**4.1 Batch test results**

Four tests were performed to examine the impact of acid dose on metal dissolution and solid liquid separation of the leached slurry. In each test concentrate was mixed with 29% w/w HCl for 30 minutes. The impact of acid dose on filtration rates is provided in **Table 3**.

The metal leach extraction for the four batch tests are shown in **Table 4**. They show very high extraction of rare earth elements. Gangue elements such as aluminium and iron are lower. This is due to the deportment of these gangue metals in the


**Table 3.**

*Acid dose and solid liquid separation performance.*

*Concentrated Hydrochloric Acid Leaching of Greenland Steenstrupine to Obviate Silica Gel… DOI: http://dx.doi.org/10.5772/intechopen.107012*


#### **Table 4.**

*Low grade concentrate metal dissolution for batch tests.*

minerals leached. Most of the iron is found in the black amphibole arfvedsonite which appears to be refractory in the leach process. The refractory aluminium and sodium may be present in sodic feldspar minerals which do not leach extensively under these leach conditions.

Due to the high slurry density it was important to observe the viscosity of the slurry mixture after the addition of concentrate and acid. For the first 1–2 minutes of mixing the mixture would remain fluid, however after this period the free liquid would disappear and the viscosity of the mixture increased to the consistency of damp solids. The damp solid became hot and with continued mixing, formed a viscous paste which progressively became more fluid with time. These observations have also been reported when leaching eudialyte concentrates [16] with strong acid.

**Figure 4** shows there is very little impact of acid dose on the dissolution of metals except thorium over the range investigated. Under all acid doses near complete extraction of rare earths and uranium was observed.

The cake form time (filtration rate) shows a strong correlation with acid dose. As the acid dose increased the cake form time decreased, demonstrating an improvement in filtration. At the higher acid doses no gel formation was observed. Both lower acid doses resulted in gel formation indicating that lower acid conditions result in favourable conditions for silica polymerisation. At higher acid doses and therefore higher acid strength in solution may result in a more crystalline silica precipitate.

**Figure 4.** *Acid dose effects on dissolution and filtration.*

The reduced filtration rate and the appearance of silica gel at the lower acid doses are all consistent with reduced rates of silica coagulation [12, 13, 17]. The rate of silica coagulation from solution has been shown to be strongly influenced by a number of factors including temperature, the presence of metals in solution and free acid [17, 18]. Generally, as temperature increases the rate of silica precipitation increases while the presence of aluminium in some systems appears to increase the rate of silica precipitation. In these series of tests, the aluminium tenor and the temperature were similar over the acid dose range investigated. As the terminal free acid increased the solid liquid separation improved. The results are consistent with improved silica coagulation at higher acid concentrations resulting in improved filtration.

Over 99% of the acid soluble silicon had precipitated under all the test conditions. Terminal free acid concentrations down to 47 g/L HCl are still capable of achieving high rare earth dissolution. A stoichiometric acid balance was performed to determine the elemental acid consumption. Most of the acid (60%) was consumed in the dissolution of Al, Na and Fe. A significant proportion (15–25%) of the acid added is not consumed in the reaction and remains in solution as free HCl. The majority of the balance of the hydrochloric acid dose is consumed by rare earth dissolution.

#### **4.2 Impact of addition method**

Previous tests were typical batch tests where all the acid and concentrate are mixed together at the start of a test. While this is a convenient procedure it can produce different results to a continuous process which represents a commercial operation. Continuous leach tests where precipitation of reactants occurs can result in significantly more heterogeneous precipitation. Heterogeneous precipitation products can have different particle size and morphology compared to homogeneous precipitation products [19].

Four tests were performed to examine acid addition methods and more continuous leach conditions. Continuous operation was simulated by dosing the reactants to the leach in staged doses during the leach. The acid dose (0.38 g/g), acid concentration (29% w/w HCl), average leach time (30 minutes) and temperature (ambient) were kept constant between tests.

The additions to the reactor were as follows:

Test 9–100%, all acid and concentrate added at the start (baseline test).

Test 11–80% of the concentrate added at the start, remainder after 10 minutes. Test 14–40% of the concentrate added at the start, remainder after 10 minutes. Test 15 - a third of the acid and concentrate added every 5 minutes to the reactor.

**Table 5** shows that the method of feed addition had little impact on metal extraction with constant acid dose. The filtration of the residues was significantly impacted by the timing of acid addition as observed in **Figure 5**.

The smaller incremental additions to the reactor are more representative of a continuous leach reactor. The results show that cake form time decreases (better filtration) as the proportion of sample added to the reactor at the start of the test fell. These are consistent with increases in heterogeneous precipitation and particle growth expectations. As incremental additions are made to the reactor; silica dissolves and reprecipitates on precipitated silica from earlier reactor additions, increasing silica precipitation particle size. Incremental feed addition to the batch reactor results in a substantial improvement in the filtration properties of the leach slurry.

