The development of products from renewable raw materials is a promising alternative to those products obtained from the traditional petrochemical route. The acacia mearnsii, popularly known as black wattle, is a leguminous tree from Australia and cultivated in Rio Grande do Sul, Brazil, with a short growth cycle of seven years, intended to be an important recuperative of poor soils depleted by intensive agriculture.

An extract rich in tannins is obtained from the black wattle bark and employed for various applications such as leather tanning and retanning, coagulant for water treatment, adhesives for wood panels and others. The aim here is to present aspects of sustainability along two dimensions: the production chain, from the seed of black wattle until obtaining the vegetable extract, and the final products.

The genetic improvement of the species; forest management and the production chain that follows the Forest Stewardship Council (FSC) certification; the industrial process with the use of water in closed circuit; energy generation from biomass burning; and the positive carbon footprint are evidence of the sustainability of this industry. The products, free from restricted substances, with lower periods of productive cycle than other species and able to replace fully or partially synthetic products, exemplify the practice of sustainability management.

The acacia mearnsii tree offers an important economic advantage for pulp production because it has higher wood density and pulp yield, which translates into higher digesting productivity and factory output, and is highly suitable for hardwood bleached craft pulping, dissolving and semi-chemical pulps. These factors make acacia mearnsii a sought-after species and have ensured a sustained demand for wattle woodchips in the Japanese market and, today, continues to attract new customers in emerging markets such as China and India. At present, the only significant secondary products are charcoal and firewood, and this tree is the only temperate acacia species grown commercially on a significant international scale.

The root of the issue

The acacia mearnsii tree was introduced in other regions of the world due to its rapid adaptation to different environmental conditions and high productivity. The species is mainly planted in South Africa and Brazil for tannin production and woodchip exports, and, in both countries, it is mainly cultivated by farmers (76–78% of the total area). Due to the high prices fetched by the wattle woodchips in the international market, acacia mearnsii is a profitable crop. In Brazil, the first plantation was carried out in 1918, and in 1928 for commercial purposes. Currently, Brazil is the world’s largest producer of this tree and it is the third-most planted tree species in Brazil, surpassed only by eucalyptus and pine.

The bark is well known for its high concentration of ‘condensed’ tannins or ‘catechol’ tannins, with flavonoid ring structures; the tree is also very resistant to pests and doesn’t need to be protected by pesticides or agrochemical inputs. As a result, products developed with these tannin extracts aren’t pollutants to the environment and they do not contain hazardous substances.

The vegetable retanning agent mimosa inhibits the formation of free radicals and the oxidation of unsaturated fatliquoring agents, as well as detaning the leather and avoiding the formation of hexavalent chromium, which is ranked 17th on the ‘Priority List of Hazardous Substances’ based on a combination of the frequency, toxicity and potential for human exposure made by Agency for Toxic Substances and Disease Registry (ATSDR) and the Environmental Protection Agency (EPA).

One of the oldest and best-known industrial applications of acacia mearnsii tannin is the transformation of hides into leather. The most widely used reactions to obtain lighter tannin is the sulphitation usually with sodium sulphite or sodium metabisulphite in acid medium. The position of sulphite can result in lower-proanthocyanidins structures that normally penetrate faster into the skin. This reaction is mainly used for obtaining cationic flocculants for water and wastewater treatment, where the group amine remains protonated in acid medium. Instead of metal salts, these flocculants are biodegradable and do not add metal to the generated residue.

Acacia nation

Tanac has its own reforestation system of acacia trees and is the largest individual planter of mimosa in the world with an agro-industrial structure of about 800 employees involved in forest management activities. It has planted up to seven million trees each year, which stay growing during a cycle of seven years, covering almost 25,000h. In addition, there are more than 12,600h of preserved native forests adjacent to its reforestation areas and 9,000h of fields. These structures are capable of providing the major part of the raw material (wattle extract) and ensure a very stable and constant quality to products. The other part is supplied by working together with independent farmers as the company supplies seeds and technical advice to them, and they provide the trees to the company to make the production volume required.

The ongoing work of advanced technologies in the production of seedlings and breeding research generates a significant increase in industrial productivity, a formation of quality forests and an appropriate environmental management. In the forestry unit planting areas, there are a consortium of acacia mearnsii trees with livestock, taking advantage of the availability of areas and native pastures; about 4,500 cattle coexist with acacia forests. The monitoring of fauna and flora is conducted permanently by professionals in order to ensure the maintenance and reproduction of the various species existing in the areas of the company.

Tanac only purchases certified or controlled forest raw material in accordance with the standards of the ‘Forest Stewardship Council: Preserve indigenous and civil rights; not to be originated from forests of high conservation value; not to be from genetically modified material; not to be illegally harvested; and not to be from converted native forests’.

