Cationic polyacrylamide (CPAM) assisted leaching of rare earth ore containing clay minerals by inhibiting swelling and promoting permeation of ammonium sulfate solution

Abstract

In-situ leaching of clay-based weathered crust elution-deposited rare earth ore (WCE-DREO) is prone to landslides resulting from the swelling of clay minerals, which compromises both ecological safety, economics and efficiency of mining. This study used the cationic polyacrylamide (CPAM), mixed with 2.0 wt% (NH₄)₂SO₄ to form a composite leaching agent to address this issue, which serves two purposes: (i) it acts as an inhibitor of clay swelling, (ii) it promotes the permeation of the leaching agent. Linear swelling tests and column leaching experiments were conducted to evaluate the effect of CPAM on swelling inhibition and permeation promotion in rare earth ore leaching. The underlying mechanisms were tested and analyzed through a series of advanced characterization methods. The findings demonstrate that the addition of CPAM significantly enhances the permeation of the leaching agent. Furthermore, the composite leaching agent of (NH₄)₂SO₄ and CPAM inhibits the swelling at higher CPAM concentrations. At a CPAM concentration of 3 × 10⁻⁴ wt%, the seepage time of the leaching agent is minimized—effectively reduced by 46 %, compared to using 2.0 wt% (NH₄)₂SO₄ alone. The CPAM structure contains amine groups and positively charged quaternary ammonium groups which enable the adsorption of CPAM onto the clay mineral interlayer and surface through hydrogen bonding and electrostatic interactions. This adsorption affects in four ways: (i) hinders the entry of water molecules into the mineral interlayers, (ii) reduces electrostatic repulsion, (iii) compresses the diffuse double layer, and (iv) promotes the aggregation of fine particles into larger clusters. These effects further inhibit the swelling of clay minerals and promote the permeation of the leaching agent. At the same time, the long-chain structure of CPAM helps to entangle fine clay mineral particles into larger clusters. This prevents clogging of the permeation channel due to the movement of fine particles along with the (NH₄)₂SO₄ solution, thereby increasing the seepage rate. Additionally, CPAM can enter the clay mineral layers to discharge internal water and further inhibit swelling. This study provides theoretical insights and technical support for the safe and highly efficient mining of WCE-DREO.