The beneficiation of secondary resources has become a key challenge to mitigate imbalances in the international market and reduce environmental impact. This supports the development of long-term economies. Accordingly, this study aims to contribute to the valorisation of sludge generated by the phosphate ore processing at Djebel Onk, Algeria. To achieve this objective, a suite of characterization methods was used, including X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma optical emission spectrometry (ICP-OES), and Fourier transform infrared spectroscopy (FTIR). All methods confirmed that apatite is the predominant phosphate mineral in the processing sludge, associated with various gangue minerals, notably calcite, dolomite, and quartz. Rare earth elements and heavy metals were also detected, but only at trace amounts, with concentrations not exceeding international standards. Desliming reduced clay and siliceous minerals and increased the apatite content, followed by reverse flotation with different collectors and depressants. Using sulfuric acid as an apatite depressant performed poorly because the release of Ca²⁺ ions into the pulp promoted the formation of CaHPO₄ on the apatite surface, which hindered surfactant adsorption and thus depression. A subsequent test used sodium–potassium tartrate as the depressant together with aluminum sulfate to minimize the effect of Ca²⁺ ions, yielding a concentrate with a grade of 28.67%. This outcome meets phosphate ore specifications and helps limit environmental impacts.