Silica fines stabilization

The observation that the quaternary ammonium monomer content of MDTHD.-DMAEMA and DMAEMA CH3C1 DMAEMA copolymers had little effect on their silica fines stabilization properties of prompted an investigation of nonionic polymers as mineral fines stabilizers (17,18). A series of N-vinylpyrrolidinone (NVP) copolymers with DMAEMA have been studied. Results are summarized in Table VII. 

Limited silica fines stabilization data indicated that increasing copolymer molecular weight from 100,000 to 1,000,000 daltons had, if anything, a negative effect on silica fines stabilization. At a molecular weight of 1,000,000 daltons, this copolymer appeared to be more effective in stabilizing silica fines than silica/kaolinite, calcite, or hematite fines. However, the results may be due in part to the larger particle size and lower surface area of the silica fines (see Table II). 

When the DMAEMA content of NVP - DMAEMA copolymers was reduced from 20% to 8%, the silica fines stabilization effectiveness appeared to improve slightly. When the 80/20 NVP - DMAEMA copolymer was converted to a terpolymer containing 8% DMAEMA (CH SO, silica fines stabilization was substantially unaffected. However, stabilization of silica/kaolinite fines was greatly improved. This suggested that the interaction of polymer quaternary nitrogen atoms with anionic sites on mineral surfaces was important for the stabilization of migrating clays but a different interaction was important for the stabilization of silica fines. Calcite fines stabilization improved while hematite fines stabilization effectiveness decreased. This also indicated the nature of the adsorbed polymer - fine particle complex varied for different minerals. 

The results summarized in Table IX indicated that another copolymer which does not contain quaternary nitrogen atoms, poly (DMAEMA - co - methyl acrylate) was also an effective silica fines stabilizer. 

Increasing the molecular weight of a copolymer containing 5% methyl acrylate (MA) from 100,000 to 1,000,000 daltons had little effect on silica stabilization effectiveness (see Table IX). Increasing the methyl acrylate content from 5% to 30% had also little effect on silica fines stabilization effectiveness. Acidizing substantially reduced the effectiveness of this class of copolymer. Results for the injection of 10,000 pore volumes of water indicated that silica fines elution from the test column was substantially reduced on a long-term basis. 

Copolymers of MDTHD and DMAPMA appeared to be the most effective silica, calcite, and hematite mineral fines stabilizers. Increasing the copolymer MDTHD content had little effect on polymer performance. Similar results were observed for a series of MDTHD -DMAEMA copolymers and a series of DMAEMA CH-C1 salt - DMAEMA copolymers (Table VI). In contrast, increasing the MDTHD content of MDTHD - NNDMAm copolymers from 67% to 90% improved copolymer performance as a silica fines and hematite fines stabilizer. 

The stability of zirconia and zirconia-based packings also allows the use of these columns at an elevated temperature. The key advantage of this is the reduction in analysis time due to the reduction in the viscosity of the mobile phase. In addition, the much broader temperature range also permits a fine-tuning of the selectivity of a separation compared with silica-based packings. 

The aluminium content of the two samples is comparable, when referred to the silica content of the original clay, and the two PILC have comparable surface areas after calcination at 300°C. The ACH bentonite was formed into small extrudates and flash-dried, whereas sample G5 was dried in a thin cake. In both cases, crushing to a fine powder was easy. Sample G5 retains a higher surface area at 800°C in spite of a higher potassium content. Therefore the K O content of the PILC is not the predominant factor for the thermal stability. 

The air-bubble generating and stabilizing process requires a minimum paste consistency. Silica fume particles are smaller than those of Portland cement and addition of silica fume therefore increases the fine fraction of the particles. The higher fraction of smaller particles then increases the surface area causing a greater binding of the water in the mix. This removes the water required for the bubble-generating process. 

For wall-coated open tubular (WCOT) columns, the stationary phase covers the inside surface of the column. The film thickness of the stationary phase can vary from 0.05 to 5 pm. It can be simply deposited on the surface, can originate from the reticulation of a polymer on the silica surface or can be bound to the silica through covalent bonds. The surface of the silica is treated before the stationary phase is deposited to avoid problems of wetability, desorption and stability over time. This treatment can involve attack by HC1 at 350 °C or the deposition of a fine coat of alumina particles. Afterward, the stationary phase is either deposited or prepared in situ by polymerisation at the inner surface of the column. Covalent bonding via Si-O—Si-C allows organic compounds to be bound to the silica surface. In the latter case, the columns are particularly stable and can be rinsed periodically allowing them to recover their initial performance. The efficiency of these columns can reach 150000 theoretical plates. 

Sumitomo Chemical Co. produces a fibre that is a mixture of alumina (85%) and silica (15%). The fibre structure consists of fine crystallites of spinel. Si02 serves to stabilize the spinel structure and prevents it from transforming to a-alumina [14], The flow diagram of this process is shown in Fig. 3.2. 

ICI Co. uses a sol-gel method to produce silica-stabilized alumina (Saffil) and calcia-stabilized zirconia fibre [15], The saffil fibre is a 8-alumina short staple fibre that has about 4% Si02 and a very fine diameter (3 pm). 

The most intriguing fact is that this process is accompanied by a manifold, from 10 to 100 times (up to 1013 cm-2), increase in the concentration of PCs ((sSi-0-)3Si-0 and (=Si-)3C radicals) in the sample. As in the RSi samples, the latter are stabilized in the near-surface silica layers, whereas oxy radicals lie at the silica surface. The oxy radicals produce a characteristic red (620 nm) luminescence excited by the light with a wavelength of 280 nm [64], so that the samples thus prepared emit red light under UV irradiation. Due to a high concentration of the centers in fine-grained silica, this luminescence can be observed even in daylight. 

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