*Concentrated Hydrochloric Acid Leaching of Greenland Steenstrupine to Obviate Silica Gel… DOI: http://dx.doi.org/10.5772/intechopen.107012*


*\*CFT = Cake Form time in seconds.*

**Table 5.**

*Impact of acid addition method.*

**Figure 5.** *Dissolution and filtration versus acid dose.*

#### **4.3 Impact of acid concentration**

Three tests were completed where the acid concentration was varied while maintaining a constant acid dose of 0.38 g HCl /g. The leach mixture was maintained at 55°C in a water bath. To better reproduce the conditions observed in continuous operation the acid and concentrate were added incrementally to a stirred reactor. The first two tests were under the same conditions except the second test used a higher acid concentration (but same acid dose). The third test was a repeat of the first test except a longer residence time (slower addition rate) was employed. The results of these tests are shown in **Tables 6** and **7**.

All tests gave similar metal extractions except for thorium which shows a significant drop in dissolution at the lower terminal free acid concentration (lower acid concentration in feed).

The acid strength had the greatest impact on the physical performance of the leach, with gel formation at the lower acid concentration which increased filtration time. Increasing leach residence time, from 32 to 69 minutes (Test 20) overcame the silica


**Table 6.**

*Impact of acid concentration on solid liquid separation.*


#### **Table 7.**

*Impact of acid concentration on metal extraction (% from concentrate).*

gelling issues resulting in improved filtration rate. These observations are consistent with the expected impact of dissolved silica precipitating as a polymerised gel.

The cake form time for test 19 is much shorter (40 seconds) than the comparable batch test (test 2, 180 seconds). This is attributed to the elevated temperature of the acid mixing stage in test 19 and increased heterogeneous silica precipitation when the acid and concentrate are added incrementally to the reactor.

#### **4.4 High grade concentrate test results**

Leach tests with high grade concentrate were completed to confirm the general relationships between the various process factors examined in this paper. The concentrate and acid were mixed using the simple batch contact and the incremental addition procedure. The acid mixing stage was maintained at 65°C with the acid added as 25% w/w HCl. Tests were conducted to identify the impact of acid dose, acid concentration and addition method on metal dissolution and filtration rate.

Metal dissolution at the lower acid dose was significantly lower regardless of addition method. The lower grade concentrate gave significantly higher metal dissolution at the 0.38 g HCl/g dose, this is attributed to the refractory nature of the gangue minerals in the low-grade concentrate which do not consume significant quantities of acid. Gangue minerals such as feldspar and arfvedsonite are only significantly present in the lower grade concentrate.

At the lower acid dose **Table 8** reveals the incremental addition method resulted in significantly better filtration which is consistent with results for the low grade concentrate.

At the higher acid dose, rare earths, uranium and thorium recovery increased while aluminium, iron and sodium dissolution were unchanged (**Table 9**). Filtration was significantly better at the higher acid dose while the method of acid addition had no discernible impact on filtration rate. The results are consistent with the higher rate *Concentrated Hydrochloric Acid Leaching of Greenland Steenstrupine to Obviate Silica Gel… DOI: http://dx.doi.org/10.5772/intechopen.107012*


#### **Table 8.**

*Impact of acid strength on the solid liquid separation of leached high grade concentrate.*


#### **Table 9.**

*High grade concentrate metal extraction (% dissolved from concentrate solids).*

of silica coagulation at the higher terminal free acid (higher acid dose) giving better filtration rates.

Metal recovery remains unchanged between tests except for thorium which shows a slight drop when a lower acid concentration was used and is attributed to the lower terminal free acid.

### **4.5 Continuous leach results**

Dry low grade concentrate and 25% w/w HCl were continuously added to a stirred reactor maintained at 80°C. Flowrates were controlled to maintain the target acid dose and nominal residence time. The test was undertaken to determine metal dissolution and observe the viscosity of the slurry in the leach reactor.

The first continuous test targeted a nominal residence time of 97 minutes at an acid dose of 0.46 g HCl/g concentrate. The second continuous test decreased the nominal residence time to 69 minutes at an acid dose of 0.39 g HCl/g concentrate. The test results of the continuous tests and the comparable batch test are presented in **Table 10**.

**Table 11** shows the metal leach extraction was similar between the continuous and batch leach tests. The filtration rates recorded in **Table 10** were consistently better for the continuous tests at similar acid dose. The continuous leach test observed a significantly lower leach viscosity compared to the batch tests. While the slurry from the batch tests had a toothpaste consistency the slurry from the continuous test was more fluid and was able to freely overflow the reactor.

In batch tests the acid/concentrate mixture initially formed a damp solid which is difficult to agitate effectively. The damp solid did not flow but as mixing continued it became more fluid achieving a consistency similar to toothpaste. If high viscosity is


#### **Table 10.**

*Comparison of continuous and batch leach methods impacts on solid liquid separation.*


#### **Table 11.**

*Comparison of metal extractions from batch and continuous leach conditions.*

observed in the continuous operation it will result in more expensive and specialised leaching equipment. The low viscosity slurry observed in the continuous tests will allow for the use of standard Continuously Stirred Tank Reactors (CSTRs) which are low cost, low risk and industry standard (**Figures 6** and **7**).

**Figure 6.** *Viscous leach conditions from batch test.*

*Concentrated Hydrochloric Acid Leaching of Greenland Steenstrupine to Obviate Silica Gel… DOI: http://dx.doi.org/10.5772/intechopen.107012*

**Figure 7.** *Fluid leach conditions from a continuous test.*