Contrary to other species used for tannin extraction (quebracho and chestnut), which are not reforested trees, the mimosa is totally derived from renewable sources, thus assuring its continuity indefinitely, and is ready to be industrialised after only seven years. The reforestation with acacia is intended to preserve the environment and occurs in grassland areas where native forests don’t occur, thus with no cutting of virgin forest and no native forest replacement by mimosa. The mimosa, besides providing the bark for tannin extraction, is today the biggest source of vegetable fuel preserving a significant portion of native forests, which otherwise would be used for that purpose. The acacia plantation is also used to save debilitated soils by fixing nitrogen.

The magnificent seven

The production process begins where the trees are cultivated for seven years and then harvested by mechanical or manual processes. The bark is then classified according to quality standards and sent to the grinding central to decrease grain size. After that, the ground bark follows through belts to the autoclaves for aqueous extraction of tannins, without using any organic solvents. The exhausted bark that passed through the autoclaving process is sent to the pressing sector and the resulting liquid returns to be used again to the next extraction. The exhausted pressed bark and the second-quality woods are used as biofuel. The excess bark is destined to be organic compost. So the entire black wattle is used, avoiding the waste generation.

The extracted liquid is submitted to an evaporation process in order to be concentrated and then chemically modified (or not) according to the final product specification. The product can be traded in liquid form, atomised by spray drier (powder) or concentrated in the low-pressure evaporator (solid block), and the water separated in the evaporation steps are also reused in the next extractions. Finally, the products are properly packaged and stored until the final distribution.

In the past ten years, a wastewater treatment system has been implemented, promoting 90% of water recyclability. The only water loss from the factory is due to the human consumption, evaporation in flue gas scrubbers and water content present in the final product. Therefore, the use of water is considered to be in a closed circuit, because there is no effluent disposal into the sewer system. Currently, the total water consumption is 100m3 an hour, in which 96m3 an hour is destined to the industrial activities and 4m3 an hour for human consumption.

There’s also a water treatment station capable of clarifying water to the whole process, and treatment for human consumption according to the ‘Ordinance 2914 from Ministry of Health’ that sets the procedures of control and surveillance of water quality for human consumption and its potability standards. All the harvesting water is treated with flocculants produced from the acacia mearnsii extract and chemicals are used to treat water for human consumption. In addition, these products are in accordance to National Sanitation Foundation (NSF), halal and kosher certification. The water intended for the industrial processes is used in the cooling factory process and in the tannin extractions. All water removed during the pressing of the exhausted bark, evaporation and atomisation steps are reused in the subsequent extractions. The cooling water, after used, is directed to the cooling towers for treatment and subsequent reuse in a closed circuit.

All the water resulting from the cleaning of the factory is then directed to the effluent treatment station, where it is treated and reused in the system of flue gas scrubbers. The release of carbon blacks and ash occurs in the burning of the exhausted bark and wood, which are collected in the flue by the action of the gas scrubbers and directed to the wastewater treatment station. The ash and carbon blacks waste are separated from the water and directed to composting with the remaining 40% of the exhausted bark.

These results show that the production of tannin extract from the bark is an environmental friendly process due to the energy generation from a renewable source, water treatment and reuse in a close cycle, low-waste generation (solid and liquid).

Acacia mearnsii extracts are the only tanning agent used in the leather industry from organised and sustainable reforestation, and companies like Tanac are continuing to search for products that are more sustainable for the leather industry. 

Company insight: Tanac

Tanac, based in southern Brazil, dedicates its activities to producing vegetable tannin extracts (modified or natural ones) and woodchips obtained from the acacia mearnsii tree. The company exports to more than 70 countries and is the worldwide market leader with a total production capacity of 36,000t of tannin extract and 700,000t of woodchip a year. The company has held the ISO 9001 quality system certification since 1996 and the ISO 14001 Environmental Management System certification since 2000. Also, its forest management is certified by the FSC, which has guaranteed the quality of the raw material since its origins. The production process of tannin extracts and derivatives is based on principles aimed at environment preservation.

Carbon footprint analysis

Since 2008, Tanac has conducted inventories of greenhouse gas emissions from its operations in Rio Grande do Sul, covering the forestry activities, industrial, transportation and logistics (wood and bark transport), administrative processes and disposal of solid waste. The inventory also serves as a management tool in order to ensure an economic, energy and operating efficiency to the company, considering all operations in three scopes:


  • Direct emissions of greenhouse gases from sources that are owned or controlled by the company as emissions from industrial processes, vehicles and production of chemical in equipment. In this scope, direct CO2 emissions from the combustion of biomass are not included.
  • Indirect emissions of greenhouse gases from electricity generation acquired and consumed by the company. In this scope, emissions occur physically at the place where electricity is generated.
  • Emissions of greenhouse gases from sources that do not belong or are not controlled by the company as transport of finished product, business travel, waste treatment and suppliers emissions.