The industry dedicated to surface treatment and, more specifically, the industry specializing in galvanization, produces a significant amount of polluting waste while requiring a high consumption of chemical reagents from the direct exploitation of natural resources. This is not sustainable and will be expensive to maintain in the medium term, in addition to being financially uncompetitive.
Consequently, this industry must make a firm commitment to implement policies based on minimizing the production of waste while, at the same time, reducing the consumption of natural resources as raw material. Bearing this in mind, the implementation of the precepts of the circular economy is proposed as a successful strategy for this type of industry: transforming waste into resources.
This is where the Chemirec® process provides innovative technology to transform the waste produced into raw materials for the process itself, as well as significantly reducing the consumption of natural resources and CO2 emissions.
The Chemirec® process has been conceived as a highly innovative solution for the recovery of hydrochloric acid (HCl) from the effluents of exhausted metal pickling baths. The aim of the pickling process is to eliminate metal oxides, manufacturing scale, annealing oxide and the rust from the metal part surface so that the parts are highly clean. Most pickling tanks initially contain hydrochloric acid, diluted to 14-16% by weight. As the bath is used, however, the concentration of hydrochloric acid decreases, requiring periodic additions of acid so the pickling speed does not significantly decrease. The system is maintained until the solubility limit of the ferric chloride (FeCl2) in the hydrochloric acid itself is reached. At this point, the bath is exhausted and the pickling cannot continue. The spent bath contains an iron concentration of least 140-150 g/L and needs to be renewed by a fresh bath. For industries which also use hot galvanizing processes, as well as containing iron, the exhausted bath also contains high levels of zinc (around 25 g/L).
Most of the hydrochloric acid recovered from the spent baths via the Chemirec® Process is re-used in the same surface treatment process; significantly reducing the purchase of this raw material. In addition, ferrous sulfate is produced in the process, which is a by-product of commercial value (as a fertilizer in horticulture and winemaking) as well as zinc chloride, which is used in the galvanizing process as one of the basic components of the fluxant (a mixture of ammonium chloride and zinc chloride).
The Chemirec® process does consume concentrated sulfuric acid; however, the cost of this acid is amply compensated for by the sale of the new resources produced, as well as the savings in using fewer raw materials (hydrochloric acid and zinc chloride) and producing less waste. Thus, the project provides very positive results, proving a return on the investment made in less than 2 years.
Condorchem Envitech has developed and patented the Chemirec® process to achieve all the described environmental and financial benefits, and make this technology available to industries dedicated to surface treatment.
At the operation level, the Chemirec® process consists of three major stages: pre-treatment, reaction and separation.
The pre-treatment stage is only necessary when the baths are exhausted, generally from industries with hot galvanizing processes, containing high levels of zinc. The objective of the pre-treatment stage is to remove all the zinc (95-98%) from the solution. If not all the zinc is removed at the beginning, it will be precipitated later with the iron, producing a product with a complex commercial outlet.
It has been shown that the most efficient technology to remove the zinc from the bath is liquid-liquid extraction using an organic solution as the extracting agent. This zinc extracting agent, in turn, is regenerated by another liquid-liquid extraction process, so it is not consumed in the Chemirec® process. Following this pre-treatment stage, two streams are obtained: the spent pickling bath, now zinc-free, and an aqueous solution of zinc chloride. This solution is returned to the galvanizing process where it is consumed in preparing the fluxant stream (a reagent of ammonium chloride and zinc chloride).
Once the zinc has been removed from the spent bath, or if the bath did not contain any zinc initially, the reaction stage can then be conducted. To recover both the free hydrochloric acid and that combined in the exhausted bath, concentrated sulfuric acid is dosed in a crystallizer reactor. This reacts with the ferric iron ions present in the solution, forming ferrous sulfate heptahydrate and hydrochloric acid. The solution is then cooled, forming ferrous sulfate heptahydrate crystals, which are easily removed by filtration from the mother liquor. This contains the ferric iron that has not precipitated, some excess sulfuric acid and the hydrochloric acid which was formed.
Finally, to separate the hydrochloric acid that has formed, the solution is subjected to a membrane distillation process that reduces the practical requirements to operate with a mixture of hydrochloric acid at high temperature. The membrane distillation stage produces the recovered hydrochloric acid, as well as an aqueous concentrate which is recycled to the reactor to return the ferric iron and unreacted sulfuric acid to the process.
The hydrochloric acid obtained – with a concentration of 18-20%, due to the azeotrope that is formed – is consumed in the surface treatment process, which means significantly less reagent is needed to be purchased, leading to savings. Thus, the Chemirec® process recovers and uses hydrochloric acid for the surface treatment industry. Also, for the hot galvanizing industry, the zinc chloride produced in the zinc separation can be reused for the production of fluxant. Finally, the ferrous sulfate heptahydrate produced has commercial value and can be sold as a fertilizer. The only requirement for the raw materials from the process is the addition of sulfuric acid, whose cost is much lower than the profits generated by the sale of the products formed and the savings from recovered resources and reducing waste management.
Thus, in summary: an exhausted bath enters the Chemirec® process and is transformed into hydrochloric acid suitable for re-use, ferrous sulfate heptahydrate crystals which can be sold and zinc chloride which can be re-used.
As a preliminary phase, Condorchem Envitech has built a Chemirec® process pilot plant with a treatment capacity of 200 L/day of exhausted pickling baths from galvanizing industries. This pilot plant has successfully demonstrated the technical feasibility of the process and the robustness of the operation. It has also been found that the hydrochloric acid recovery obtained experimentally is excellent, at over 95% of the initial hydrochloric acid amount (free plus combined).
At a later stage, Condorchem Envitech built an industrial plant, with a treatment capacity of 2 m3/day of exhausted pickling baths from galvanizing industries; thus demonstrating that the process successfully tested on a pilot scale also works successfully on an industrial scale; even surpassing the levels of efficiency achieved on a pilot scale, due to greater automation of the process: around 98% recovery of initial hydrochloric acid (free plus combined).
In addition to being a clearly viable process financially, with the investment involved in its implementation being recovered in a very short time, the environmental aspect involved in operating the Chemirec® process implies is of great relevance. On the one hand, a large amount of waste stops being produced, and the purchase of raw materials is significantly reduced; thus, leading to a large reduction in CO2 emissions from reducing the amount of goods being transported. Secondly, 43% of the world’s zinc is used in the galvanizing industry, so the recovery and subsequent re-use of the zinc from pickling baths in the galvanization process both saves on the purchase of raw material and reduces the extraction of natural resources.
Although the Chemirec® process has been designed for exhausted pickling baths, it is also useful for all effluents with an appreciable amount of hydrochloric acid, both free and combined. Unlike other alternatives, the Chemirec® process is technically feasible, even when the bath contains large amounts of zinc.
The Chremirec® technology is oriented to being used in effluent treatment in the following processes: