**3. Leaching methods**

#### **3.1 Batch leach tests**

High acidity and high temperature are known to assist with the precipitation of silica in a solid crystalline form rather than polymerising to form silica gel [11–13]. Higher leach slurry densities offer higher acid concentrations in solution and lower mass throughputs. Lower mass throughputs reduce construction costs for the leaching equipment. Concentrated HCl was selected to produce a high solids density and high acidity in the leach. The use of concentrated HCl will increase the temperature of the leach due to the heat generated by acid dilution.

The program focused on leaching low grade concentrate under various atmospheric conditions with hydrochloric acid to determine metal dissolution and the relative filtration performance of the resultant leached slurry. The general acid leach test procedure is shown in **Figure 2**. All acid doses are quoted in grams of anhydrous HCl per gram of dry concentrate.

The concentrate was leached by manually mixing concentrate and concentrated HCl (20–29% w/w HCl) for 15–30 minutes. Dilute HCl (10 g/L HCl) is then added to generate a slurry which is sufficiently fluid to enable it to be poured onto a vacuum filter. The quantity of 10 g/L HCl added to the leach slurry was varied so that the mass of slurry and the slurry % solids to the filter was constant regardless of the acid concentration or acid dose used in the leach stage. The diluted slurry was then vacuum *Concentrated Hydrochloric Acid Leaching of Greenland Steenstrupine to Obviate Silica Gel… DOI: http://dx.doi.org/10.5772/intechopen.107012*

**Figure 2.** *Batch laboratory work flow.*

filtered and the cake form time determined. Filter cake form time was used as a proxy for filtration rate.

The filtrate was collected and a subsample immediately diluted 1:10 with 1 M HCl, to stabilise the sample prior to submitting for elemental analysis. The filter cake was dispersed in D.I water which had been acidified to pH 1 (HCl) and vacuum filtered. Once the surface moisture had disappeared from the filter cake, additional pH 1 wash water was applied to the cake to remove entrained dissolved salts. The solids were then dried to constant mass at 80°C and submitted for elemental analysis.

#### **3.2 Continuous leach tests**

Continuous leach tests were conducted once suitable conditions for concentrate leaching were identified from the batch testwork program. Dry concentrate was continuously fed (vibratory feeder) to a 5.8 L stirred baffled reactor. Concentrated (25% w/w) HCl was continuously pumped in via a peristaltic pump to achieve the required acid dose. The reactor temperature was maintained at 80°C using indirect steam through titanium steam coil. A thermocouple was inserted into the slurry and indirect steam added through a solenoid valve connected to an automatic controller.

The reactor had a nominal residence time of between 69 and 97 minutes depending on feed flowrates. Samples were taken from the reactor overflow and prepared for assay as previously described.

#### **3.3 Continuous leach tests**

Analysis of the solid samples was performed using a four-acid digest followed by ICP-OES and ICP-MS finishes. The digest solutions were appropriately diluted before being analysed. Solid samples were also analysed by sodium peroxide fusion digest, followed by an analysis of the resulting solutions by ICP-OES and ICP-MS. Solids were assayed using sodium fusion to ensure any refractory minerals were dissolved. Good agreement was achieved between the four acid digest and peroxide fusion methods and suggests that any refractory minerals present in the sample was minimal.

Elemental assays of liquors were determined by ICP-OES and ICP-MS. The liquors were diluted 1:10 with 1 M HCl prior to submission to stabilise silicon in solution.

Free acid was determined by titration with standard NaOH using Calcium EDTA as a complexing agent, the procedure was modified from that described in [15] and

**Figure 3.** *Continuous leach set up.*

required correction for phosphoric acid in the sample, based on the phosphorous assay of the solution (**Figure 3**).
