ENVIRONMENTAL ENGINEERING – TROUGH A YOUNG EYE Vol.2, 2013

M.Sc., Ewelina Piątkowska, M.Sc., Aleksandra Pala

Silesian University of Technology,

Department of Technologies and Installations for Waste Management St. Konarskiego 18, 44-100 Gliwice

,

Effect of phosphogypsum on selected properties of municipal and industrial waste composite

Key words: phosphogypsum, waste management, municipal waste, industrial waste, waste mix, waste composite

Abstract: Phosphogypsum (waste code 06 09 80) is an inorganic industrial by-product, formed during theproduction of extractive phosphoric acid. For each Mg of phosphoric acid produced falls 4-5 Mg waste phosphogypsum, which is mostly directed to landfill. Properties of phosphogypsum, which characterise low pH and small moisture content constitutes the key of the research. In view of this aspect the investigation of possibilities of use phosphogypsum as an ingredient of waste composition has been undertaken. Article aims to determine the impact of the selected add-on (phosphogypsum) on the parameters of composite of municipal and industrial waste, which will have its application in rehabilitation of degraded areas; and to help the roughly further research on innovative methods of management of phosphogypsum.

  1. Introduction

Phosphogypsum (calcium sulfate) is a solid waste generated from natural phosphate rock in phosphoric acid production (H3PO4), an essential component of many modern fertilizers. Mainly composed of gypsum (CaSO4·2H2O) but also includes some impurities of environmental concern, such as P2O5, fluorides and metal elements (e.g., iron, manganese, aluminium, magnesium, etc.). Currently, a large number of phosphogypsum (PG) are discarded without any treatment, leading to appreciable land occupation and serious environmental contamination [1-3]. For every tone of phosphoric acid made, about five tones of phosphogypsum are manufactured [4]. It is evaluated that more than 100-280 million tones are generated annually worldwide [5]. There are four methods are being known to dispose of this by-product: discharging to water bodies; backfilling inmine pits; dry stacking; and wet stacking [2]. Currently, only 15% of world PG production is recycled as building materials, agricultural fertilizers or soil stabilization allowances and as set controller in the manufacture of Portland cement. The remained 85% is stored into stock-piles near the chemical plant, occupying significant land areas and causing large environmental looses [3]. Therefore, recycling and minimizing the negative impacts of this waste increasingly arouses the interest of researchers all around the world.

1.1. PG parameters

Due to the residual phosphoric, sulphuric and hydrofluoric acids contained within the porous PG, it is considered as an acidic by-product with power hydrogen (pH) underneath three [2, 6]. The properties of phosphogypsum are determined by: the nature of the phosphate ore used, the type of wet process employed, the plant operation efficiency, the disposal method, and the age, location and depth of the landfil or stack where the PG is dumped [7]. Usually PG, which originates from filter cake, has a free moisture content of 25–30%. PG is a powdery material that has little or no plasticity. The free water content may vary very much, depending on how long the PG has been allowed to drain after stacking and on local weather conditions. PG solubility depend on its pH, and it is highly soluble in saltwater (~4.1 g/dm3)

ENVIRONMENTAL ENGINEERING – TROUGH A YOUNG EYE Vol.2, 2013

[8]. Its particle density ranges between 2.27 and 2.40 g/cm3 and its bulk density between 0.9 and 1.7 g/cm3. Morphological investigation showed, that PG has a prevailing particle size ranging between 0.250 and 0.045 mm in diameter. Size of pieces depend on the source of the phosphate rock and the reactor conditions. PG has crystal structure, predominantly of rhombic and hexagonal forms [7].

1.2. PG management

In view of the characteristics of PG waste, which includes more than 95% of CaSO4·2H2O, of its attractive economic potential, and of continuously increasing concerns about environmental pollution as well, at the present time there is a great interest in utilizing PG as an alternative raw material for many applications [7]. Civil engineering, agriculture, microbiology, waste management constitutes examples of the most popular fields in which PG is used as principal component.

Giving an example for first application, total conversion of phosphogypsum industrial waste into portlandite (calcium hydroxide) and sodium sulphate has been investigated [9]. Portlandite was produced by dissolution of dihydrated calcium sulphate from the phosphogypsum and reaction with sodium hydroxide. Phosphogypsum waste serving as a calcium source might be a solution for reducing old stockpiles of phosphogypsum wastes, but also resulting in more sustainable production for future phosphoric acid and other gypsum-producing industrial processes. Second example may be presented by practical utilization of compositions based on waste phosphogypsum in the construction of a parking lot [10]. Another interesting instance concerns efforts, which have been made to make high strength alpha plaster from phosphogypsum. It was found that with small quantity of chemical admixture (sodium succinate/potassium citrate/sodium sulphate), alpha plaster of high strength can be produced [11]. Utilization of PG as raw and calcined material in manufacturing of other building products, to prepare non-fired bricks, mixing PG with fly ash for building roads etc. was also investigated [4, 12, 13].

