Phosphatic waste clay: Origin, composition, physicochemical properties, challenges, values and possible remedies – A review

Highlights

• Over two billion tons of waste clay has been accumulated in Florida to date.

• This reserve contains about 600 million tons of P and 600 thousand tons of REEs.

• This reserve can satisfy a great portion of U.S. domestic demand for REEs and P.

• Waste clay poses severe environmental problems along with economic loss.

• Waste clay has been considered as an ultimate processing challenge in the industry.

Abstract

The daily growing demand for critical materials has further highlighted the importance of secondary sources such as industrial waste streams. Waste clay, a phosphate ore process tailing, contains a remarkable amount of critical materials such as P and REEs so that comparing to different phosphate ore process streams, waste clay presents the highest concentration of REEs after phosphate rock. Due to the enormous volume of this waste accumulated in Florida to date, this reserve can satisfy a great portion of U.S. domestic demand for REEs, as an example. However, due to its troublesome nature, this reserve poses severe environmental problems along with economic loss. Two required attempts are the removal of extremely fine-sized clays, followed by the recovery of phosphate content, which can pave the path for the recovery of REE-bearing phases. Different possible remedies or combination of them have been considered by various research/ industrial trials, including froth flotation, selective-flocculation, floc-flotation, cycloning, gravity separation, magnetic separation, leaching, etc., most of which have shown no promising solution because of failing to address economic and of course environmental concerns. Moving from mostly chemical separation processes to the primarily physical/ physicochemical processes with low operational costs and environmental impacts could be a general solution. This requires detailed mineralogical and elemental characterization, physicochemical, rheological, electrochemistry, surface chemistry, crystal chemistry, solution chemistry, and quantum chemistry investigations on each single and then mixed-phase systems composing waste clay. Such insights can help develop the fundamental knowledge, upon which more versatile and efficient solutions can be established.

Introduction

As the world population and global economies continue to grow at an increasingly fast pace, and due to advancements in science and technology, the demand for critical materials is rapidly rising. In such a context, critical materials are those with particularly high importance and, at the same time, with a high risk of supply disruptions (Blengini et al., 2019). Such ever-growing demand makes the recovery of these critical materials from wastes through reusing and recycling more attractive. As well, the transition to a circular economy is essential to develop a sustainable, low carbon, resource-efficient, and competitive economy in a country (Blengini et al., 2019, Matinde et al., 2018). The concept of circular economy mandates the reduction, recycling, and re-use of mining and metallurgical wastes (Matinde et al., 2018). A sustainable future for human beings must include the effective recycling of waste streams to meet the demands and avoid adverse environmental consequences. Humanity faces many environmental challenges in the 21st century. In light of this fact, during the World Summit on Sustainable Development in 2002, governments reaffirmed the importance of solid-waste management, calling for priority attention to be given to prevention, minimization, reusing, and recycling of wastes (Lottermoser, 2011). Despite remarkable efforts worldwide to reduce the amount of waste produced, solid mineral wastes from the processing plants remain one of the world’s largest waste streams (Bian et al., 2012). Changing circumstances may turn a particular waste into a valuable commodity, either because of the technology advancement or market demands that fuel the interest in waste reprocessing and the extraction of values from these kinds of sources. Mineral processing plant tailings may be regarded as worthless at the time of production, yet they can contain mineral and energy resources that may become valuable over time. An important example in this context is the phosphate mining and beneficiation industry, which strives day in day out to meet the needs and supplies for the production of adequate food for the ever-growing world population, which according to the Food and Agriculture Organization (FAO), is estimated to reach 9 billion in 2050 (Karunanithi et al., 2015). This means that there is a challenging task ahead for the fertilizer and agriculture industries to ensure food availability for the world population. Such a necessity for the inevitable growth of the phosphate industry has led to the ever-increasing volume of waste streams such as the phosphatic waste clay (also known as waste clay), a phosphate process tailing that contains massive quantities of phosphorous (P) and other critical and valuable elements (Zhang et al., 2017). In Florida, for example, more than one ton of waste clay is produced per each ton of phosphate rock product, which means about 20 million tons of this waste annually, and over two billion tons totally accumulated in Florida to date. Waste clay has been considered as an ultimate processing challenge in the industry, which now is considered as a precious reserve for P and rare earth elements (REEs), and other critical elements. Assuming 9% P₂O₅, 300 ppm REEs, and 40 ppm Uranium (U), this reserve contains approximately 600 million tons of phosphate, 600 thousand tons of REEs including 200 thousand tons of Yttrium (Y) solely, 1.2 million tons of Magnesium (Mg), and 80 million kilograms of U. This clearly shows the importance of this reserve. However, waste clay poses severe challenges to the beneficiation process and the environment due to its extremely fine particle size (below 20 µm) (Zhang et al., 2017).

