TECHNICAL FIELD The present disclosure relates to a method of recycling resources when manufacturing a lithium secondary battery. In particular, the present disclosure relates to an apparatus for recovering an electrode active material from an electrode scrap generated 10 in a lithium secondary battery manufacturing process or from a lithium secondary battery that is discarded after use, and a method of reusing the recovered active material. The present application claims priority to Korean Patent Application No. 10-2020-0101962 filed on August 13, 2020 in the Republic of Korea, the disclosure of which is incorporated herein by reference. 15 BACKGROUND ART Lithium secondary batteries which may be repeatedly charged and discharged are in the spotlight as an alternative to fossil energy. Lithium secondary batteries have been mainly used in traditional handheld devices such as cell phones, video cameras, and power 20 tools. However, recently, application fields of lithium secondary batteries have been gradually increasing to electric vehicles (EVs, HEVs, and PHEVs), large capacity energy storage systems (ESSs), uninterruptible power supply systems (UPS), etc. A lithium secondary battery includes an electrode assembly in which unit cells 3 having a structure in which a positive electrode plate and a negative electrode plate coated with an active material on a current collector are arranged with a separator interposed therebetween, and an exterior material sealing and accommodating the electrode assembly together with an electrolyte, that is, a battery case. The positive electrode active material 5 of the lithium secondary battery mainly uses a lithium-based oxide, and the negative electrode active material uses a carbon material. The lithium-based oxide contains a metal such as cobalt, nickel, or manganese. In particular, cobalt, nickel, and manganese are very expensive valuable metals. Among these, cobalt is a strategic metal, and each country in the world has a special interest in supply and demand of cobalt. Since the number of cobalt 10 producing countries is limited, it is known as a metal whose supply and demand is unstable worldwide. If an imbalance in the supply and demand of raw materials of strategic metals occurs, raw material prices are highly likely to rise. Conventionally, research on recovering and recycling these valuable metals from lithium secondary batteries (waste batteries) which are discarded when their lifespan is 15 completed after use has been mainly conducted. In addition to waste batteries, it is more preferable if resources may be recovered from wastes discarded after the positive electrode plate is punched or from the positive electrode in which a defect occurs during the process. Currently, when manufacturing a lithium secondary battery, as shown in FIG. 1, a positive electrode sheet 30 is manufactured by forming a positive electrode active material 20 layer 20 in which a long sheet type positive electrode current collector 10 such as aluminum (Al) foil is coated with a positive electrode slurry in which a positive electrode active material, a conductive material, a binder, a solvent, etc. are mixed, and then a positive electrode plate 40 is punched out with a certain size. A part remaining after punching is 4 discarded as a positive electrode scrap 50. If it is possible to recover the positive electrode active material from the positive electrode scrap 50 and reuse the positive electrode active material, it would be very desirable from an industrial-economic point of view and an environmental point of view. 5 Conventionally, in most cases, a method of recovering the positive active material is performed by dissolving a positive electrode in hydrochloric acid, sulfuric acid, nitric acid, etc., and then extracting an active material element such as cobalt, nickel, manganese, etc., and the extracted active material element is reused as a raw material for the synthesis of the positive active material. However, the method of extracting the active material element using 10 an acid has disadvantages in that a process of recovering pure raw materials is not environmentally friendly as well as requires a neutralization process and a wastewater treatment process, which increases the process cost. In addition, the method has a disadvantage in that lithium, which is one of the main elements of the positive electrode active material, may not be recovered. In order to solve these disadvantages, a method of 15 directly reusing the active material without dissolving the positive electrode active material and extracting the active material in element form is required. DISCLOSURE Technical Problem 20 The present disclosure is directed to providing an active material recovery apparatus capable of easily recovering an electrode active material from an electrode scrap in its intrinsic shape. The present disclosure is also directed to providing a positive electrode active 5 material reuse method using the active material recovery apparatus. Technical Solution In one aspect of the present disclosure, there is provided an active material recovery 5 apparatus which is a rotary firing apparatus comprising a rod of a screw type therein includes a heat treatment bath and a screening wall arranged in a line along an axis of the rod, wherein the heat