Reclamation of saline and sodic soil by using divided doses of phosphogypsum has been tested in cultivated condition. Physically and chemically degenerated soils were reclaimed by addition of repetitive added application of a divided quantity of PG using means of leaching water in cultivated plots [14].

Addition of phosphogypsum was used in the patented technology of sludge from municipal wastewater treatment plants. Adding minced phosphogypsum with suitable particle size improved physico-chemical treatment of functional properties of the product by supplying phosphorus as phosphates. Uniform dosage of the soil layer and the same enrichment in the whole layer was obtained [15].

Interesting example of applications of PG may refer to microbiology, where a mixed culture of sulfate-reducing bacteria was used to removing sulfate from PG [16, 17]. Treatment tests of PG have been made by leaching of radionuclides by using TBP–TOPO in kerosene, biotransformation of PG has been applied in wastewaters from dairy industry [18, 19]. All examples confirm increasing interest around PG.

The original investigation of possibilities of use phosphogypsum as an ingredient of waste composite has been undertaken in present article. The impact of selected add-on (phosphogypsum) on the parameters of composite of municipal and industrial waste is the aim

ENVIRONMENTAL ENGINEERING – TROUGH A YOUNG EYE Vol.2, 2013

of the research. Successful series of experiments will find its application in reclamation of degraded land areas.

  1. Material and methods

All determinations were performed at the Department of Technologies and Installations for Waste Management in Silesian University of Technology. The study was performed on components and mixtures, which are detailed later in this article, in accordance with applicable standards and acts.

Critical parameter of waste composite is reaction level, which have to perform the land reclamation requirements in range 6.5 - 9.0 pH [20]. Moisture content (MC) of the composite constitutes second meaningful parameter, which is demand to be less than 20%.

Principle of test method for the determination of pH was to prepare a suspension of the sample in an amount of five times the volume with one of the reagents such as: water, potassium chloride at a concentration of 1 mol/l KCl, or calcium chloride at a concentration of 0.01 mol/dm3 CaCl2; according to standard PN-ISO 10390:1997. The determination was provided with water and pH was measured using a pH meter.

The method for determining moisture content, by weight of the total weight determination, was based on a percentage weight loss of solid dried at 105 °C. The result of the determination should be an arithmetic mean of two parallel determinations, which do not differ from each other by more than 0.5; according to standard PN-Z-15008-02:1993.

Materials used for blending waste composite are schematically presented in Graph 1, which for simplification have been called "ash", "brick", "soil", "sludge" and they are representing in sequence: a mixture of fly ash and solid waste from the calcium-based flue gas desulphurization (10 01 82); mix concrete and concrete rubble (17 01 01) and crashed bricks (17 01 02); soil and the stones other than those mentioned in 17 05 03 (17 05 04); stabilized municipal sewage sludge (19 08 05). Every kind of waste has assigned waste code, according to European Waste Catalogue and Hazardous Waste List [21].

Graph 1. Materials used for waste composite and the variability of samples parameters

• pH 11-13• pH 8-9

• MC 1.5-2.5%• MC 3.5-9%

ASHBRICK

SOILSLUDGE

• pH 6-9• pH 7-9

• MC 40-95%• MC 10-14%

Source: Own work

Final parameters of mixture depend upon compounds parameters, which are different for different waste samples (Graph 1). Parameters of every single sample of each component

ENVIRONMENTAL ENGINEERING – TROUGH A YOUNG EYE Vol.2, 2013

have been experimentally tested. Indications for our samples were varied: for ash pH between 11-13, MC between 1.5-2.5%; for brick pH between 8-9, MC between 3.5-9.0%; for soil pH in range 6-9, MC in range 40-95%; and for sludge pH 7-9, MC 10-14% depending on the source of fresh samples. Variability of component's parameters affects directly the parameters of final mixture. Composite of introduced at Graph 1 wastes was made repeatedly in different compounds mass fraction and their key-parameters of mixture oscillated about pH 12.0 and about 16-36% MC. Even when low MC has been achieved, the pH was still strongly alkaline, exceeding the limits of standards for land reclamation. Sludge and ash are a decisive components, which influences the mixture pH and it looseness in view of it moisture content.

