Animal Bone Processing for a Circular Bioeconomy: Methods, Products, Applications, and Policy (2020–2025 Review)

Animal bones are abundant co-products of the meat industry and a strategic source of calcium and phosphorus alongside valuable proteins (collagen/gelatin). This review synthesizes advances (2020–2025) across the full processing chain—from preparation and cleaning to four principal routes: (i) rendering to bone meal/flour for feed and slow-release fertilizers; (ii) calcination to bone ash with tunable hydroxyapatite (HAp), β-tricalcium phosphate (β-TCP) and α-TCP for fertilizers and bioceramics; (iii) hydrothermal/chemical approaches for collagen/gelatin extraction and soluble phosphates (e.g., DCP/MCP); and (iv) biotechnological pathways, including phosphate-solubilizing microbes and hydrothermal treatment + anaerobic digestion with nutrient recovery. We consolidate operating windows (e.g., ~700–950 °C for HAp-rich ash; ~900–1050 °C for BCP; >1125 °C for α-TCP; 120–180 °C for subcritical-water extraction), decision points, and quality metrics (XRD/FTIR phase analysis, BET/porosity, citrate/NAC or DGT phosphorus availability, Bloom strength for gelatin). Industrial applications span agriculture (recycled P fertilizers), food (gelatin/collagen), biomedical (HAp/TCP biomaterials), and energy (bone char, biogas), with environmental and regulatory sections outlining sanitary safeguards (ABP categories, feed-ban rules) and EU fertiliser market entry (FPR/CMC pathways). We identify research gaps in harmonized QA/QC across routes, decision-grade LCA/TEA for electrified or intensified processing, field-scale agronomy for bone-derived P fertilizers, and regulatory interoperability to enable cross-border trade. Overall, bones represent a high-leverage circular feedstock; with fit-for-purpose processing and robust compliance, they can deliver environmental benefits, economic resilience, and advanced materials performance.