A closed-loop system and process is used for applying fire retardant chemicals to substrates including fibers, yarns, fabrics or mixtures thereof. Substrates are preferably positioned in a vessel such as a dye machine which circulates the fire retardant chemicals. After absorption of the fire retardant composition, non-absorbed fire retardant chemicals are recovered and re-used on subsequent batches of fibers. The re-use of fire retardant chemicals can take place in the same vessel which is used to treat successive batches of substrates. Alternatively, recovery can be achieved by directing the non-absorbed fire retardant composition into a second dye machine containing additional fibers, or by extracting the fire retardant composition by centrifugation or other means, or by a combination of the two processes. The process is environmentally friendly, and allows for higher throughput on impregnating fibers with fire retardant chemicals.