In the present study, the ingredients used had the following parameters (Table 1):

Table 1. Parameters of composite compounds

Waste / pH / Moisture content %
Ash / 12.76 / 1.48
Brick / 8.71 / 8.88
Soil / 8.71 / 14.17
Sludge / 7.13 / 84.98

Source: Own work

Sludge had almost neutral reaction, brick and soil slightly alkaline and reaction of ash was highly acid. Inversely to MC, which for ash was very low (1.48%) and for sludge very high (84.98%). Waste compounds had been mixed in optimal proportion and gives composite with very alkaline reaction (12.47 pH) and powdery consistence (19.71%), which not gives satisfying results. To reduce pH level an additive of phosphogypsum (waste code 06 09 80) was applied, which characterize acid reaction and not high humidity (Table 2).

Table 2. Parameters of phosphogypsum additive

PG / pH / Moisture content %
PG Egypt / 2.89 / 39.42

Source: Own work

Phosphogypsum sample resulting from the production of phosphoric acid from raw material of sedimentary phosphate rock originates from Egypt [22]. In Egypt, hard works were done on the characterization and beneficiation of PG. The complete chemical analysis of PG confirmed that the waste was free from the radioactive elements and heavy metals such as cadmium, which being the most polluting element could hinder the utilization of the waste [23].

The pH of PG water extract amount to 2.89 and with 39.42% of moisture content (Table 2). In order to bring pH of the mixture closer to neutral, next proportions in 1 kg of prepared composite with phosphogypsum add-on (CPG) were established (Graph2).

ENVIRONMENTAL ENGINEERING – TROUGH A YOUNG EYE Vol.2, 2013

Graph 2. Mass fraction of composite components expressed as a percentage

Ash

7%

Brick PG 11% 32%

Soil

32%

Sludge 18%

Source: Own work

Due to the highly alkaline ash, its share was estimated as 7% and sludge due to its high humidity as 18%. Brick counted 11% of share. Participation of first three ingredients has been dominated with PG, which counted 32% of composite mass fraction. The same amount of 32% was predicted for soil to simulate the natural ground in the best way. It has been successfully estimated, that the PG add-on will effectively decrease pH of final waste composite.

  1. Results and discussions

The new composition of waste mixture with PG add-on reached parameters giving reaction level average equal 7.84 and 33.52% of moisture content (Table 3). Achieved reaction level of CPG was above 30% lower than mixtures in previous laboratory experiments, which did not include PG.

Table 3. Parameters of final waste composite

Composite symbol / pH / MC %
CPG / 7.84 / 33.52

Source: Own work

Moisture content of CPG with effect of 33.52% is not satisfying, because the final consistent of composite is not enough powdery and a little bit sticky, what is preclude usage an installations for spreading composite at degenerate lands. The most wet component in composite was sludge, with 84.98% (Table 1), which had an key-influence on the final MC. Sludge quality is the most diversified material in all mixture. It is highly probable that a mixture of the same proportions with 70% wet sewage sludge could significantly improve the flow properties of the final composite. The chemical constitution of sewage sludge is very variable and depends on many factors including the type of treated water used in the cleaning process. The share of industrial wastewater in municipal wastewater treatment plants and the nature of these can have a decisive impact on the quality of the resulting sludge.

ENVIRONMENTAL ENGINEERING – TROUGH A YOUNG EYE Vol.2, 2013

It is difficult to obtain similar results regularly with such diversified parameters of input waste samples. Laboratory work and chemical determinations are demanded on every single sample, what can be costly and time-consuming.

This research is original and none scientific works were found, which could confront achieved results. Those studies, which were provided on phosphogypsum utilities in agricultural use, excluded its toxicity for plants (in particular dosages), and considered it as ecologically useful [3, 4, 14, 24-26]. Concentrations of undesirable compounds of ready waste composite still cannot be ignored such as heavy metals, radionuclides or salinity and they must to be determined in accordance to Regulation of the Polish Minister of the Environment on the recovery process of R10 (distribution of waste on land for use as fertilizer or soil improved) and Regulation of the Minister of the Environment on soil quality standards and quality standards land [27, 28].

  1. Conclusions
  1. Effect of phosphogypsum on selected properties (pH, moisture content) of waste composite, announcing new method for disposal of municipal and industrial waste, has been state as positive, giving considerable reduction of composite's pH level and closing it to the neutral reaction.
  1. The entire composition consists of the waste, which after meeting further investigations might become a brand new product.
  1. Obtained material have a neutral pH, is rich in bio-available phosphorus for plants and it is predicted to be used for degenerated land reclamation.
  1. Attained results allow continuing research on waste composite and assign further directions of experiments.
  1. In aspect of parameters diversity of composite's components, it is recommended to develop a statistical optimizing method facilitating the selection of ingredient's mass fraction.
  1. Except pH and moisture content parameters, which have to be fulfill before the composite will be utilized in the degraded lands, determination of toxicity using garden cress seeds, determination of heavy metals, salt content, rate of phosphorus leaching, study of pH changes as a function of time need to be proceed.

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