Nowadays, industrial applications for recycling of hard metals focus on the zinc process and on feeding scrap into the primary, hydrometallurgical route. The latter is only suitable for pure hard scrap of sufficiently small particle size due to the required process time. The zinc process works based on the fact that zinc forms voluminous intermetallic phases with cobalt metal that acts as a binder in most common hard metals. In summary, it effectuates only disintegration without separation of impurities. Also, the matrix used in drill bits for oil and gas exploration consists of a complex alloy made of copper, manganese, nickel, and zinc.
For these reasons, the industrial implementation of recycling of the PDC drill bits as a whole (see Figure 1) has proven to be complicated. Thus, about 1,000 tons of tungsten per year are lost for the value-added chain. Compared to the annual worldwide primary production of 80,000 tons this poses a significant amount of unutilized high-grade resource with potential economic value.
Therefore, it is the goal of this project to devise an efficient method to recover tungsten carbide as well as the constituents of the complex binder alloy as re-usable products.
Of the possible techniques for recycling of bulky hard metal scrap, selective chemical digestion of the binder alloy is examined first. In screening tests, different digestion solutions in combination with oxidizing or complexing agents are evaluated for their efficiency to dissolve the binder while leaving the tungsten carbide intact. In order to bring additional value to the process, a technique for recovering the high contents of copper, manganese, nickel, and zinc from the solution will be developed (see Figure 2).
With the goal of understanding the digestion process in greater depth, tests regarding time dependence of leaching of the complex binder phase alone and of the infiltrated cast tungsten carbide are scheduled. The leaching rate and the influence regarding formation of a product layer thereon will be determined. The focus lies on comprehending the mutual interference of binder metals and tungsten carbide.
In further optimization analyses, the operation conditions are then adjusted in a way that they result in the most effective selective digestion. The final goal is an industrially applicable method enabling a high recovery of tungsten carbide free of contaminations and re-usable binder metals.
A test plan covering all significant parameters for the optimization tests is targeted with the help of design of experiments and the software MODDE 11™. Moreover, thermodynamic simulations will be carried out with FactSage 7.0™ and HSC 8™.
Other approaches for recycling of drill bits that will be examined in the scope of this project are complete chemical digestion of binder alloy and tungsten carbide as well as selective electrochemical leaching.