treatment bath constitutes a heating zone, and the screening wall constitutes a cooling zone; and an exhaust injection and degassing system, wherein the heat treatment bath removes a binder and a conductive material in an active material layer by performing 10 heat treatment in air on an electrode scrap comprising the active material layer on a current collector while rotating the electrode scrap around the axis of the rod, and separates the current collector from the active material layer, and an active material in the active material layer passes through the screening wall and is recovered as an active material in powder form, and the current collector that does not pass through the screening wall is recovered 15 separately. The heat treatment bath may also rotate around the axis of the rod. An angle of the entire active material recovery apparatus may be adjusted so that the axis is inclined with respect to a ground. The active material recovery apparatus may have a vibration function. 20 Input of a new electrode scrap and recovery of the active material may be continuously performed. Preferably, the heat treatment bath has a tubular shape with both ends open so that the electrode scrap is put therein and the separated current collector and active material are 6 transferred to the screening wall, and a tube is an open type system through which air enters and exits. Preferably, the screening wall has a tubular shape with both ends open so that the separated current collector and active material are put therein and the current collector is 5 discharged. The heat treatment bath is preferably an open type system in which air of 10 mL/min to 100 L/min is added or injected per 100 g of the electrode scrap that is put in. Air inlets may be preferably formed in a plurality of places in the heat treatment bath. 10 In one aspect of the present disclosure, there is provided a positive electrode active material reuse method including preparing an active material recovery apparatus according to the present disclosure; putting a positive electrode scrap in a heat treatment bath comprising a lithium composite transition metal oxide positive electrode active material layer on a current collector; removing a binder and a conductive material in the active 15 material layer by performing heat treatment in air on the positive electrode scrap while rotating the positive electrode scrap around an axis of a rod in the heat treatment bath and separating the current collector from the active material layer; recovering an active material in powder form that has passed through a screening wall; and annealing the active material in the air at 400 to 1000 °C to obtain a reusable active material. 20 At this time, the heat treatment may be performed at 300 to 650 ℃. Heat treatment may be performed at 550 °C as a temperature increase rate of 5 °C/min for 30 minutes. A carbon component generated by carbonization of the binder or the conductive material may not remain on a surface of the recovered active material. 7 The positive electrode active material reuse method may further include, before the annealing, cleaning the recovered active material with a lithium compound solution showing basicity in an aqueous solution state. In that case, before the annealing, a lithium precursor is preferably added to the cleaned active material. The lithium compound aqueous solution 5 may be prepared to contain a lithium compound more than 0% and equal to or less than 15%, and preferably uses LiOH. The cleaning may be performed within one hour. The cleaning may be performed by impregnating the recovered active material in the lithium compound aqueous solution and at the same time stirring the recovered active material. For another example, the positive electrode active material reuse method may 10 further include, after the cleaning, obtaining the active material to which a lithium precursor is added and of which particles are adjusted, by mixing the cleaned active material with a lithium precursor solution and spray drying the active material. The positive electrode active material reuse method may further include performing surface coating on the annealed active material. 15 The lithium precursor used in annealing may include at least one of LiOH, Li2CO3, LiNO3 and Li2O. The lithium precursor is added by an amount that may be added as much as a ratio of lithium lost compared to a ratio of lithium to other metals in a raw material active material used in the active material layer. For example, the lithium precursor may be added by an 20 amount of lithium added at a molar ratio of 0.001 to 0.4. Furthermore, the lithium precursor may be added by an amount of lithium that may be further added at a molar ratio of 0.0001 to 0.1 with respect to 1:1 that is a molar ratio of lithium to other metals. A temperature of the annealing may exceed a melting point of the lithium precursor. 8 The performing of the surface coating may include coating at least one of a metal, an organic metal and a carbon component on a surface in a solid or liquid method and then performing heat treatment at 100 to 1200 ℃. The reusable active material is represented by Chemical Formula 1 below, 5 LiaNixMnyCozMwO2+δ (in Chemical Formula 1 above, M includes at least one selected from the group consisting of B, W, Al, Ti and Mg, 1