectra of the samples before and after each treatment showed a relative decrease in the D-band compared to the G-band (**Figure 7a**). This change was most significant after the spent medium treatment indicating that the lignin-degrading enzymes might have preferentially oxidized aromatic C=C carbon with some type of physical or chemical defects [43].

After the sequential treatment, the gold recovered before and after the spent medium treatment was unchanged at 77%, indicating that some of the by-products of this treatment were interfering with the recovery (DAC, **Figure 8**). Therefore, 1 M NaOH washing was incorporated into the sequential treatment to remove what

**Figure 8.** *Gold recovery in each step of sequential biotreatment of DRGO (modified [13]).*

#### **Figure 9.**

*Schematic illustration of agglomerated particles in DAC after enzyme treatment, which are rich in C, Al and Si. After enzyme treatment, humic-like substances are formed to interact with clay minerals through metallic ions like Fe3+.*

*Biotechnological Approaches to Facilitate Gold Recovery from Double Refractory Gold Ores DOI: http://dx.doi.org/10.5772/intechopen.94334*

was identified by 3D fluorescence spectroscopy as humic-like substances interfering during the gold recovery process, which lead to the observed changes in the Raman spectra and increase in gold recovery to 92% (**Figure 7b**) [43]. Thus, it was proposed that sediment-like C-Si-Al phase could have been produced by an electrostatic aggregation of clay minerals like illite and the humic-like substances. Under the condition of pH 4.0, where the enzyme treatment is performed, the surface of illite is negatively charged (isoelectric point 3.0) (**Figure 9**). While the humic-like substances have a large, irregular structure, and thus, the acid dissociation constant does not have a uniform value and has a width of 2.9–5.5. At pH 4.0, if the humiclike substances have a positive charge, then it might direct form an agglomerate with the negatively charged illite. However, if charged negative, then some of the cations in the solution like Fe3+ might be involved in the agglomeration of C-Si-Al.
