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Vermiculite-lizardite ameliorants from mining waste

Beneficiation products of phlogopite mining waste have been studied in terms of their use in soil engineering to improve soil hydro-physical and physicochemical properties. Vermiculite and lizardite minerals, parts of the obtained products, can interact with acid solutions and can be used to reduce the acidity of agricultural soils as well as to rehabilitate acidified industrial landscapes. The plant testing results indicate a positive effect of vermiculite-lizardite materials on plant growth and development.


Introduction
The negative effects of deposit development include disturbance of natural landscapes resulting from the displacement of large amounts of overburden rocks, alienation of territories for waste storage, and pollution of adjacent water and soil objects due to particles from the disintegrated rock mass.
On the other hand, overburden rocks can be recycled if their composition allows effective use in the national economy. The given paper suggests the use of products of phlogopite mining waste processing in Kovdor in the Murmansk region that contains vermiculite and serpentine minerals as components of soil structures.
Ameliorants improve and condition degraded, including polluted, soils. The products containing vermiculite and serpentine mineral lizardite can be used for land-improvement purposes because of their main mineral components' ability to interact with acidic solutions; to adsorb copper, nickel, and other metals [1,2]; and to increase the amount of magnesium and silicon available for plants [3].

Mineral composition of obtained materials
The initial mining waste was divided into three products: granular, slime, and offgrade (so-called processing "tailings"). Figure 1 shows a general view of the products. The distribution of the main minerals in the processing products is presented in table 1. The mineral content in the products is indicated in mass percent.  The granular product is a material with a particle size up to 2 mm and is mostly white (the color of lizardite). The content of target lizardite and vermiculite minerals is 31% and 33%, with an extraction of 52% and 60%, respectively (table 1). The product contains about 36% impurities, predominantly particles of olivine-pyroxene rocks and fine-crystalline phenites or syenites as well as diopside, quartz, and feldspar particles. The yield of granular product is 34.4%.
The slime product is a fine-grained material with a particle size up to 0.3 mm. The content of lizardite and vermiculite is approximately the same and amounts to about 30% each. In this product, vermiculite does not contain packs and is represented by small thin leaves. The yield of the slime product is 22%, with the recovery of lizardite and vermiculite 32-35%.
The tailings contain a small amount of lizardite and vermiculite (7% and 2%, respectively) and are not of interest in terms of a substrate that can change the soil's chemical composition; however, they can be used to adjust the hydro-physical soil properties. The tailings can be used to increase the proportion of physical sand (particles greater than 0.01 mm in size), which increases the water permeability of the soil and changes its hydrological characteristics [9].
Besides initial granular material, the authors studied a thermally activated (expanded) granular product. The roasting was performed with the aim of obtaining material containing expanded vermiculite, which is widely used in agricultural engineering. Figure 2 presents data on the granulometric composition of vermiculite-lizardite products. The linear nature of cumulative dependence for the granular product (figure 2a), which indicates the uniform distribution of particle sizes, draws the attention. Compared to the granular product, slime product and tailings are characterized by a higher content of fine particles. Figure 2b presents the differential curves for the granulometric composition. The data analysis shows the domination of pulverescent particles with a size of less than 0.05 mm in the slime product and the domination of fine sand (particle size 0.05-0.25 mm) in the tailings.

Physical and chemical properties of obtained materials
The moisture retaining power of the products was evaluated by such a practically important indicator as the field capacity [9]. The field capacity is the moisture of pre-saturated soil that is established after draining excess water. Field capacity of the slime and granular products is almost the same and amounts to 43-45% (table 2). Expanded vermiculite doubles field capacity of the roasted granular product up to 90%.
The chemical composition of vermiculite-lizardite products is presented in table 2. The main components are magnesium and silicon (24-42%) as well as iron and calcium (4-14%). Titanium, manganese, and nickel are present as microcomponents. The chemical analysis has shown the presence of carbonates in the vermiculite-lizardite sample; the CO2 content is about 3.8-4.8%.

Acid neutralization ability of obtained materials
Carbonates are minerals that are the least resistant to acidic solutions. A well-known property of serpentine minerals, including lizardite, is an ability to interact with acidic solutions. Figure 3 presents the results of the determination of the samples' acid-neutralizing ability B (mg-eq g -1 ).  The data obtained indicate that all vermiculite-lizardite products interact with the acid solution, and the reaction proceeds slowly; equilibrium is reached within 20 to 30 days. The initial granular product is less active compared to the other two products. The relatively high activity of the slime product is explained by its high dispersion. The more intense release of the alkaline potential by the thermally activated granular product may be due to both partial amorphization of lizardite and dehydroxylation of the vermiculite surface.
Theoretically, the maximum value of acid-neutralizing ability can be estimated by the content of magnesium oxide. The degree of interaction of the reagent with the acid solution was calculated as the percentage ratio of the amount of the alkaline component that entered the neutralization reaction after 20 days to the maximum amount of the alkaline component. The calculated data are given in table 2. The highest degree of interaction of alkaline components with acid is 86%; it was fixed for a finedispersed slime product. With an increase in particle size, this indicator decreases to 71% for the initial granular product. Roasting the granular product results in a regular increase (up to 78%) of the interaction degree.