REEs, as another value contained in the waste streams such as waste clay, are also becoming increasingly important in the transition to a low-carbon circular economy, considering their essential role in permanent magnets, lamp phosphors, rechargeable nickel metal hydride batteries, catalysts, and other green economy applications. The increasing popularity of hybrid and electric cars, wind turbines, and compact fluorescent lamps is causing an increase in the demand and price of REEs (Binnemans et al., 2013). The European Commission considers the REEs as the most critical raw materials group, with the highest supply risk (European Commission, 2018). According to the medium-term criticality matrix of the U.S. Department of Energy (DOE), the five most critical REEs are neodymium (Nd), europium (Eu), terbium (Tb), dysprosium (Dy), and Y (U.S. Department of Energy, 2011). China is presently producing more than 90% of all rare earths, although this country possesses less than 40% of the proven reserves. Due to tremendous and increasing domestic demands, China tightened its REE export quota from 50,145 tons in 2009 to only 30,258 tons in 2010, and so forth (Mancheri et al., 2019). These export quota may cause severe problems for REE users outside China, and therefore, also for the development of a more sustainable, low-carbon circular economy. The uncertainties upon satisfying the growing global demand for these elements highlight the importance of nontraditional sources for these strategically prominent elements (Matinde et al., 2018, Mancheri et al., 2019). Phosphate ore and phosphate processing wastes have been identified as possible alternative sources for REEs. Waste streams such as waste clay may contain low REE concentrations, but are available in huge volumes, as pointed out earlier. This implies that these waste streams could provide significant amounts of rare earths if efficient recycling flow sheets can be developed (He and Kappler, 2017). Therefore, recycling critical elements like P and REEs from secondary sources like waste streams is a possible solution that can alleviate the disparity between supply and demand.

On the other hand, the P and the other certain elements present in these waste streams pose a threat to the environment through nutrient enrichment resulting in various complex ecological problems, including fresh and coastal water eutrophication (Kleinman et al., 2011). To date, extensive research has been conducted to mitigate the environmental impacts and increase the valorization potential of the wastes. The most urgent problem facing scientists working on recycling and reusing of mining and processing wastes is the quantification and distribution of elements in wastes. The chemistry and mineralogy of the wastes, which can be heterogeneous depending on their geographical origin and processing practice, should be precisely described in order to understand their long-term behavior (Bian et al., 2012). We need to drastically improve our scientific effort to explain the occurrence and distribution of elements and minerals in wastes on all scales since such knowledge is imperative to establish the recycling and reusing potential of wastes (Lottermoser, 2011). Because of vastly improved mineral processing technologies, which in turn leads to economic and environmental benefits, the targeted extraction of value from waste streams has become possible. Such careful extraction of targeted elements requires detailed knowledge of the waste’s mineralogical, geochemical, and bulk physical properties (Geise et al., 2011).

In line with these emerging paradigms of environmental responsibility and sustainable development, the objective of this paper is to explore the origin, nature, physicochemical properties, processing challenges, potential values, and possible remedies for one of the current ultimate mineral processing challenges, waste clay. This study also provides a critical review of current and emerging research and industrial practices on the processing and valorization of waste clay in the phosphate industry, along with the reclamation policies and regulations. This study aims to enhance the scientists’ and engineers’ understanding of different properties and aspects of the issue, and therefore better define the possible routes to reach success in the recovery of values from waste clay and mitigating its environmental impacts.